# Top ¶

## 1 Introduction ¶

### 1.1 Overview ¶

Copyright © 1993 by the Regents of the University of California

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see <https://www.gnu.org/licenses/>.

This is a program that is still being written. Many things are missing, including adequate documentation. This manual assumes that you already know how to program in Logo, and merely presents the details of this new implementation.

UCBLogo was originally implemented in 1988 on VAX computers at Berkeley, based on experience gained from implementing LSRHS Logo on PDP-11 computers in the 1970s. Since then UCBLogo has been ported to other computer systems and operating systems.

Read Computer Science Logo Style, Volume 1: Symbolic Computing by Brian Harvey (MIT Press, 1997) for a tutorial on Logo programming with emphasis on symbolic computation.

Here are the special features of this dialect of Logo:

    Source file compatible among Unix, Windows, and Mac platforms.

    Random-access arrays.

    Variable number of inputs to user-defined procedures.

    Mutators for list structure (dangerous).

    Pause on error, and other improvements to error handling.

    Comments and continuation lines; formatting is preserved when
    procedure definitions are saved or edited.

    Terrapin-style tokenization (e.g., [2+3] is a list with one member)
    but LCSI-style syntax (no special forms except TO).  The best of
    both worlds.

    First-class instruction and expression templates (see APPLY).

    Macros.

Features not found in Berkeley Logo include robotics, music, animation, parallelism, and multimedia. For those, a different implementation of Logo is needed.

### 1.2 Getter/Setter Variable Syntax ¶

Logo distinguishes procedures from variables. A procedure is a set of instructions to carry out some computation; a variable is a named container that holds a data value such as a number, word, list, or array.

In traditional Logo syntax, a non-numeric word typed without punctuation represents a request to invoke the procedure named by that word. A word typed with a preceding quotation mark represents the word itself. For example, in the instruction

    	PRINT FIRST "WORD

the procedures named `FIRST` and `PRINT` are invoked, but the procedure named WORD is not invoked; the word W-O-R-D is the input to FIRST.

What about variables? There are two things one can do with a variable: give it a value, and find out its value. To give a variable a value, Logo provides the primitive procedure `MAKE`, which requires two inputs: the name of the variable and the new value to be assigned. The first input, the name of the variable, is just a word, and if (as is almost always the case) the programmer wants to assign a value to a specific variable whose name is known in advance, that input is quoted, just as any known specific word would be:

    	MAKE "MY.VAR FIRST "WORD

gives the variable named `MY.VAR` the value `W` (the first letter of `WORD`).

To find the value of a variable, Logo provides the primitive procedure `THING`, which takes a variable name as its input, and outputs the value of the accessible variable with that name. Thus

    	PRINT THING "MY.VAR

will print `W` (supposing the `MAKE` above has been done). Since finding the value of a specific, known variable name is such a common operation, Logo also provides an abbreviated notation that combines `THING` with quote:

    	PRINT :MY.VAR

The colon (which Logo old-timers pronounce "dots") replaces `THING` and `"` in the earlier version of the instruction.

Newcomers to Logo often complain about the need for all this punctuation. In particular, Logo programmers who learned about dots and quotes without also learning about `THING` wonder why an instruction such as

    	MAKE "NEW.VAR :OLD.VAR

uses two different punctuation marks to identify the two variables. (Having read the paragraphs above, you will understand that actually both variable names are quoted, but the procedure `THING` is invoked to find the value of `OLD.VAR`, since it’s that value, not `OLD.VAR`’s name, that `MAKE` needs to know. It wouldn’t make sense to ask for `THING` of `NEW.VAR`, since we haven’t given `NEW.VAR` a value yet.)

Although Logo’s punctuation rules make sense once understood, they do form a barrier to entry for the Logo beginner. Why, then, couldn’t Logo be designed so that an unpunctuated word would represent a procedure if there is a procedure by that name, or a variable if there is a variable by that name? Then we could say

    	PRINT MY.VAR

and Logo would realize that `MY.VAR` is the name of a variable, not of a procedure. The traditional reason not to use this convention is that Logo allows the same word to name a procedure and a variable at the same time. This is most often important for words that name data types, as in the following procedure:

    	TO PLURAL :WORD
    	OUTPUT WORD :WORD "S
    	END

Here the name `WORD` is a natural choice for the input to `PLURAL`, since it describes the kind of input that `PLURAL` expects. Within the procedure, we use `WORD` to represent Logo’s primitive procedure that combines two input words to form a new, longer word; we use `:WORD` to represent the variable containing the input, whatever actual word is given when `PLURAL` is invoked.

    	? PRINT PLURAL "COMPUTER
    	COMPUTERS

However, if a Logo instruction includes an unquoted word that is not the name of a procedure, Logo could look for a variable of that name instead. This would allow a "punctuationless" Logo, provided that users who want to work without colons for variables choose variable names that are not also procedure names.

What about assigning a value to a variable? Could we do without the quotation mark on `MAKE`’s first input? Alas, no. Although the first input to `MAKE` is usually a constant, known variable name, sometimes it isn’t, as in this example:

    	TO INCREMENT :VAR
    	MAKE :VAR (THING :VAR)+1	; Note: it's not "VAR here!
    	END

    	? MAKE "X 5
    	? INCREMENT "X
    	? PRINT :X
    	6

The procedure `INCREMENT` takes a variable name as its input and changes the value of that variable. In this example there are two variables; the variable whose name is `VAR`, and whose value is the word `X`; and the variable whose name is `X` and whose value changes from 5 to 6. Suppose we changed the behavior of `MAKE` so that it took the word after `MAKE` as the name of the variable to change; we would be unable to write `INCREMENT`:

    	TO INCREMENT :VAR		; nonworking!
    	MAKE VAR (THING VAR)+1
    	END

This would assign a new value to `VAR`, not to `X`.

What we can do is to allow an alternative to `MAKE`, a "setter" procedure for a particular variable. The notation will be

    	? SETFOO 7
    	? PRINT FOO
    	7

`SETFOO` is a "setter procedure" that takes one input (in this case the input 7) and assigns its value to the variable named `FOO`.

Berkeley Logo allows users to choose either the traditional notation, in which case the same name can be used both for a procedure and for a variable, or the getter/setter notation, in which variable `FOO` is set with `SETFOO` and examined with `FOO`, but the same name can’t be used for procedure and variable.

Here is how this choice is allowed: Berkeley Logo uses traditional notation, with procedures distinct from variables. However, if there is a variable named `AllowGetSet` whose value is TRUE (which there is, by default, when Logo starts up), then if a Logo instruction refers to a nonexistent procedure (so that the error message "I don’t know how to ..." would result), Logo tries the following two steps:

    	1.  If the name is at least four characters long, and the first three
    	characters are the letters SET (upper or lower case), and if the name
    	is followed in the instruction by another value, and if the name
    	without the SET is the name of a variable that already exists, then
    	Logo will invoke MAKE with its first input being the name without the
    	SET, and its second input being the following value.

    	2.  If step 1’s conditions are not met, but the name is
    	the name of an accessible variable, then Logo will invoke
    	THING with that name as input, to find the variable’s value.

Step 1 requires that the variable already exist so that misspellings of names of `SETxxx` primitives (e.g., `SETHEADING`) will still be caught, instead of silently creating a new variable. The command `GLOBAL` can be used to create a variable without giving it a value.

One final point: The `TO` command in Logo has always been a special case; the rest of the line starting with `TO` is not evaluated as ordinary Logo expressions are. In particular, the colons used to mark the names of inputs to the procedure do not cause `THING` to be invoked. They are merely mnemonic aids, reminding the Logo user that these words are names of variables. (Arguably, this nonstantard behavior of `TO` adds to Logo beginners’ confusion about colons.) To a programmer using colonless variable references, the colons in the `TO` line are unnecessary and meaningless. Berkeley Logo therefore makes the colons optional:

    	TO FOO :IN1 :IN2

and

    	TO FOO IN1 IN2

are both allowed.

### 1.3 Entering and Leaving Logo ¶

The process to start Logo depends on your operating system:

Unix:

  Type the word `logo` to the shell. (The directory in which you’ve installed Logo must be in your path.)

DOS:

  Change directories to the one containing Logo (probably `C:\UCBLOGO`). Then type `UCBLOGO` for the large memory version, or `BL` for the 640K version.

Mac:

  Double-click on the `LOGO` icon within the "UCB Logo" folder.

Windows:

  Double-click on the `UCBWLOGO` icon in the `UCBLOGO` folder.

To leave Logo, enter the command `bye`.

On startup, Logo looks for a file named `startup.lg` in the system Logo library and, if found, loads it. Then it looks for `startup.lg` in the user’s home directory, or the current directory, depending on the operating system, and loads that. These startup files can be used to predefine procedures, e.g., to provide non-English names for primitive procedures.

Under Unix, DOS, or Windows, if you include one or more filenames on the command line when starting Logo, those files will be loaded before the interpreter starts reading commands from your terminal. If you load a file that executes some program that includes a `bye` command, Logo will run that program and exit. You can therefore write standalone programs in Logo and run them with shell/batch scripts. To support this technique, Logo does not print its usual welcoming and parting messages if you give file arguments to the logo command.

If a command line argument is just a hyphen, then all command line arguments after the hyphen are not taken as filenames, but are instead collected in a list, one word per argument; the buried variable `COMMAND.LINE` contains that list of arguments, or the empty list if there are none. On my Linux system, if the first line of an executable shell script is `#!/usr/local/bin/logo -` (note the hyphen) then the script can be given command line arguments and they all end up in `:COMMAND.LINE` along with the script’s path. Experiment.

If you type your interrupt character (see table below) Logo will stop what it’s doing and return to top-level, as if you did `THROW "TOPLEVEL`. If you type your quit character Logo will pause as if you did `PAUSE`.

                wxWidgets      Unix       DOS/Windows     Mac Classic

    toplevel      alt-S   usually ctrl-C    ctrl-Q     command-. (period)

    pause         alt-P   usually ctrl-\    ctrl-W     command-, (comma)

If you have an environment variable called `LOGOLIB` whose value is the name of a directory, then Logo will use that directory instead of the default library. If you invoke a procedure that has not been defined, Logo first looks for a file in the current directory named `proc.lg` where proc is the procedure name in lower case letters. If such a file exists, Logo loads that file. If the missing procedure is still undefined, or if there is no such file, Logo then looks in the library directory for a file named `proc` (no `.lg`) and, if it exists, loads it. If neither file contains a definition for the procedure, then Logo signals an error. Several procedures that are primitive in most versions of Logo are included in the default library, so if you use a different library you may want to include some or all of the default library in it.

### 1.4 Tokenization ¶

Names of procedures, variables, and property lists are case-insensitive. So are the special words `END`, `TRUE`, and `FALSE`. Case of letters is preserved in everything you type, however.

Within square brackets, words are delimited only by spaces and square brackets. `[2+3]` is a list containing one word. Note, however, that the Logo primitives that interpret such a list as a Logo instruction or expression (`RUN`, `IF`, etc.) reparse the list as if it had not been typed inside brackets.

After a quotation mark outside square brackets, a word is delimited by a space, a square bracket, or a parenthesis.

A word not after a quotation mark or inside square brackets is delimited by a space, a bracket, a parenthesis, or an infix operator `+-*/=<>`. Note that words following colons are in this category. Note that quote and colon are not delimiters. Each infix operator character is a word in itself, except that the two-character sequences `<=`, `>=`, and `<>` (the latter meaning not-equal) with no intervening space are recognized as a single word.

A word consisting of a question mark followed by a number (e.g., `?37`), when runparsed (i.e., where a procedure name is expected), is treated as if it were the sequence

    ( ? 37 )

making the number an input to the ? procedure. (See the discussion of templates, below.) This special treatment does not apply to words read as data, to words with a non-number following the question mark, or if the question mark is backslashed.

A line (an instruction line or one read by `READLIST` or `READWORD`) can be continued onto the following line if its last character is a tilde (`~`). `READWORD` preserves the tilde and the newline; `READLIST` does not.

Lines read with `READRAWLINE` are never continued.

An instruction line or a line read by `READLIST` (but not by `READWORD`) is automatically continued to the next line, as if ended with a tilde, if there are unmatched brackets, parentheses, braces, or vertical bars pending. However, it’s an error if the continuation line contains only the word `END`; this is to prevent runaway procedure definitions. Lines explicitly continued with a tilde avoid this restriction.

If a line being typed interactively on the keyboard is continued, either with a tilde or automatically, Logo will display a tilde as a prompt character for the continuation line.

A semicolon begins a comment in an instruction line. Logo ignores characters from the semicolon to the end of the line. A tilde as the last character still indicates a continuation line, but not a continuation of the comment. For example, typing the instruction

    print "abc;comment ~
    def

will print the word `abcdef`. Semicolon has no special meaning in data lines read by `READWORD` or `READLIST`, but such a line can later be reparsed using `RUNPARSE` and then comments will be recognized.

The two-character sequence `#!` at the beginning of a line also starts a comment. Unix users can therefore write a file containing Logo commands, starting with the line

    #! /usr/local/bin/logo

(or wherever your Logo executable lives) and the file will be executable directly from the shell.

To include an otherwise delimiting character (including semicolon or tilde) in a word, precede it with backslash (`\`). If the last character of a line is a backslash, then the newline character following the backslash will be part of the last word on the line, and the line continues onto the following line. To include a backslash in a word, use `\\`. If the combination backslash-newline is entered at the terminal, Logo will issue a backslash as a prompt character for the continuation line. All of this applies to data lines read with `READWORD` or `READLIST` as well as to instruction lines.

A line read with `READRAWLINE` has no special quoting mechanism; both backslash and vertical bar (described below) are just ordinary characters.

An alternative notation to include otherwise delimiting characters in words is to enclose a group of characters in vertical bars. All characters between vertical bars are treated as if they were letters. In data read with `READWORD` the vertical bars are preserved in the resulting word. In data read with `READLIST` (or resulting from a `PARSE` or `RUNPARSE` of a word) the vertical bars do not appear explicitly; all potentially delimiting characters (including spaces, brackets, parentheses, and infix operators) appear unmarked, but tokenized as though they were letters. Within vertical bars, backslash may still be used; the only characters that must be backslashed in this context are backslash and vertical bar themselves.

Characters entered between vertical bars are forever special, even if the word or list containing them is later reparsed with `PARSE` or `RUNPARSE`. Characters typed after a backslash are treated somewhat differently: When a quoted word containing a backslashed character is runparsed, the backslashed character loses its special quality and acts thereafter as if typed normally. This distinction is important only if you are building a Logo expression out of parts, to be `RUN` later, and want to use parentheses. For example,

    PRINT RUN (SE "\( 2 "+ 3 "\))

will print 5, but

    RUN (SE "MAKE ""|(| 2)

will create a variable whose name is open-parenthesis. (Each example would fail if vertical bars and backslashes were interchanged.)

A normally delimiting character entered within vertical bars is `EQUALP` to the same character without the vertical bars, but can be distinguished by the `VBARREDP` predicate. (However, `VBARREDP` returns `TRUE` only for characters for which special treatment is necessary: whitespace, parentheses, brackets, infix operators, backslash, vertical bar, tilde, quote, question mark, colon, and semicolon.)

## 2 Data Structure Primitives ¶

### 2.1 Constructors ¶

#### word ¶

    WORD word1 word2
    (WORD word1 word2 word3 ...)

outputs a word formed by concatenating its inputs.

#### list ¶

    LIST thing1 thing2
    (LIST thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, which can be any Logo datum (word, list, or array).

#### sentence ¶

    SENTENCE thing1 thing2
    SE thing1 thing2
    (SENTENCE thing1 thing2 thing3 ...)
    (SE thing1 thing2 thing3 ...)

outputs a list whose members are its inputs, if those inputs are not lists, or the members of its inputs, if those inputs are lists.

#### fput ¶

    FPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the beginning. If the second input is a word, then the first input must be a one-letter word, and FPUT is equivalent to WORD.

#### lput ¶

    LPUT thing list

outputs a list equal to its second input with one extra member, the first input, at the end. If the second input is a word, then the first input must be a one-letter word, and LPUT is equivalent to WORD with its inputs in the other order.

#### array ¶

    ARRAY size
    (ARRAY size origin)

outputs an array of size members (must be a positive integer), each of which initially is an empty list. Array members can be selected with `ITEM` and changed with `SETITEM`. The first member of the array is member number 1 unless an origin input (must be an integer) is given, in which case the first member of the array has that number as its index. (Typically 0 is used as the origin if anything.) Arrays are printed by `PRINT` and friends, and can be typed in, inside curly braces; indicate an origin with `{a b c}@0`.

See item , setitem .

#### mdarray ¶

    MDARRAY sizelist				(library procedure)
    (MDARRAY sizelist origin)

outputs a multi-dimensional array. The first input must be a list of one or more positive integers. The second input, if present, must be a single integer that applies to every dimension of the array.

Ex: `(MDARRAY [3 5] 0)` outputs a two-dimensional array whose members range from `[0 0]` to `[2 4]`.

#### listtoarray ¶

    LISTTOARRAY list
    (LISTTOARRAY list origin)

outputs an array of the same size as the input list, whose members are the members of the input list.

#### arraytolist ¶

    ARRAYTOLIST array

outputs a list whose members are the members of the input array. The first member of the output is the first member of the array, regardless of the array’s origin.

#### combine ¶

    COMBINE thing1 thing2				(library procedure)

if thing2 is a word, outputs `WORD thing1 thing2`. If thing2 is a list, outputs `FPUT thing1 thing2`.

See word , fput

#### reverse ¶

    REVERSE list					(library procedure)

outputs a list whose members are the members of the input list, in reverse order.

#### gensym ¶

    GENSYM						(library procedure)

outputs a unique word each time it’s invoked. The words are of the form `G1`, `G2`, etc.

### 2.2 Data Selectors ¶

#### first ¶

    FIRST thing

if the input is a word, outputs the first character of the word. If the input is a list, outputs the first member of the list. If the input is an array, outputs the origin of the array (that is, the index of the first member of the array).

#### firsts ¶

    FIRSTS list

outputs a list containing the `FIRST` of each member of the input list. It is an error if any member of the input list is empty. (The input itself may be empty, in which case the output is also empty.) This could be written as

    to firsts :list
    output map "first :list
    end

but is provided as a primitive in order to speed up the iteration tools `MAP`, `MAP.SE`, and `FOREACH`.

    to transpose :matrix
    if emptyp first :matrix [op []]
    op fput firsts :matrix transpose bfs :matrix
    end

#### last ¶

    LAST wordorlist

if the input is a word, outputs the last character of the word. If the input is a list, outputs the last member of the list.

#### butfirst ¶

    BUTFIRST wordorlist
    BF wordorlist

if the input is a word, outputs a word containing all but the first character of the input. If the input is a list, outputs a list containing all but the first member of the input.

#### butfirsts ¶

    BUTFIRSTS list
    BFS list

outputs a list containing the `BUTFIRST` of each member of the input list. It is an error if any member of the input list is empty or an array. (The input itself may be empty, in which case the output is also empty.) This could be written as

    to butfirsts :list
    output map "butfirst :list
    end

but is provided as a primitive in order to speed up the iteration tools `MAP`, `MAP.SE`, and `FOREACH`.

#### butlast ¶

    BUTLAST wordorlist
    BL wordorlist

if the input is a word, outputs a word containing all but the last character of the input. If the input is a list, outputs a list containing all but the last member of the input.

#### item ¶

    ITEM index thing

if the thing is a word, outputs the indexth character of the word. If the thing is a list, outputs the indexth member of the list. If the thing is an array, outputs the indexth member of the array. Index starts at 1 for words and lists; the starting index of an array is specified when the array is created.

#### mditem ¶

    MDITEM indexlist array				(library procedure)

outputs the member of the multidimensional array selected by the list of numbers indexlist.

#### pick ¶

    PICK list					(library procedure)

outputs a randomly chosen member of the input list.

#### remove ¶

    REMOVE thing list				(library procedure)

outputs a copy of list with every member equal to thing removed.

#### remdup ¶

    REMDUP list					(library procedure)

outputs a copy of list with duplicate members removed. If two or more members of the input are equal, the rightmost of those members is the one that remains in the output.

#### quoted ¶

    QUOTED thing					(library procedure)

outputs its input, if a list; outputs its input with a quotation mark prepended, if a word.

### 2.3 Data Mutators ¶

#### setitem ¶

    SETITEM index array value

command. Replaces the indexth member of array with the new value. Ensures that the resulting array is not circular, i.e., value may not be a list or array that contains array.

#### mdsetitem ¶

    MDSETITEM indexlist array value			(library procedure)

command. Replaces the member of array chosen by indexlist with the new value.

#### .setfirst ¶

    .SETFIRST list value

command. Changes the first member of list to be value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETFIRST` can lead to circular list structures, which will get some Logo primitives into infinite loops, and to unexpected changes to other data structures that share storage with the list being modified.

#### .setbf ¶

    .SETBF list value

command. Changes the butfirst of list to be value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETBF` can lead to circular list structures, which will get some Logo primitives into infinite loops; unexpected changes to other data structures that share storage with the list being modified; or to Logo crashes and coredumps if the butfirst of a list is not itself a list.

#### .setitem ¶

    .SETITEM index array value

command. Changes the indexth member of array to be value, like `SETITEM`, but without checking for circularity.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of `.SETITEM` can lead to circular arrays, which will get some Logo primitives into infinite loops.

See setitem.

#### push ¶

    PUSH stackname thing				(library procedure)

command. Adds the thing to the stack that is the value of the variable whose name is stackname. This variable must have a list as its value; the initial value should be the empty list. New members are added at the front of the list.

#### pop ¶

    POP stackname					(library procedure)

outputs the most recently `PUSH`ed member of the stack that is the value of the variable whose name is stackname and removes that member from the stack.

#### queue ¶

    QUEUE queuename thing				(library procedure)

command. Adds the thing to the queue that is the value of the variable whose name is queuename. This variable must have a list as its value; the initial value should be the empty list. New members are added at the back of the list.

#### dequeue ¶

    DEQUEUE queuename				(library procedure)

outputs the least recently `QUEUE`d member of the queue that is the value of the variable whose name is queuename and removes that member from the queue.

### 2.4 Predicates ¶

#### wordp ¶

    WORDP thing
    WORD? thing

outputs `TRUE` if the input is a word, `FALSE` otherwise.

#### listp ¶

    LISTP thing
    LIST? thing

outputs `TRUE` if the input is a list, `FALSE` otherwise.

#### arrayp ¶

    ARRAYP thing
    ARRAY? thing

outputs `TRUE` if the input is an array, `FALSE` otherwise.

#### emptyp ¶

    EMPTYP thing
    EMPTY? thing

outputs `TRUE` if the input is the empty word or the empty list, `FALSE` otherwise.

#### equalp ¶

    EQUALP thing1 thing2
    EQUAL? thing1 thing2
    thing1 = thing2

outputs `TRUE` if the inputs are equal, `FALSE` otherwise. Two numbers are equal if they have the same numeric value. Two non-numeric words are equal if they contain the same characters in the same order. If there is a variable named `CASEIGNOREDP` whose value is `TRUE`, then an upper case letter is considered the same as the corresponding lower case letter. (This is the case by default.) Two lists are equal if their members are equal. An array is only equal to itself; two separately created arrays are never equal even if their members are equal. (It is important to be able to know if two expressions have the same array as their value because arrays are mutable; if, for example, two variables have the same array as their values then performing `SETITEM` on one of them will also change the other.)

See caseignoredp , setitem

#### notequalp ¶

    NOTEQUALP thing1 thing2
    NOTEQUAL? thing1 thing2
    thing1 <> thing2

outputs `FALSE` if the inputs are equal, `TRUE` otherwise. See `EQUALP` for the meaning of equality for different data types.

#### beforep ¶

    BEFOREP word1 word2
    BEFORE? word1 word2

outputs `TRUE` if word1 comes before word2 in ASCII collating sequence (for words of letters, in alphabetical order). Case-sensitivity is determined by the value of `CASEIGNOREDP`. Note that if the inputs are numbers, the result may not be the same as with `LESSP`; for example, `BEFOREP 3 12` is false because 3 collates after 1.

See caseignoredp , lessp

#### .eq ¶

    .EQ thing1 thing2

outputs `TRUE` if its two inputs are the same datum, so that applying a mutator to one will change the other as well. Outputs `FALSE` otherwise, even if the inputs are equal in value.

WARNING: Primitives whose names start with a period are dangerous. Their use by non-experts is not recommended. The use of mutators can lead to circular data structures, infinite loops, or Logo crashes.

#### memberp ¶

    MEMBERP thing1 thing2
    MEMBER? thing1 thing2

if thing2 is a list or an array, outputs `TRUE` if thing1 is `EQUALP` to a member of thing2, `FALSE` otherwise. If thing2 is a word, outputs `TRUE` if thing1 is a one-character word `EQUALP` to a character of thing2, `FALSE` otherwise.

See equalp .

#### substringp ¶

    SUBSTRINGP thing1 thing2
    SUBSTRING? thing1 thing2

if thing1 or thing2 is a list or an array, outputs `FALSE`. If thing2 is a word, outputs `TRUE` if thing1 is `EQUALP` to a substring of thing2, `FALSE` otherwise.

See equalp .

#### numberp ¶

    NUMBERP thing
    NUMBER? thing

outputs `TRUE` if the input is a number, `FALSE` otherwise.

#### vbarredp ¶

    VBARREDP char
    VBARRED? char
    BACKSLASHEDP char                               (library procedure)
    BACKSLASHED? char                               (library procedure)

outputs `TRUE` if the input character was originally entered into Logo within vertical bars (|) to prevent its usual special syntactic meaning, `FALSE` otherwise. (Outputs `TRUE` only if the character is a backslashed space, tab, newline, or one of `()[]+-*/=<>":;\~?|` )

The names `BACKSLASHEDP` and `BACKSLASHED?` are included in the Logo library for backward compatibility with the former names of this primitive, although it does not output `TRUE` for characters originally entered with backslashes.

### 2.5 Queries ¶

#### count ¶

    COUNT thing

outputs the number of characters in the input, if the input is a word; outputs the number of members in the input, if it is a list or an array. (For an array, this may or may not be the index of the last member, depending on the array’s origin.)

#### ascii ¶

    ASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code. Interprets control characters as representing vbarred punctuation, and returns the character code for the corresponding punctuation character without vertical bars. (Compare `RAWASCII`.)

#### rawascii ¶

    RAWASCII char

outputs the integer (between 0 and 255) that represents the input character in the ASCII code. Interprets control characters as representing themselves. To find out the ASCII code of an arbitrary keystroke, use `RAWASCII RC`.

#### char ¶

    CHAR int

outputs the character represented in the ASCII code by the input, which must be an integer between 0 and 255.

See ascii .

#### member ¶

    MEMBER thing1 thing2

if thing2 is a word or list and if `MEMBERP` with these inputs would output `TRUE`, outputs the portion of thing2 from the first instance of thing1 to the end. If `MEMBERP` would output `FALSE`, outputs the empty word or list according to the type of thing2. It is an error for thing2 to be an array.

See memberp .

#### lowercase ¶

    LOWERCASE word

outputs a copy of the input word, but with all uppercase letters changed to the corresponding lowercase letter.

#### uppercase ¶

    UPPERCASE word

outputs a copy of the input word, but with all lowercase letters changed to the corresponding uppercase letter.

#### standout ¶

    STANDOUT thing

outputs a word that, when printed, will appear like the input but displayed in standout mode (boldface, reverse video, or whatever your version does for standout). The word contains machine-specific magic characters at the beginning and end; in between is the printed form (as if displayed using `TYPE`) of the input. The output is always a word, even if the input is of some other type, but it may include spaces and other formatting characters. Note: a word output by `STANDOUT` while Logo is running on one machine will probably not have the desired effect if printed on another type of machine.

In the Macintosh classic version, the way that standout works is incompatible with the use of characters whose ASCII code is greater than 127. Therefore, you have a choice to make: The instruction

    CANINVERSE 0

disables standout, but enables the display of ASCII codes above 127, and the instruction

    CANINVERSE 1

restores the default situation in which standout is enabled and the extra graphic characters cannot be printed.

#### parse ¶

    PARSE word

outputs the list that would result if the input word were entered in response to a `READLIST` operation. That is, `PARSE READWORD` has the same value as `READLIST` for the same characters read.

See readlist .

#### runparse ¶

    RUNPARSE wordorlist

outputs the list that would result if the input word or list were entered as an instruction line; characters such as infix operators and parentheses are separate members of the output. Note that sublists of a runparsed list are not themselves runparsed.

## 3 Objects ¶

Note: The object implementation in UCBLogo is new and still being worked on. It also is optional and may not be compiled into the version you are using. Bug reports and patches are greatly appreciated.

An object is a data structure, like a list, except that instead of just a sequence of elements, an object has named elements, which are either variables or procedures. (A procedure that belongs to an object is sometimes called a method.) In the object metaphor, instead of just having the one computer that does everything, we think of each object as capable of doing things. One object can ask another object to do something for it.

At any moment, there is a current object. That object might have its own version of procedures that are available globally and documented in this manual. So workspace operations such as `pots` or `erase` might have different effects depending on the current object.

The initial current object is called `logo`; there is an operation that outputs that object. Every object has one or more parents, and ultimately the parent of the parent... of an object has to be `logo`. The primitive procedures in this manual belong to the `logo` object, as do global variables.

Turtles are objects. This is valuable in part so that you can customize the behavior of a turtle. For example, if you’re simulating a kaleidoscope, you might want half your turtles to swap the meanings of the `left` and `right` commands. But given what’s said above about there being a current object, and that the initial current object is `logo`, which isn’t a turtle, you might think that you can’t run `forward 100` without first creating a turtle object and making it the current object. But a goal of this object system is that things that were easy in traditional Logo should still be easy. And so, in addition to the current object, there is also a default turtle; if a turtle procedure is addressed to an object that isn’t a turtle, there is a `logo` procedure for each turtle procedure that does the equivalent of

    to forward :distance
    ask defaultturtle [forward :distance]
    end

Unlike some other object oriented languages, Berkeley Logo does not distinguish between classes (kinds of objects, which mostly specify the procedures (the methods) of objects of that kind) and instances (individual concrete objects, with state memory in the form of variables belonging to the instance). Instead we use prototyping OOP, in which any object (instance) can serve as the model, the prototype (the class), for other objects.

Class/instance OOP is the right thing for large teams of programmers who need a detailed specification (in the form of class definitions) of the desired program behavior before they start writing code. But prototyping OOP is the right thing for tinkering, for having a glimmer of an idea and playing around with it without a rigid specification. So, you want to have dogs in your program. You start by building a dog, one you can see on the screen as you invent behaviors such as `roll.over` and `wag.tail`. Then you make a bunch of objects with that dog as their parent, using that dog as a prototype. But the prototype dog is still a particular dog, with a particular position, color, and so on.

Having said that, sometimes you do want to distinguish a class object, which will mostly have procedures, with few or no variables, and its instance objects, which mostly inherit procedures from the class, but have their own individual variables. To accommodate that style of work, we distinguish two ways to make a child object from a parent object. `Kindof` takes an object as input, and outputs a new object that inherits from the input object. This is how to make a subclass that is mostly like the given class, but overrides certain behaviors. `Oneof` makes a child object, but `ask`s the new object to `exist` before outputting it. Every object has an `exist` procedure, because it inherits one from `logo` if no intermediate ancestor specifies one. The details come a little later in this section, in the Constructors subsection, but this is the general idea of how class/instance programming can be accommodated in a prototyping OOP language.

### 3.1 Constructors ¶

#### kindof ¶

    KINDOF object
    KINDOF objectlist
    (KINDOF object1 object2 ...)

creates and outputs an object whose parent is object, or whose parents are object1, object2, etc., or the elements of objectlist. There must be at least one input, and if a list, it may not be empty.

#### something ¶

    SOMETHING

creates and outputs an object whose parent is the `Logo` object. `make "foo something` is equivalent to `make "foo kindof logo`.

#### oneof ¶

    ONEOF object
    ONEOF objectlist
    (ONEOF object input1 input2 ...)
    (ONEOF objectlist input1 input2 ...)

creates an object whose parent is object or whose parents are the elements of objectlist. `Ask`s the new object to `exist`, and then outputs the object. Before the object is `Ask`ed to `exist`, all remaining inputs after the first are collected into a list, which is made the value of the global variable `initlist`. The convention is that `initlist` contains alternating names and values, which serve to initialize the newly created object.

The output of `oneof` is usually called an instance.

See exist.

### 3.2 Mutators ¶

#### exist ¶

    exist

Initializes a new object. It is run automatically by `oneof`.

The default `exist` procedure creates object-local variables with the odd-numbered elements of `initlist` as names, and the corresponding even-numbered elements as values. Thus

    make "obj (oneof logo "foo "bar "baz "garply)

will create a new object and then tell it to `exist`, which will in effect tell it to

    havemake "foo "bar
    havemake "baz "garply

Example: Objects do not usually keep track of their instances. To make one that does, you can specialize `exist`:

    ? make "recordingobject something
    ? ask :recordingobject [havemake "instances []]
    ? ask :recordingobject [to exist]
    > usual.exist
    > make "instances [fput self :instances]
    > end

See oneof. usual, havemake.

#### have ¶

    HAVE name
    HAVE namelist

tells the current object to create an object variable named name, or object variables whose names are the elements of namelist.

See havemake.

#### havemake ¶

    HAVEMAKE name value

tells the current object to create an object variable named name, and give it the value value. Since procedure input values are computed before the procedure is run, the value expression may refer to the object’s parent’s variable named name:

    havemake "size :size/2

See have, make.

### 3.3 selectors ¶

All selectors apply to the current object. If you want to select from another object, use `ask`.

#### self ¶

    SELF

outputs the current object (not its name!).

#### parents ¶

    PARENTS

outputs a list containing the parent(s) of the current object. All objects except for `logo` have at least one parent. Note that the elements of the output list are objects, not names of objects.

Examples:

    to grandparents
    output remdup (map.se [ask ? [parents]] parents)
    end

    to ancestors
    if emptyp parents [output (list self)]   ; can happen only if self=logo
    output remdup sentence parents map.se [ask ? [ancestors]] parents
    end

#### mynames ¶

    MYNAMES

output a list of the names of the object variables owned (not inherited) by the current object.

#### mynamep ¶

    MYNAMEP name
    MYNAME? name

outputs `true` if name is the name of an object variable owned (not inherited) by the current object, `false` otherwise.

See mynames.

#### myprocs ¶

    MYPROCS

outputs a list of the names of the procedures (methods) owned by (not inherited by) the current object.

#### myprocp ¶

    MYPROCP name
    MYPROC? name

outputs `true` if name is the name of a procedure (a method) owned (not inherited) by the current object, `false` otherwise.

See myprocs.

### 3.4 Messages ¶

The procedures in this section are for changing the current object, either permanently (unless changed again) or just to send one message.

#### talkto ¶

    TALKTO object

changes the current object to object. (Note that the input is an object, not the name of an object.) `Talkto` can be used only at toplevel or within a `pause` (when typing into a Logo prompt, not inside a procedure).

See ask.

#### ask ¶

    ASK object runlist

command or operation. Temporarily sets the current object to object while running the instructions or expression in runlist. If runlist is an expression, then `ask` outputs its value. As soon as runlist finishes, the current object is set back to its previous value.

See talkto.

### 3.5 Queries ¶

These procedures are particularly useful when debugging.

#### whosename ¶

    WHOSENAME name

outputs the object that owns the currently accessible variable named name. If there is no such accessible variable, or it’s a procedure-local variable, an error is signalled.

#### whoseproc ¶

    WHOSEPROC name

outputs the object that owns the currently accessible procedure named name. If there is no such procedure, an error is signalled.

### 3.6 usual ¶

    to foo ...
    ...
    USUAL.foo ...
    ...
    end

`Usual` is not a procedure. It’s a special notation that can be used only inside a procedure definition; the word `usual` must be followed by a period and then the name of the procedure being defined. (That is, `usual.foo` can be used only inside the definition of `foo`.) It refers to the procedure that would be inherited from a parent if this (re)definition didn’t exist; it allows a specialized method to invoke the ordinary version.

    make "bigturtle kindof turtle
    ask :bigturtle [to forward :length]
    usual.forward 2 * :length
    end

If there is no inherited procedure of the same name, then calling `usual.___` does nothing if used as a command, or outputs an empty list if used as an operation.

If an object has multiple parents, the behavior of `usual` may be confusing. Suppose the current object has two parents, `A` and `B`, in that order. Then `usual.foo` in the current object’s own `foo` method refers to object `A`’s `foo`, but `usual.foo` within object `A`’s `foo` refers to object `B`’s `foo` in this situation, even though `A`’s parent isn’t `B`.

    make "dashedturtle kindof turtle
    ask :dashedturtle [to forward :length]
    if not pendownp [usual.forward :length stop]
    if :length <= 5 [usual.forward :length stop]
    usual.forward 5
    penup
    usual.forward (ifelse :length <= 10 [:length-5] [5])
    pendown
    if :length > 10 [forward :length-10]    ; Note no USUAL here.
    end

    ask :dashedturtle [forward 25]
    - - -►

    make "bigdashed oneof (list :bigturtle :dashedturtle)
    make "dashedbig oneof (list :dashedturtle :bigturtle)
    ask :bigdashed [forward 25]
    - - - - - ►
    ask :dashedbig [forward 25]
    --  --  --►

## 4 Communication ¶

### 4.1 Transmitters ¶

Note: If there is a variable named `PRINTDEPTHLIMIT` with a nonnegative integer value, then complex list and array structures will be printed only to the allowed depth. That is, members of members of... of members will be allowed only so far. The members omitted because they are just past the depth limit are indicated by an ellipsis for each one, so a too-deep list of two members will print as `[... ...]`.

If there is a variable named `PRINTWIDTHLIMIT` with a nonnegative integer value, then only the first so many members of any array or list will be printed. A single ellipsis replaces all missing data within the structure. The width limit also applies to the number of characters printed in a word, except that a `PRINTWIDTHLIMIT` between 0 and 9 will be treated as if it were 10 when applied to words. This limit applies not only to the top-level printed datum but to any substructures within it.

See printdepthlimit , printwidthlimit

If there is a variable named `FULLPRINTP` whose value is `TRUE`, then words that were created using backslash or vertical bar (to include characters that would otherwise not be treated as part of a word) are printed with the backslashes or vertical bars shown, so that the printed result could be re-read by Logo to produce the same value. If `FULLPRINTP` is `TRUE` then the empty word (however it was created) prints as `||`. (Otherwise it prints as nothing at all.)

See fullprintp .

#### print ¶

    PRINT thing
    PR thing
    (PRINT thing1 thing2 ...)
    (PR thing1 thing2 ...)

command. Prints the input or inputs to the current write stream (initially the screen). All the inputs are printed on a single line, separated by spaces, ending with a newline. If an input is a list, square brackets are not printed around it, but brackets are printed around sublists. Braces are always printed around arrays.

#### type ¶

    TYPE thing
    (TYPE thing1 thing2 ...)

command. Prints the input or inputs like `PRINT`, except that no newline character is printed at the end and multiple inputs are not separated by spaces. Note: printing to the screen is ordinarily line buffered; that is, the characters you print using `TYPE` will not actually appear on the screen until either a newline character is printed (for example, by `PRINT` or `SHOW`) or Logo tries to read from the keyboard (either at the request of your program or after an instruction prompt). This buffering makes the program much faster than it would be if each character appeared immediately, and in most cases the effect is not disconcerting. To accommodate programs that do a lot of positioned text display using `TYPE`, Logo will force printing whenever `CURSOR` or `SETCURSOR` is invoked. This solves most buffering problems. Still, on occasion you may find it necessary to force the buffered characters to be printed explicitly; this can be done using the `WAIT` command. `WAIT 0` will force printing without actually waiting.

See setcursor , wait

#### show ¶

    SHOW thing
    (SHOW thing1 thing2 ...)

command. Prints the input or inputs like `PRINT`, except that if an input is a list it is printed inside square brackets.

See print .

### 4.2 Receivers ¶

#### readlist ¶

    READLIST
    RL

reads a line from the read stream (initially the keyboard) and outputs that line as a list. The line is separated into members as though it were typed in square brackets in an instruction. If the read stream is a file, and the end of file is reached, `READLIST` outputs the empty word (not the empty list). `READLIST` processes backslash, vertical bar, and tilde characters in the read stream; the output list will not contain these characters but they will have had their usual effect. `READLIST` does not, however, treat semicolon as a comment character.

#### readword ¶

    READWORD
    RW

reads a line from the read stream and outputs that line as a word. The output is a single word even if the line contains spaces, brackets, etc. If the read stream is a file, and the end of file is reached, `READWORD` outputs the empty list (not the empty word). `READWORD` processes backslash, vertical bar, and tilde characters in the read stream. In the case of a tilde used for line continuation, the output word does include the tilde and the newline characters, so that the user program can tell exactly what the user entered. Vertical bars in the line are also preserved in the output. Backslash characters are not preserved in the output.

#### readrawline ¶

    READRAWLINE

reads a line from the read stream and outputs that line as a word. The output is a single word even if the line contains spaces, brackets, etc. If the read stream is a file, and the end of file is reached, `READRAWLINE` outputs the empty list (not the empty word). `READRAWLINE` outputs the exact string of characters as they appear in the line, with no special meaning for backslash, vertical bar, tilde, or any other formatting characters.

See readword .

#### readchar ¶

    READCHAR
    RC

reads a single character from the read stream and outputs that character as a word. If the read stream is a file, and the end of file is reached, `READCHAR` outputs the empty list (not the empty word). If the read stream is the keyboard, echoing is turned off when `READCHAR` is invoked, and remains off until `READLIST` or `READWORD` is invoked or a Logo prompt is printed. Backslash, vertical bar, and tilde characters have no special meaning in this context.

See readlist .

#### readchars ¶

    READCHARS num
    RCS num

reads num characters from the read stream and outputs those characters as a word. If the read stream is a file, and the end of file is reached, `READCHARS` outputs the empty list (not the empty word). If the read stream is the keyboard, echoing is turned off when `READCHARS` is invoked, and remains off until `READLIST` or `READWORD` is invoked or a Logo prompt is printed. Backslash, vertical bar, and tilde characters have no special meaning in this context.

See readlist , readword

#### shell ¶

    SHELL command
    (SHELL command wordflag)

Under Unix, outputs the result of running command as a shell command. (The command is sent to ‘`/bin/sh`’, not ‘`csh`’ or other alternatives.) If the command is a literal list in the instruction line, and if you want a backslash character sent to the shell, you must use `\\` to get the backslash through Logo’s reader intact. The output is a list containing one member for each line generated by the shell command. Ordinarily each such line is represented by a list in the output, as though the line were read using `READLIST`. If a second input is given, regardless of the value of the input, each line is represented by a word in the output as though it were read with `READWORD`. Example:

    to dayofweek
    output first first shell [date]
    end

This is `first first` to extract the first word of the first (and only) line of the shell output.

Under MacOS X, `SHELL` works as under Unix. `SHELL` is not available under Mac Classic.

Under DOS, `SHELL` is a command, not an operation; it sends its input to a DOS command processor but does not collect the result of the command.

Under Windows, the wxWidgets version of Logo behaves as under Unix (except that DOS-style commands are understood; use `dir` rather than `ls`). The non-wxWidgets version behaves like the DOS version.

### 4.3 File Access ¶

#### setprefix ¶

    SETPREFIX string

command. Sets a prefix that will be used as the implicit beginning of filenames in `OPENREAD`, `OPENWRITE`, `OPENAPPEND`, `OPENUPDATE`, `LOAD`, and `SAVE` commands. Logo will put the appropriate separator character (slash for Unix, backslash for DOS/Windows, colon for MacOS Classic) between the prefix and the filename entered by the user. The input to `SETPREFIX` must be a word, unless it is the empty list, to indicate that there should be no prefix.

See openread , See openwrite , See openappend , See openupdate , See load , See save .

#### prefix ¶

    PREFIX

outputs the current file prefix, or [] if there is no prefix.

See setprefix .

#### openread ¶

    OPENREAD filename

command. Opens the named file for reading. The read position is initially at the beginning of the file.

#### openwrite ¶

    OPENWRITE filename

command. Opens the named file for writing. If the file already existed, the old version is deleted and a new, empty file created.

`OPENWRITE`, but not the other `OPEN` variants, will accept as input a two-element list, in which the first element must be a variable name, and the second must be a positive integer. A character buffer of the specified size will be created. When a `SETWRITE` is done with this same list (in the sense of .EQ, not a copy, so you must do something like

    ? make "buf [foo 100]
    ? openwrite :buf
    ? setwrite :buf
        [...]
    ? close :buf

and not just

    ? openwrite [foo 100]
    ? setwrite [foo 100]

and so on), the printed characters are stored in the buffer; when a `CLOSE` is done with the same list as input, the characters from the buffer (treated as one long word, even if spaces and newlines are included) become the value of the specified variable.

#### openappend ¶

    OPENAPPEND filename

command. Opens the named file for writing. If the file already exists, the write position is initially set to the end of the old file, so that newly written data will be appended to it.

#### openupdate ¶

    OPENUPDATE filename

command. Opens the named file for reading and writing. The read and write position is initially set to the end of the old file. Note: each open file has only one position, for both reading and writing. If a file opened for update is both `READER` and `WRITER` at the same time, then `SETREADPOS` will also affect `WRITEPOS` and vice versa. Also, if you alternate reading and writing the same file, you must `SETREADPOS` between a write and a read, and `SETWRITEPOS` between a read and a write.

See reader , writer , setreadpos , setwritepos

#### close ¶

    CLOSE filename

command. Closes the named file. If the file was currently the reader or writer, then the reader or writer is changed to the keyboard or screen, as if `SETREAD []` or `SETWRITE []` had been done.

#### allopen ¶

    ALLOPEN

outputs a list whose members are the names of all files currently open. This list does not include the dribble file, if any.

#### closeall ¶

    CLOSEALL					(library procedure)

command. Closes all open files. Abbreviates `FOREACH ALLOPEN [CLOSE ?]`

See foreach , close

#### erasefile ¶

    ERASEFILE filename
    ERF filename

command. Erases (deletes, removes) the named file, which should not currently be open.

#### dribble ¶

    DRIBBLE filename

command. Creates a new file whose name is the input, like `OPENWRITE`, and begins recording in that file everything that is read from the keyboard or written to the terminal. That is, this writing is in addition to the writing to `WRITER`. The intent is to create a transcript of a Logo session, including things like prompt characters and interactions.

See openwrite , writer

#### nodribble ¶

    NODRIBBLE

command. Stops copying information into the dribble file, and closes the file.

#### setread ¶

    SETREAD filename

command. Makes the named file the read stream, used for `READLIST`, etc. The file must already be open with `OPENREAD` or `OPENUPDATE`. If the input is the empty list, then the read stream becomes the keyboard, as usual. Changing the read stream does not close the file that was previously the read stream, so it is possible to alternate between files.

See readlist , openread , openupdate

#### setwrite ¶

    SETWRITE filename

command. Makes the named file the write stream, used for `PRINT`, etc. The file must already be open with `OPENWRITE`, `OPENAPPEND`, or `OPENUPDATE`. If the input is the empty list, then the write stream becomes the screen, as usual. Changing the write stream does not close the file that was previously the write stream, so it is possible to alternate between files.

If the input is a list, then its first element must be a variable name, and its second and last element must be a positive integer; a buffer of that many characters will be allocated, and will become the writestream. If the same list (same in the `.EQ` sense, not a copy) has been used as input to `OPENWRITE`, then the already-allocated buffer will be used, and the writer can be changed to and from this buffer, with all the characters accumulated as in a file. When the same list is used as input to `CLOSE`, the contents of the buffer (as an unparsed word, which may contain newline characters) will become the value of the named variable. For compatibility with earlier versions, if the list has not been opened when the `SETWRITE` is done, it will be opened implicitly, but the first `SETWRITE` after this one will implicitly close it, setting the variable and freeing the allocated buffer.

See print , openwrite ; openappend ; openupdate

#### reader ¶

    READER

outputs the name of the current read stream file, or the empty list if the read stream is the terminal.

#### writer ¶

    WRITER

outputs the name of the current write stream file, or the empty list if the write stream is the screen.

#### setreadpos ¶

    SETREADPOS charpos

command. Sets the file pointer of the read stream file so that the next `READLIST`, etc., will begin reading at the charposth character in the file, counting from 0. (That is, `SETREADPOS 0` will start reading from the beginning of the file.) Meaningless if the read stream is the keyboard.

See readlist .

#### setwritepos ¶

    SETWRITEPOS charpos

command. Sets the file pointer of the write stream file so that the next `PRINT`, etc., will begin writing at the charposth character in the file, counting from 0. (That is, `SETWRITEPOS 0` will start writing from the beginning of the file.) Meaningless if the write stream is the screen.

See print .

#### readpos ¶

    READPOS

outputs the file position of the current read stream file.

#### writepos ¶

    WRITEPOS

outputs the file position of the current write stream file.

#### eofp ¶

    EOFP
    EOF?

predicate, outputs `TRUE` if there are no more characters to be read in the read stream file, `FALSE` otherwise.

#### filep ¶

    FILEP filename
    FILE? filename					(library procedure)

predicate, outputs `TRUE` if a file of the specified name exists and can be read, `FALSE` otherwise.

### 4.4 Terminal Access ¶

#### keyp ¶

    KEYP
    KEY?

predicate, outputs `TRUE` if there are characters waiting to be read from the read stream. If the read stream is a file, this is equivalent to `NOT EOFP`. If the read stream is the terminal, then echoing is turned off and the terminal is set to `cbreak` (character at a time instead of line at a time) mode. It remains in this mode until some line-mode reading is requested (e.g., `READLIST`). The Unix operating system forgets about any pending characters when it switches modes, so the first `KEYP` invocation will always output `FALSE`.

See eofp , readlist

#### linep ¶

    LINEP
    LINE?

predicate, outputs `TRUE` if there is a line waiting to be read from the read stream. If the read stream is a file, this is equivalent to `NOT EOFP`. If the read stream is the terminal, then typed characters will be displayed and may be edited until `RET` is pressed.

See eofp .

#### cleartext ¶

    CLEARTEXT
    CT

command. Clears the text window.

#### setcursor ¶

    SETCURSOR vector

command. The input is a list of two numbers, the x and y coordinates of a text window position (origin in the upper left corner, positive direction is southeast). The text cursor is moved to the requested position. This command also forces the immediate printing of any buffered characters.

#### cursor ¶

    CURSOR

outputs a list containing the current x and y coordinates of the text cursor. Logo may get confused about the current cursor position if, e.g., you type in a long line that wraps around or your program prints escape codes that affect the screen strangely.

#### setmargins ¶

    SETMARGINS vector

command. The input must be a list of two numbers, as for `SETCURSOR`. The effect is to clear the screen and then arrange for all further printing to be shifted down and to the right according to the indicated margins. Specifically, every time a newline character is printed (explicitly or implicitly) Logo will type x_margin spaces, and on every invocation of `SETCURSOR` the margins will be added to the input x and y coordinates. (`CURSOR` will report the cursor position relative to the margins, so that this shift will be invisible to Logo programs.) The purpose of this command is to accommodate the display of terminal screens in lecture halls with inadequate TV monitors that miss the top and left edges of the screen.

See setcursor .

#### settextcolor ¶

    SETTEXTCOLOR foreground background
    SETTC foreground background

command (wxWidgets only). The inputs are color numbers, or RGB color lists, as for turtle graphics. The foreground and background colors for the textscreen/splitscreen text window are changed to the given values. The change affects text already printed as well as future text printing; there is only one text color for the entire window.

Command (non-wxWidgets Windows and DOS extended only). The inputs are color numbers, as for turtle graphics. Future printing to the text window will use the specified colors for foreground (the characters printed) and background (the space under those characters). Using `STANDOUT` will revert to the default text window colors. In the DOS extended (`ucblogo.exe`) version, colors in textscreen mode are limited to numbers 0-7, and the coloring applies only to text printed by the program, not to the echoing of text typed by the user. Neither limitation applies to the text portion of splitscreen mode, which is actually drawn as graphics internally.

See standout .

#### increasefont ¶

    INCREASEFONT
    DECREASEFONT

command (wxWidgets only). Increase or decrease the size of the font used in the text and edit windows to the next larger or smaller available size.

#### settextsize ¶

    SETTEXTSIZE height

command (wxWidgets only). Set the "point size" of the font used in the text and edit windows to the given integer input. The desired size may not be available, in which case the nearest available size will be used. Note: There is only a slight correlation between these integers and pixel sizes. Our rough estimate is that the number of pixels of height is about 1.5 times the point size, but it varies for different fonts. See `SETLABELHEIGHT` for a different approach used for the graphics window.

#### textsize ¶

    TEXTSIZE

(wxWidgets only) outputs the "point size" of the font used in the text and edit windows. See `SETTEXTSIZE` for a discussion of font sizing. See `LABELSIZE` for a different approach used for the graphics window.

#### setfont ¶

    SETFONT fontname

command (wxWidgets only). Set the font family used in all windows to the one named by the input. Try `Courier` or `Monospace` as likely possibilities. Not all computers have the same fonts installed. It’s a good idea to stick with monospace fonts (ones in which all characters have the same width).

#### font ¶

    FONT

(wxWidgets only) outputs the name of the font family used in all windows.

## 5 Arithmetic ¶

### 5.1 Numeric Operations ¶

#### sum ¶

    SUM num1 num2
    (SUM num1 num2 num3 ...)
    num1 + num2

outputs the sum of its inputs.

#### difference ¶

    DIFFERENCE num1 num2
    num1 - num2

outputs the difference of its inputs. Minus sign means infix difference in ambiguous contexts (when preceded by a complete expression), unless it is preceded by a space and followed by a nonspace. (See also `MINUS`.)

#### minus ¶

    MINUS num
    - num

outputs the negative of its input. Minus sign means unary minus if the previous token is an infix operator or open parenthesis, or it is preceded by a space and followed by a nonspace. There is a difference in binding strength between the two forms:

    MINUS 3 + 4     means   -(3+4)
    - 3 + 4         means   (-3)+4

#### product ¶

    PRODUCT num1 num2
    (PRODUCT num1 num2 num3 ...)
    num1 * num2

outputs the product of its inputs.

#### quotient ¶

    QUOTIENT num1 num2
    (QUOTIENT num)
    num1 / num2

outputs the quotient of its inputs. The quotient of two integers is an integer if and only if the dividend is a multiple of the divisor. (In other words, `QUOTIENT 5 2` is 2.5, not 2, but `QUOTIENT 4 2` is 2, not 2.0 — it does the right thing.) With a single input, `QUOTIENT` outputs the reciprocal of the input.

#### remainder ¶

    REMAINDER num1 num2

outputs the remainder on dividing num1 by num2; both must be integers and the result is an integer with the same sign as num1.

#### modulo ¶

    MODULO num1 num2

outputs the remainder on dividing num1 by num2; both must be integers and the result is an integer with the same sign as num2.

#### int ¶

    INT num

outputs its input with fractional part removed, i.e., an integer with the same sign as the input, whose absolute value is the largest integer less than or equal to the absolute value of the input.

#### round ¶

    ROUND num

outputs the nearest integer to the input.

#### sqrt ¶

    SQRT num

outputs the square root of the input, which must be nonnegative.

#### power ¶

    POWER num1 num2

outputs num1 to the num2 power. If num1 is negative, then num2 must be an integer.

#### exp ¶

    EXP num

outputs e (2.718281828+) to the input power.

#### log10 ¶

    LOG10 num

outputs the common logarithm of the input.

#### ln ¶

    LN num

outputs the natural logarithm of the input.

#### sin ¶

    SIN degrees

outputs the sine of its input, which is taken in degrees.

#### radsin ¶

    RADSIN radians

outputs the sine of its input, which is taken in radians.

#### cos ¶

    COS degrees

outputs the cosine of its input, which is taken in degrees.

#### radcos ¶

    RADCOS radians

outputs the cosine of its input, which is taken in radians.

#### arctan ¶

    ARCTAN num
    (ARCTAN x y)

outputs the arctangent, in degrees, of its input. With two inputs, outputs the arctangent of y/x, if x is nonzero, or 90 or –90 depending on the sign of y, if x is zero.

#### radarctan ¶

    RADARCTAN num
    (RADARCTAN x y)

outputs the arctangent, in radians, of its input. With two inputs, outputs the arctangent of y/x, if x is nonzero, or pi/2 or –pi/2 depending on the sign of y, if x is zero.

The expression `2*(RADARCTAN 0 1)` can be used to get the value of pi.

#### iseq ¶

    ISEQ from to					(library procedure)

outputs a list of the integers from from to to, inclusive.

    ? show iseq 3 7
    [3 4 5 6 7]
    ? show iseq 7 3
    [7 6 5 4 3]

#### rseq ¶

    RSEQ from to count				(library procedure)

outputs a list of count equally spaced rational numbers between from and to, inclusive.

    ? show rseq 3 5 9
    [3 3.25 3.5 3.75 4 4.25 4.5 4.75 5]
    ? show rseq 3 5 5
    [3 3.5 4 4.5 5]

### 5.2 Numeric Predicates ¶

#### lessp ¶

    LESSP num1 num2
    LESS? num1 num2
    num1 < num2

outputs `TRUE` if its first input is strictly less than its second.

#### greaterp ¶

    GREATERP num1 num2
    GREATER? num1 num2
    num1 > num2

outputs `TRUE` if its first input is strictly greater than its second.

#### lessequalp ¶

    LESSEQUALP num1 num2
    LESSEQUAL? num1 num2
    num1 <= num2

outputs `TRUE` if its first input is less than or equal to its second.

#### greaterequalp ¶

    GREATEREQUALP num1 num2
    GREATEREQUAL? num1 num2
    num1 >= num2

outputs `TRUE` if its first input is greater than or equal to its second.

### 5.3 Random Numbers ¶

#### random ¶

    RANDOM num
    (RANDOM start end)

with one input, outputs a random nonnegative integer less than its input, which must be a positive integer.

With two inputs, `RANDOM` outputs a random integer greater than or equal to the first input, and less than or equal to the second input. Both inputs must be integers, and the first must be less than the second. `(RANDOM 0 9)` is equivalent to `RANDOM 10`; `(RANDOM 3 8)` is equivalent to `(RANDOM 6)+3`.

#### rerandom ¶

    RERANDOM
    (RERANDOM seed)

command. Makes the results of `RANDOM` reproducible. Ordinarily the sequence of random numbers is different each time Logo is used. If you need the same sequence of pseudo-random numbers repeatedly, e.g. to debug a program, say `RERANDOM` before the first invocation of `RANDOM`. If you need more than one repeatable sequence, you can give `RERANDOM` an integer input; each possible input selects a unique sequence of numbers.

### 5.4 Print Formatting ¶

#### form ¶

    FORM num width precision

outputs a word containing a printable representation of num, possibly preceded by spaces (and therefore not a number for purposes of performing arithmetic operations), with at least width characters, including exactly precision digits after the decimal point. (If precision is 0 then there will be no decimal point in the output.)

As a debugging feature, (`FORM num -1 format`) will print the floating point num according to the C printf format, to allow

    to hex :num
    op form :num -1 "|%08X %08X|
    end

to allow finding out the exact result of floating point operations. The precise format needed may be machine-dependent.

### 5.5 Bitwise Operations ¶

#### bitand ¶

    BITAND num1 num2
    (BITAND num1 num2 num3 ...)

outputs the bitwise and of its inputs, which must be integers.

See and .

#### bitor ¶

    BITOR num1 num2
    (BITOR num1 num2 num3 ...)

outputs the bitwise or of its inputs, which must be integers.

See or .

#### bitxor ¶

    BITXOR num1 num2
    (BITXOR num1 num2 num3 ...)

outputs the bitwise exclusive or of its inputs, which must be integers.

#### bitnot ¶

    BITNOT num

outputs the bitwise not of its input, which must be an integer.

See not .

#### ashift ¶

    ASHIFT num1 num2

outputs num1 arithmetic-shifted to the left by num2 bits. If num2 is negative, the shift is to the right with sign extension. The inputs must be integers.

#### lshift ¶

    LSHIFT num1 num2

outputs num1 logical-shifted to the left by num2 bits. If num2 is negative, the shift is to the right with zero fill. The inputs must be integers.

## 6 Logical Operations ¶

### 6.1 And, Or, and Not ¶

#### and ¶

    AND tf1 tf2
    (AND tf1 tf2 tf3 ...)

outputs `TRUE` if all inputs are `TRUE`, otherwise `FALSE`. All inputs must be `TRUE` or `FALSE`. (Comparison is case-insensitive regardless of the value of `CASEIGNOREDP`. That is, `true` or `True` or `TRUE` are all the same.) An input can be a list, in which case it is taken as an expression to run; that expression must produce a `TRUE` or `FALSE` value. List expressions are evaluated from left to right; as soon as a `FALSE` value is found, the remaining inputs are not examined. Example:

    MAKE "RESULT AND [NOT (:X = 0)] [(1 / :X) > .5]

to avoid the division by zero if the first part is false.

#### or ¶

    OR tf1 tf2
    (OR tf1 tf2 tf3 ...)

outputs `TRUE` if any input is `TRUE`, otherwise `FALSE`. All inputs must be `TRUE` or `FALSE`. (Comparison is case-insensitive regardless of the value of `CASEIGNOREDP`. That is, `true` or `True` or `TRUE` are all the same.) An input can be a list, in which case it is taken as an expression to run; that expression must produce a `TRUE` or `FALSE` value. List expressions are evaluated from left to right; as soon as a `TRUE` value is found, the remaining inputs are not examined. Example:

    IF OR :X=0 [some.long.computation] [...]

to avoid the long computation if the first condition is met.

#### not ¶

    NOT tf

outputs `TRUE` if the input is `FALSE`, and vice versa. The input can be a list, in which case it is taken as an expression to run; that expression must produce a `TRUE` or `FALSE` value. The following example prints `true`:

    PRINT NOT "FALSE

## 7 Graphics ¶

Berkeley Logo provides traditional Logo turtle graphics with one turtle. Multiple turtles, dynamic turtles, and collision detection are not supported. This is the most hardware-dependent part of Logo; some features may exist on some machines but not others. Nevertheless, the goal has been to make Logo programs as portable as possible, rather than to take fullest advantage of the capabilities of each machine. In particular, Logo attempts to scale the screen so that turtle coordinates [–100 –100] and [100 100] fit on the graphics window, and so that the aspect ratio is 1:1.

The center of the graphics window (which may or may not be the entire screen, depending on the machine used) is turtle location [0 0]. Positive X is to the right; positive Y is up. Headings (angles) are measured in degrees clockwise from the positive Y axis. (This differs from the common mathematical convention of measuring angles counterclockwise from the positive X axis.) The turtle is represented as an isoceles triangle; the actual turtle position is at the midpoint of the base (the short side). However, the turtle is drawn one step behind its actual position, so that the display of the base of the turtle’s triangle does not obscure a line drawn perpendicular to it (as would happen after drawing a square).

Colors are, of course, hardware-dependent. However, Logo provides partial hardware independence by interpreting color numbers 0 through 7 uniformly on all computers:

    0  black        1  blue         2  green        3  cyan
    4  red          5  magenta      6  yellow       7 white

Where possible, Logo provides additional user-settable colors; how many are available depends on the hardware and operating system environment. If at least 16 colors are available, Logo tries to provide uniform initial settings for the colors 8-15:

     8  brown        9  tan         10  forest      11  aqua
    12  salmon      13  purple      14  orange      15  grey

Logo begins with a black background and white pen.

### 7.1 Turtle Motion ¶

#### forward ¶

    FORWARD dist
    FD dist

moves the turtle forward, in the direction that it’s facing, by the specified distance (measured in turtle steps).

#### back ¶

    BACK dist
    BK dist

moves the turtle backward, i.e., exactly opposite to the direction that it’s facing, by the specified distance. (The heading of the turtle does not change.)

#### left ¶

    LEFT degrees
    LT degrees

turns the turtle counterclockwise by the specified angle, measured in degrees (1/360 of a circle).

#### right ¶

    RIGHT degrees
    RT degrees

turns the turtle clockwise by the specified angle, measured in degrees (1/360 of a circle).

#### setpos ¶

    SETPOS pos

moves the turtle to an absolute position in the graphics window. The input is a list of two numbers, the X and Y coordinates.

#### setxy ¶

    SETXY xcor ycor

moves the turtle to an absolute position in the graphics window. The two inputs are numbers, the X and Y coordinates.

#### setx ¶

    SETX xcor

moves the turtle horizontally from its old position to a new absolute horizontal coordinate. The input is the new X coordinate.

#### sety ¶

    SETY ycor

moves the turtle vertically from its old position to a new absolute vertical coordinate. The input is the new Y coordinate.

#### setheading ¶

    SETHEADING degrees
    SETH degrees

turns the turtle to a new absolute heading. The input is a number, the heading in degrees clockwise from the positive Y axis.

#### home ¶

    HOME

moves the turtle to the center of the screen. Equivalent to `SETPOS [0 0] SETHEADING 0.`

See setpos , See setheading .

#### arc ¶

    ARC angle radius

draws an arc of a circle, with the turtle at the center, with the specified radius, starting at the turtle’s heading and extending clockwise through the specified angle. The turtle does not move.

### 7.2 Turtle Motion Queries ¶

#### pos ¶

    POS

outputs the turtle’s current position, as a list of two numbers, the X and Y coordinates.

#### xcor ¶

    XCOR						(library procedure)

outputs a number, the turtle’s X coordinate.

#### ycor ¶

    YCOR						(library procedure)

outputs a number, the turtle’s Y coordinate.

#### heading ¶

    HEADING

outputs a number, the turtle’s heading in degrees.

#### towards ¶

    TOWARDS pos

outputs a number, the heading at which the turtle should be facing so that it would point from its current position to the position given as the input.

#### scrunch ¶

    SCRUNCH

outputs a list containing two numbers, the X and Y scrunch factors, as used by `SETSCRUNCH`. (But note that `SETSCRUNCH` takes two numbers as inputs, not one list of numbers.)

See setscrunch .

### 7.3 Turtle and Window Control ¶

#### showturtle ¶

    SHOWTURTLE
    ST

makes the turtle visible.

#### hideturtle ¶

    HIDETURTLE
    HT

makes the turtle invisible. It’s a good idea to do this while you’re in the middle of a complicated drawing, because hiding the turtle speeds up the drawing substantially.

#### clean ¶

    CLEAN

erases all lines that the turtle has drawn on the graphics window. The turtle’s state (position, heading, pen mode, etc.) is not changed.

#### clearscreen ¶

    CLEARSCREEN
    CS

erases the graphics window and sends the turtle to its initial position and heading. Like `HOME` and `CLEAN` together.

See home .

#### wrap ¶

    WRAP

tells the turtle to enter wrap mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will "wrap around" and reappear at the opposite edge of the window. The top edge wraps to the bottom edge, while the left edge wraps to the right edge. (So the window is topologically equivalent to a torus.) This is the turtle’s initial mode. Compare `WINDOW` and `FENCE`.

See fence .

#### window ¶

    WINDOW

tells the turtle to enter window mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will move offscreen. The visible graphics window is considered as just part of an infinite graphics plane; the turtle can be anywhere on the plane. (If you lose the turtle, `HOME` will bring it back to the center of the window.) Compare `WRAP` and `FENCE`.

#### fence ¶

    FENCE

tells the turtle to enter fence mode: From now on, if the turtle is asked to move past the boundary of the graphics window, it will move as far as it can and then stop at the edge with an "out of bounds" error message. Compare `WRAP` and `WINDOW`.

See wrap .

#### fill ¶

    FILL

fills in a region of the graphics window containing the turtle and bounded by lines that have been drawn earlier. This is not portable; it doesn’t work for all machines, and may not work exactly the same way on different machines.

#### filled ¶

    FILLED color instructions

runs the instructions, remembering all points visited by turtle motion commands, starting and ending with the turtle’s initial position. Then draws (ignoring penmode) the resulting polygon, in the current pen color, filling the polygon with the given color, which can be a color number or an RGB list. The instruction list cannot include another FILLED invocation. (wxWidgets only)

#### label ¶

    LABEL text

takes a word or list as input, and prints the input on the graphics window, starting at the turtle’s position.

#### setlabelheight ¶

    SETLABELHEIGHT height

command (wxWidgets only). Takes a positive integer argument and tries to set the font size so that the character height (including descenders) is that many turtle steps. This will be different from the number of screen pixels if `SETSCRUNCH` has been used. Also, note that `SETSCRUNCH` changes the font size to try to preserve this height in turtle steps. Note that the query operation corresponding to this command is `LABELSIZE`, not `LABELHEIGHT`, because it tells you the width as well as the height of characters in the current font.

#### textscreen ¶

    TEXTSCREEN
    TS

rearranges the size and position of windows to maximize the space available in the text window (the window used for interaction with Logo). The details differ among machines. Compare `SPLITSCREEN` and `FULLSCREEN`.

See splitscreen .

#### fullscreen ¶

    FULLSCREEN
    FS

rearranges the size and position of windows to maximize the space available in the graphics window. The details differ among machines. Compare `SPLITSCREEN` and `TEXTSCREEN`.

Since there must be a text window to allow printing (including the printing of the Logo prompt), Logo automatically switches from fullscreen to splitscreen whenever anything is printed.

In the DOS version, switching from fullscreen to splitscreen loses the part of the picture that’s hidden by the text window. [This design decision follows from the scarcity of memory, so that the extra memory to remember an invisible part of a drawing seems too expensive.]

#### splitscreen ¶

    SPLITSCREEN
    SS

rearranges the size and position of windows to allow some room for text interaction while also keeping most of the graphics window visible. The details differ among machines. Compare `TEXTSCREEN` and `FULLSCREEN`.

See textscreen .

#### setscrunch ¶

    SETSCRUNCH xscale yscale

adjusts the aspect ratio and scaling of the graphics display. After this command is used, all further turtle motion will be adjusted by multiplying the horizontal and vertical extent of the motion by the two numbers given as inputs. For example, after the instruction `SETSCRUNCH 2 1` motion at a heading of 45 degrees will move twice as far horizontally as vertically. If your squares don’t come out square, try this. (Alternatively, you can deliberately misadjust the aspect ratio to draw an ellipse.)

For all modern computers, both scale factors are initially 1. For DOS machines, the scale factors are initially set according to what the hardware claims the aspect ratio is, but the hardware sometimes lies. For DOS, the values set by `SETSCRUNCH` are remembered in a file (called `scrunch.dat`) and are automatically put into effect when a Logo session begins.

#### refresh ¶

    REFRESH

(command) tells Logo to remember the turtle’s motions so that they can be used for high-resolution printing (wxWidgets) or to refresh the graphics window if it is moved, resized, or overlayed (non-wxWidgets). This is the default.

#### norefresh ¶

    NOREFRESH

(command) tells Logo not to remember the turtle’s motions, which may be useful to save time and memory if your program is interactive or animated, rather than drawing a static picture you’ll want to print later (wxWidgets). In non-wxWidgets versions, using NOREFRESH may prevent Logo from restoring the graphics image after the window is moved, resized, or overlayed.

### 7.4 Turtle and Window Queries ¶

#### shownp ¶

    SHOWNP
    SHOWN?

outputs `TRUE` if the turtle is shown (visible), `FALSE` if the turtle is hidden. See `SHOWTURTLE` and `HIDETURTLE`.

See showturtle , hideturtle .

#### screenmode ¶

    SCREENMODE

outputs the word `TEXTSCREEN`, `SPLITSCREEN`, or `FULLSCREEN` depending on the current screen mode.

#### turtlemode ¶

    TURTLEMODE

outputs the word `WRAP`, `FENCE`, or `WINDOW` depending on the current turtle mode.

#### labelsize ¶

    LABELSIZE

(wxWidgets only) outputs a list of two positive integers, the width and height of characters displayed by `LABEL` measured in turtle steps (which will be different from screen pixels if `SETSCRUNCH` has been used). There is no `SETLABELSIZE` because the width and height of a font are not separately controllable, so the inverse of this operation is `SETLABELHEIGHT`, which takes just one number for the desired height.

### 7.5 Pen and Background Control ¶

The turtle carries a pen that can draw pictures. At any time the pen can be UP (in which case moving the turtle does not change what’s on the graphics screen) or DOWN (in which case the turtle leaves a trace). If the pen is down, it can operate in one of three modes: PAINT (so that it draws lines when the turtle moves), ERASE (so that it erases any lines that might have been drawn on or through that path earlier), or REVERSE (so that it inverts the status of each point along the turtle’s path).

#### pendown ¶

    PENDOWN
    PD

sets the pen’s position to `DOWN`, without changing its mode.

#### penup ¶

    PENUP
    PU

sets the pen’s position to `UP`, without changing its mode.

#### penpaint ¶

    PENPAINT
    PPT

sets the pen’s position to `DOWN` and mode to `PAINT`.

#### penerase ¶

    PENERASE
    PE

sets the pen’s position to `DOWN` and mode to `ERASE`.

#### penreverse ¶

    PENREVERSE
    PX

sets the pen’s position to `DOWN` and mode to `REVERSE`. (This may interact in system-dependent ways with use of color.)

#### setpencolor ¶

    SETPENCOLOR colornumber.or.rgblist
    SETPC colornumber.or.rgblist

sets the pen color to the given number, which must be a nonnegative integer. There are initial assignments for the first 16 colors:

     0  black	 1  blue	 2  green	 3  cyan
     4  red		 5  magenta	 6  yellow	 7 white
     8  brown	 9  tan		10  forest	11  aqua
    12  salmon	13  purple	14  orange	15  grey

but other colors can be assigned to numbers by the `PALETTE` command. Alternatively, sets the pen color to the given RGB values (a list of three nonnegative numbers less than 100 specifying the percent saturation of red, green, and blue in the desired color).

#### setpalette ¶

    SETPALETTE colornumber rgblist

sets the actual color corresponding to a given number, if allowed by the hardware and operating system. Colornumber must be an integer greater than or equal to 8. (Logo tries to keep the first 8 colors constant.) The second input is a list of three nonnegative numbers less than 100 specifying the percent saturation of red, green, and blue in the desired color.

#### setpensize ¶

    SETPENSIZE size

sets the thickness of the pen. The input is either a single positive integer or a list of two positive integers (for horizontal and vertical thickness). Some versions pay no attention to the second number, but always have a square pen.

#### setpenpattern ¶

    SETPENPATTERN pattern

sets hardware-dependent pen characteristics. This command is not guaranteed compatible between implementations on different machines.

#### setpen ¶

    SETPEN list					(library procedure)

sets the pen’s position, mode, thickness, and hardware-dependent characteristics according to the information in the input list, which should be taken from an earlier invocation of `PEN`.

See pen .

#### setbackground ¶

    SETBACKGROUND colornumber.or.rgblist
    SETBG colornumber.or.rgblist

set the screen background color by slot number or RGB values. See `SETPENCOLOR` for details.

See setpencolor .

### 7.6 Pen Queries ¶

#### pendownp ¶

    PENDOWNP
    PENDOWN?

outputs `TRUE` if the pen is down, `FALSE` if it’s up.

#### penmode ¶

    PENMODE

outputs one of the words `PAINT`, `ERASE`, or `REVERSE` according to the current pen mode.

#### pencolor ¶

    PENCOLOR
    PC

outputs a color number, a nonnegative integer that is associated with a particular color, or a list of RGB values if such a list was used as the most recent input to `SETPENCOLOR`. There are initial assignments for the first 16 colors:

     0  black        1  blue         2  green        3  cyan
     4  red          5  magenta      6  yellow       7 white
     8  brown        9  tan         10  forest      11  aqua
    12  salmon      13  purple      14  orange      15  grey

but other colors can be assigned to numbers by the `PALETTE` command.

#### palette ¶

    PALETTE colornumber

outputs a list of three nonnegative numbers less than 100 specifying the percent saturation of red, green, and blue in the color associated with the given number.

#### pensize ¶

    PENSIZE

outputs a list of two positive integers, specifying the horizontal and vertical thickness of the turtle pen. (In some implementations, including wxWidgets, the two numbers are always equal.)

    PENPATTERN

outputs system-specific pen information.

#### pen ¶

    PEN						(library procedure)

outputs a list containing the pen’s position, mode, thickness, and hardware-specific characteristics, for use by `SETPEN`.

See setpen .

#### background ¶

    BACKGROUND
    BG

outputs the graphics background color, either as a slot number or as an RGB list, whichever way it was set. (See `PENCOLOR`.)

### 7.7 Saving and Loading Pictures ¶

#### savepict ¶

    SAVEPICT filename

command. Writes a file with the specified name containing the state of the graphics window, including any nonstandard color palette settings, in Logo’s internal format. This picture can be restored to the screen using `LOADPICT`. The format is not portable between platforms, nor is it readable by other programs. epspict to export Logo graphics for other programs.

#### loadpict ¶

    LOADPICT filename

command. Reads the specified file, which must have been written by a `SAVEPICT` command, and restores the graphics window and color palette settings to the values stored in the file. Any drawing previously on the screen is cleared.

See savepict .

#### epspict ¶

    EPSPICT filename

command. Writes a file with the specified name, containing an Encapsulated Postscript (EPS) representation of the state of the graphics window. This file can be imported into other programs that understand EPS format. Restrictions: the drawing cannot use `FILL`, `PENERASE`, or `PENREVERSE`; any such instructions will be ignored in the translation to Postscript form.

### 7.8 Mouse Queries ¶

#### mousepos ¶

    MOUSEPOS

outputs the coordinates of the mouse, provided that it’s within the graphics window, in turtle coordinates. If the mouse is outside the graphics window, then the last position within the window is returned. Exception: If a mouse button is pressed within the graphics window and held while the mouse is dragged outside the window, the mouse’s position is returned as if the window were big enough to include it.

#### clickpos ¶

    CLICKPOS

outputs the coordinates that the mouse was at when a mouse button was most recently pushed, provided that that position was within the graphics window, in turtle coordinates. (wxWidgets only)

#### buttonp ¶

    BUTTONP
    BUTTON?

outputs `TRUE` if a mouse button is down and the mouse is over the graphics window. Once the button is down, `BUTTONP` remains true until the button is released, even if the mouse is dragged out of the graphics window.

#### button ¶

    BUTTON

outputs 0 if no mouse button has been pushed inside the Logo window since the last call to `BUTTON`. Otherwise, it outputs an integer between 1 and 3 indicating which button was most recently pressed. Ordinarily 1 means left, 2 means right, and 3 means center, but operating systems may reconfigure these.

## 8 Workspace Management ¶

### 8.1 Procedure Definition ¶

#### to ¶

    TO procname :input1 :input2 ...			(special form)

command. Prepares Logo to accept a procedure definition. The procedure will be named procname and there must not already be a procedure by that name. The inputs will be called input1 etc. Any number of inputs are allowed, including none. Names of procedures and inputs are case-insensitive.

Unlike every other Logo procedure, TO takes as its inputs the actual words typed in the instruction line, as if they were all quoted, rather than the results of evaluating expressions to provide the inputs. (That’s what special form means.)

This version of Logo allows variable numbers of inputs to a procedure. After the procedure name come four kinds of things, in this order:

        1.   0 or more REQUIRED inputs    :FOO :FROBOZZ
        2.   0 or more OPTIONAL inputs    [:BAZ 87] [:THINGO 5+9]
        3.   0 or 1 REST input            [:GARPLY]
        4.   0 or 1 DEFAULT number        5

Every procedure has a minimum, default, and maximum number of inputs. (The latter can be infinite.)

The minimum number of inputs is the number of required inputs, which must come first. A required input is indicated by the

    :inputname

notation.

After all the required inputs can be zero or more optional inputs, each of which is represented by the following notation:

    [:inputname default.value.expression]

When the procedure is invoked, if actual inputs are not supplied for these optional inputs, the default value expressions are evaluated to set values for the corresponding input names. The inputs are processed from left to right, so a default value expression can be based on earlier inputs. Example:

    to proc :inlist [:startvalue first :inlist]

If the procedure is invoked by saying

    proc [a b c]

then the variable `inlist` will have the value `[A B C]` and the variable `startvalue` will have the value `A`. If the procedure is invoked by saying

    (proc [a b c] "x)

then `inlist` will have the value `[A B C]` and `startvalue` will have the value `X`.

After all the required and optional input can come a single rest input, represented by the following notation:

    [:inputname]

This is a rest input rather than an optional input because there is no default value expression. There can be at most one rest input. When the procedure is invoked, the value of this inputname will be a list containing all of the actual inputs provided that were not used for required or optional inputs. Example:

    to proc :in1 [:in2 "foo] [:in3 "baz] [:in4]

If this procedure is invoked by saying

    proc "x

then `in1` has the value `X`, `in2` has the value `FOO`, `in3` has the value `BAZ`, and `in4` has the value `[]` (the empty list). If it’s invoked by saying

    (proc "a "b "c "d "e)

then `in1` has the value `A`, `in2` has the value `B`, `in3` has the value `C`, and `in4` has the value `[D E]`.

The maximum number of inputs for a procedure is infinite if a rest input is given; otherwise, it is the number of required inputs plus the number of optional inputs.

The default number of inputs for a procedure, which is the number of inputs that it will accept if its invocation is not enclosed in parentheses, is ordinarily equal to the minimum number. If you want a different default number you can indicate that by putting the desired default number as the last thing on the `TO` line. example:

    to proc :in1 [:in2 "foo] [:in3] 3

This procedure has a minimum of one input, a default of three inputs, and an infinite maximum.

Logo responds to the `TO` command by entering procedure definition mode. The prompt character changes from `?` to `>` and whatever instructions you type become part of the definition until you type a line containing only the word `END`.

#### define ¶

    DEFINE procname text

command. Defines a procedure with name procname and text text. If there is already a procedure with the same name, the new definition replaces the old one. The text input must be a list whose members are lists. The first member is a list of inputs; it looks like a `TO` line but without the word `TO`, without the procedure name, and without the colons before input names. In other words, the members of this first sublist are words for the names of required inputs and lists for the names of optional or rest inputs. The remaining sublists of the text input make up the body of the procedure, with one sublist for each instruction line of the body. (There is no `END` line in the text input.) It is an error to redefine a primitive procedure unless the variable `REDEFP` has the value `TRUE`.

See redefp .

#### text ¶

    TEXT procname

outputs the text of the procedure named procname in the form expected by `DEFINE`: a list of lists, the first of which describes the inputs to the procedure and the rest of which are the lines of its body. The text does not reflect formatting information used when the procedure was defined, such as continuation lines and extra spaces.

#### fulltext ¶

    FULLTEXT procname

outputs a representation of the procedure procname in which formatting information is preserved. If the procedure was defined with `TO`, `EDIT`, or `LOAD`, then the output is a list of words. Each word represents one entire line of the definition in the form output by `READWORD`, including extra spaces and continuation lines. The last member of the output represents the `END` line. If the procedure was defined with `DEFINE`, then the output is a list of lists. If these lists are printed, one per line, the result will look like a definition using `TO`. Note: the output from `FULLTEXT` is not suitable for use as input to `DEFINE`!

See text.

#### copydef ¶

    COPYDEF newname oldname

command. Makes newname a procedure identical to oldname. The latter may be a primitive. If newname was already defined, its previous definition is lost. If newname was already a primitive, the redefinition is not permitted unless the variable `REDEFP` has the value `TRUE`.

Note: dialects of Logo differ as to the order of inputs to `COPYDEF`. This dialect uses "`MAKE` order," not "`NAME` order."

See redefp , save , po , pot .

### 8.2 Variable Definition ¶

#### make ¶

    MAKE varname value

command. Assigns the value value to the variable named varname, which must be a word. Variable names are case-insensitive. If a variable with the same name already exists, the value of that variable is changed. If not, a new global variable is created.

#### name ¶

    NAME value varname				(library procedure)

command. Same as `MAKE` but with the inputs in reverse order.

#### local ¶

    LOCAL varname
    LOCAL varnamelist
    (LOCAL varname1 varname2 ...)

command. Accepts as inputs one or more words, or a list of words. A variable is created for each of these words, with that word as its name. The variables are local to the currently running procedure. Logo variables follow dynamic scope rules; a variable that is local to a procedure is available to any subprocedure invoked by that procedure. The variables created by `LOCAL` have no initial value; they must be assigned a value (e.g., with `MAKE`) before the procedure attempts to read their value.

See make .

#### localmake ¶

    LOCALMAKE varname value				(library procedure)

command. Makes the named variable local, like `LOCAL`, and assigns it the given value, like `MAKE`.

See local , See make .

#### thing ¶

    THING varname
    :quoted.varname

outputs the value of the variable whose name is the input. If there is more than one such variable, the innermost local variable of that name is chosen. The colon notation is an abbreviation not for `THING` but for the combination

    thing "

so that `:FOO` means `THING "FOO`.

#### global ¶

    GLOBAL varname
    GLOBAL varnamelist
    (GLOBAL varname1 varname2 ...)

command. Accepts as inputs one or more words, or a list of words. A global variable is created for each of these words, with that word as its name. The only reason this is necessary is that you might want to use the "setter" notation `SETXYZ` for a variable `XYZ` that does not already have a value; `GLOBAL "XYZ` makes that legal. Note: If there is currently a local variable of the same name, this command does *not* make Logo use the global value instead of the local one.

### 8.3 Property Lists ¶

Note: Names of property lists are always case-insensitive. Names of individual properties are case-sensitive or case-insensitive depending on the value of `CASEIGNOREDP`, which is `TRUE` by default.

See caseignoredp .

In principle, every possible name is the name of a property list, which is initially empty. So Logo never gives a "no such property list" error, as it would for undefined procedure or variable names. But the primitive procedures that deal with "all" property lists (`CONTENTS`, `PLISTS`, etc.) list only nonempty ones. To "erase" a property list erase means to make it empty, removing all properties from it.

#### pprop ¶

    PPROP plistname propname value

command. Adds a property to the plistname property list with name propname and value value.

#### gprop ¶

    GPROP plistname propname

outputs the value of the propname property in the plistname property list, or the empty list if there is no such property.

#### remprop ¶

    REMPROP plistname propname

command. Removes the property named propname from the property list named plistname.

#### plist ¶

    PLIST plistname

outputs a list whose odd-numbered members are the names, and whose even-numbered members are the values, of the properties in the property list named plistname. The output is a copy of the actual property list; changing properties later will not magically change a list output earlier by `PLIST`.

### 8.4 Workspace Predicates ¶

#### procedurep ¶

    PROCEDUREP name
    PROCEDURE? name

outputs `TRUE` if the input is the name of a procedure.

#### primitivep ¶

    PRIMITIVEP name
    PRIMITIVE? name

outputs `TRUE` if the input is the name of a primitive procedure (one built into Logo). Note that some of the procedures described in this document are library procedures, not primitives.

#### definedp ¶

    DEFINEDP name
    DEFINED? name

outputs `TRUE` if the input is the name of a user-defined procedure, including a library procedure.

#### namep ¶

    NAMEP name
    NAME? name

outputs `TRUE` if the input is the name of a variable.

#### plistp ¶

    PLISTP name
    PLIST? name

outputs `TRUE` if the input is the name of a nonempty property list. (In principle every word is the name of a property list; if you haven’t put any properties in it, `PLIST` of that name outputs an empty list, rather than giving an error message.)

### 8.5 Workspace Queries ¶

Note: All procedures whose input is indicated as contentslist will accept a single word (taken as a procedure name), a list of words (taken as names of procedures), or a list of three lists as described under the `CONTENTS` command above.

#### contents ¶

    CONTENTS

outputs a "contents list," i.e., a list of three lists containing names of defined procedures, variables, and property lists respectively. This list includes all unburied named items in the workspace.

#### buried ¶

    BURIED

outputs a contents list including all buried named items in the workspace.

#### traced ¶

    TRACED

outputs a contents list including all traced named items in the workspace.

#### stepped ¶

    STEPPED

outputs a contents list including all stepped named items in the workspace.

#### procedures ¶

    PROCEDURES

outputs a list of the names of all unburied user-defined procedures in the workspace. Note that this is a list of names, not a contents list. (However, procedures that require a contents list as input will accept this list.)

#### primitives ¶

    PRIMITIVES

outputs a list of the names of all primitive procedures in the workspace. Note that this is a list of names, not a contents list. (However, procedures that require a contents list as input will accept this list.)

#### names ¶

    NAMES

outputs a contents list consisting of an empty list (indicating no procedure names) followed by a list of all unburied variable names in the workspace.

#### plists ¶

    PLISTS

outputs a contents list consisting of two empty lists (indicating no procedures or variables) followed by a list of all unburied nonempty property lists in the workspace.

#### namelist ¶

    NAMELIST varname				(library procedure)
    NAMELIST varnamelist

outputs a contents list consisting of an empty list followed by a list of the name or names given as input. This is useful in conjunction with workspace control procedures that require a contents list as input.

#### pllist ¶

    PLLIST plname					(library procedure)
    PLLIST plnamelist

outputs a contents list consisting of two empty lists followed by a list of the name or names given as input. This is useful in conjunction with workspace control procedures that require a contents list as input.

See contents .

#### arity ¶

    ARITY procedurename

outputs a list of three numbers: the minimum, default, and maximum number of inputs for the procedure whose name is the input. It is an error if there is no such procedure. A maximum of -1 means that the number of inputs is unlimited.

#### nodes ¶

    NODES

outputs a list of two numbers. The first represents the number of nodes of memory currently in use. The second shows the maximum number of nodes that have been in use at any time since the last invocation of `NODES`. (A node is a small block of computer memory as used by Logo. Each number uses one node. Each non-numeric word uses one node, plus some non-node memory for the characters in the word. Each array takes one node, plus some non-node memory, as well as the memory required by its elements. Each list requires one node per element, as well as the memory within the elements.) If you want to track the memory use of an algorithm, it is best if you invoke `GC` at the beginning of each iteration, since otherwise the maximum will include storage that is unused but not yet collected.

### 8.6 Workspace Inspection ¶

#### po ¶

    PRINTOUT contentslist
    PO contentslist

command. Prints to the write stream the definitions of all procedures, variables, and property lists named in the input contents list.

#### poall ¶

    POALL						(library procedure)

command. Prints all unburied definitions in the workspace. Abbreviates `PO CONTENTS`.

See contents .

#### pops ¶

    POPS						(library procedure)

command. Prints the definitions of all unburied procedures in the workspace. Abbreviates `PO PROCEDURES`.

See po , procedures .

#### pons ¶

    PONS						(library procedure)

command. Prints the definitions of all unburied variables in the workspace. Abbreviates `PO NAMES`.

See po , names .

#### popls ¶

    POPLS						(library procedure)

command. Prints the contents of all unburied nonempty property lists in the workspace. Abbreviates `PO PLISTS`.

See po , plists .

#### pon ¶

    PON varname					(library procedure)
    PON varnamelist

command. Prints the definitions of the named variable(s).
Abbreviates `PO NAMELIST varname(list)`.

See po , namelist .

#### popl ¶

    POPL plname					(library procedure)
    POPL plnamelist

command. Prints the definitions of the named property list(s).
Abbreviates `PO PLLIST plname(list)`.

See po , pllist .

#### pot ¶

    POT contentslist

command. Prints the title lines of the named procedures and the definitions of the named variables and property lists. For property lists, the entire list is shown on one line instead of as a series of `PPROP` instructions as in `PO`.

See pprop , po .

#### pots ¶

    POTS						(library procedure)

command. Prints the title lines of all unburied procedures in the workspace. Abbreviates `POT PROCEDURES`.

See procedures .

### 8.7 Workspace Control ¶

#### erase ¶

    ERASE contentslist
    ER contentslist

command. Erases from the workspace the procedures, variables, and property lists named in the input. Primitive procedures may not be erased unless the variable `REDEFP` has the value `TRUE`.

See redefp .

#### erall ¶

    ERALL

command. Erases all unburied procedures, variables, and property lists from the workspace. Abbreviates `ERASE CONTENTS`.

See contents .

#### erps ¶

    ERPS

command. Erases all unburied procedures from the workspace.
Abbreviates `ERASE PROCEDURES`.

See erase , procedures .

#### erns ¶

    ERNS

command. Erases all unburied variables from the workspace. Abbreviates `ERASE NAMES`.

See erase , names .

#### erpls ¶

    ERPLS

command. Erases all unburied property lists from the workspace.
Abbreviates `ERASE PLISTS`.

See erase , plists .

#### ern ¶

    ERN varname					(library procedure)
    ERN varnamelist

command. Erases from the workspace the variable(s) named in the input. Abbreviates `ERASE NAMELIST varname(list)`.

See erase , namelist .

#### erpl ¶

    ERPL plname					(library procedure)
    ERPL plnamelist

command. Erases from the workspace the property list(s) named in the input. Abbreviates `ERASE PLLIST plname(list)`.

See erase , pllist .

#### bury ¶

    BURY contentslist

command. Buries the procedures, variables, and property lists named in the input. A buried item is not included in the lists output by `CONTENTS`, `PROCEDURES`, `VARIABLES`, and `PLISTS`, but is included in the list output by `BURIED`. By implication, buried things are not printed by `POALL` or saved by `SAVE`.

See contents , procedures , pons , plists , poall , save .

#### buryall ¶

    BURYALL                                         (library procedure)

command. Abbreviates `BURY CONTENTS`.

See contents .

#### buryname ¶

    BURYNAME varname				(library procedure)
    BURYNAME varnamelist

command. Abbreviates `BURY NAMELIST `varname(list)``.

See bury , namelist .

#### unbury ¶

    UNBURY contentslist

command. Unburies the procedures, variables, and property lists named in the input. That is, the named items will be returned to view in `CONTENTS`, etc.

See contents .

#### unburyall ¶

    UNBURYALL					(library procedure)

command. Abbreviates `UNBURY BURIED`.

See buried .

#### unburyname ¶

    UNBURYNAME varname				(library procedure)
    UNBURYNAME varnamelist

command. Abbreviates `UNBURY NAMELIST varname(list)`.

See unbury , namelist .

#### buriedp ¶

    BURIEDP contentslist
    BURIED? contentslist

outputs `TRUE` if the first procedure, variable, or property list named in the contents list is buried, `FALSE` if not. Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can `BURIEDP [[] [variable]]` or `BURIEDP [[] [] [proplist]]`.

#### trace ¶

    TRACE contentslist

command. Marks the named items for tracing. A message is printed whenever a traced procedure is invoked, giving the actual input values, and whenever a traced procedure `STOP`s or `OUTPUT`s. A message is printed whenever a new value is assigned to a traced variable using `MAKE`. A message is printed whenever a new property is given to a traced property list using `PPROP`.

See stop , output , make , pprop .

#### untrace ¶

    UNTRACE contentslist

command. Turns off tracing for the named items.

#### tracedp ¶

    TRACEDP contentslist
    TRACED? contentslist

outputs `TRUE` if the first procedure, variable, or property list named in the contents list is traced, `FALSE` if not. Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can `TRACEDP [[] [variable]]` or `TRACEDP [[] [] [proplist]]`.

#### step ¶

    STEP contentslist

command. Marks the named items for stepping. Whenever a stepped procedure is invoked, each instruction line in the procedure body is printed before being executed, and Logo waits for the user to type a newline at the terminal. A message is printed whenever a stepped variable name is shadowed because a local variable of the same name is created either as a procedure input or by the `LOCAL` command.

See local .

#### unstep ¶

    UNSTEP contentslist

command. Turns off stepping for the named items.

#### steppedp ¶

    STEPPEDP contentslist
    STEPPED? contentslist

outputs `TRUE` if the first procedure, variable, or property list named in the contents list is stepped, `FALSE` if not. Only the first thing in the list is tested; the most common use will be with a word as input, naming a procedure, but a contents list is allowed so that you can `STEPPEDP [[] [variable]]` or `STEPPEDP [[] [] [proplist]]`.

#### edit ¶

    EDIT contentslist
    ED contentslist
    (EDIT)
    (ED)

command. If invoked with an input, `EDIT` writes the definitions of the named items into a temporary file and edits that file, using an editor that depends on the platform you’re using. In wxWidgets, and in the MacOS Classic version, there is an editor built into Logo. In the non-wxWidgets versions for Unix, MacOS X, Windows, and DOS, Logo uses your favorite editor as determined by the `EDITOR` environment variable. If you don’t have an `EDITOR` variable, edits the definitions using `jove`. If invoked without an input, `EDIT` edits the same file left over from a previous `EDIT` or `EDITFILE` instruction. When you leave the editor, Logo reads the revised definitions and modifies the workspace accordingly. It is not an error if the input includes names for which there is no previous definition.

If there is a variable `LOADNOISILY` whose value is `TRUE`, then, after leaving the editor, `TO` commands in the temporary file print ‘`procname defined`’ (where procname is the name of the procedure being defined); if `LOADNOISILY` is `FALSE` or undefined, `TO` commands in the file are carried out silently.

If there is an environment variable called `TEMP`, then Logo uses its value as the directory in which to write the temporary file used for editing.

Exceptionally, the `EDIT` command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

See loadnoisily , See editfile .

#### editfile ¶

    EDITFILE filename

command. Starts the Logo editor, like `EDIT`, but instead of editing a temporary file it edits the file specified by the input. When you leave the editor, Logo reads the revised file, as for `EDIT`. `EDITFILE` also remembers the filename, so that a subsequent `EDIT` command with no input will re-edit the same file.

`EDITFILE` is intended as an alternative to `LOAD` and `SAVE`. You can maintain a workspace file yourself, controlling the order in which definitions appear, maintaining comments in the file, and so on.

In the wxWidgets version, `EDITFILE` asks whether or not you want to load the file into Logo when you finish editing. This allows you to use `EDITFILE` to edit data files without leaving Logo.

#### edall ¶

    EDALL						(library procedure)

command. Abbreviates `EDIT CONTENTS`.

See contents .

#### edps ¶

    EDPS						(library procedure)

command. Abbreviates `EDIT PROCEDURES`.

See edit , procedures .

#### edns ¶

    EDNS						(library procedure)

command. Abbreviates `EDIT NAMES`.

See edit , names .

#### edpls ¶

    EDPLS						(library procedure)

command. Abbreviates `EDIT PLISTS`.

See edit , plists .

#### edn ¶

    EDN varname					(library procedure)
    EDN varnamelist

command. Abbreviates `EDIT NAMELIST varname(list)`.

See edit , namelist .

#### edpl ¶

    EDPL plname					(library procedure)
    EDPL plnamelist

command. Abbreviates `EDIT PLLIST plname(list)`.

See edit , pllist .

#### save ¶

    SAVE filename

command. Saves the definitions of all unburied procedures, variables, and nonempty property lists in the named file. Equivalent to

    to save :filename
    local "oldwriter
    make "oldwriter writer
    openwrite :filename
    setwrite :filename
    poall
    setwrite :oldwriter
    close :filename
    end

Exceptionally, `SAVE` can be used with no input and without parentheses if it is the last thing on the command line. In this case, the filename from the most recent `LOAD` or `SAVE` command will be used. (It is an error if there has been no previous `LOAD` or `SAVE`.)

#### savel ¶

    SAVEL contentslist filename			(library procedure)

command. Saves the definitions of the procedures, variables, and property lists specified by contentslist to the file named filename.

#### load ¶

    LOAD filename

command. Reads instructions from the named file and executes them. The file can include procedure definitions with `TO`, and these are accepted even if a procedure by the same name already exists. If the file assigns a list value to a variable named `STARTUP`, then that list is run as an instructionlist after the file is loaded. If there is a variable `LOADNOISILY` whose value is `TRUE`, then `TO` commands in the file print ‘`procname defined`’ (where procname is the name of the procedure being defined); if `LOADNOISILY` is `FALSE` or undefined, `TO` commands in the file are carried out silently.

See startup , See loadnoisily .

#### cslsload ¶

    CSLSLOAD name

command. Loads the named file, like `LOAD`, but from the directory containing the Computer Science Logo Style programs instead of the current user’s directory.

See load .

#### help ¶

    HELP name
    (HELP)

command. Prints information from the reference manual about the primitive procedure named by the input. With no input, lists all the primitives about which help is available. If there is an environment variable `LOGOHELP`, then its value is taken as the directory in which to look for help files, instead of the default help directory.

If `HELP` is called with the name of a defined procedure for which there is no help file, it will print the title line of the procedure followed by lines from the procedure body that start with semicolon, stopping when a non-semicolon line is seen.

Exceptionally, the `HELP` command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

#### seteditor ¶

    SETEDITOR path

command. Tells Logo to use the specified program as its editor instead of the default editor. The format of a path depends on your operating system.

#### setlibloc ¶

    SETLIBLOC path

command. Tells Logo to use the specified directory as its library instead of the default. (Note that many Logo "primitive" procedures are actually found in the library, so they may become unavailable if your new library does not include them!) The format of a path depends on your operating system.

#### setcslsloc ¶

    SETCSLSLOC path

command. Tells Logo to use the specified directory for the `CSLSLOAD` command, instead of the default directory. The format of a path depends on your operating system.

See cslsload .

#### sethelploc ¶

    SETHELPLOC path

command. Tells Logo to look in the specified directory for the information provided by the HELP command, instead of the default directory. The format of a path depends on your operating system.

#### settemploc ¶

    SETTEMPLOC path

command. Tells Logo to write editor temporary files in the specified directory rather than in the default directory. You must have write permission for this directory. The format of a path depends on your operating system.

#### gc ¶

    GC
    (GC anything)

command. Runs the garbage collector, reclaiming unused nodes. Logo does this when necessary anyway, but you may want to use this command to control exactly when Logo does it. In particular, the numbers output by the `NODES` operation will not be very meaningful unless garbage has been collected. Another reason to use `GC` is that a garbage collection takes a noticeable fraction of a second, and you may want to schedule collections for times before or after some time-critical animation. If invoked with an argument (of any value), `GC` runs a full garbage collection, including GCTWA (Garbage Collect Truly Worthless Atoms, which means that it removes from Logo’s memory words that used to be procedure or variable names but aren’t any more); without an argument, `GC` does a generational garbage collection, which means that only recently created nodes are examined. (The latter is usually good enough.)

#### .setsegmentsize ¶

    .SETSEGMENTSIZE num

command. Sets the number of nodes that Logo allocates from the operating system at once to num, which must be a positive integer. The name is dotted because bad things will happen if you use a number that’s too small or too large for your computer. The initial value is 16,000 for most systems, but is smaller for 68000-based Macs. Making it larger will speed up computations (by reducing the number of garbage collections) at the cost of allocating more memory than necessary.

## 9 Control Structures ¶

### 9.1 Control ¶

Note: in the following descriptions, an instructionlist can be a list or a word. In the latter case, the word is parsed into list form before it is run. Thus, `RUN READWORD` or `RUN READLIST` will work. The former is slightly preferable because it allows for a continued line (with `~`) that includes a comment (with `;`) on the first line.

A tf input must be the word `TRUE`, the word `FALSE`, or a list. If it’s a list, then it must be a Logo expression, which will be evaluated to produce a value that must be `TRUE` or `FALSE`. The comparisons with `TRUE` and `FALSE` are always case-insensitive.

A runlist can consist of either a single expression (that produces a value) or zero or more instructions (that do something, rather than output a value), depending on the context:

    PRINT IFELSE :X<0 ["NEGATIVE] ["POSITIVE]  ; one value in each case
    REPEAT 4 [PRINT "A PRINT "B]  ; two instructions

#### run ¶

    RUN instructionlist

command or operation. Runs the Logo instructions in the input list; outputs if the list contains an expression that outputs.

See readword , readlist .

#### runresult ¶

    RUNRESULT instructionlist

runs the instructions in the input; outputs an empty list if those instructions produce no output, or a list whose only member is the output from running the input instructionlist. Useful for inventing command-or-operation control structures:

    local "result
    make "result runresult [something]
    if emptyp :result [stop]
    output first :result

#### repeat ¶

    REPEAT num instructionlist

command. Runs the instructionlist repeatedly, num times.

#### forever ¶

    FOREVER instructionlist

command. Runs the "instructionlist" repeatedly, until something inside the instructionlist (such as `STOP` or `THROW`) makes it stop.

See stop , See throw .

#### repcount ¶

    REPCOUNT

outputs the repetition count of the innermost current `REPEAT` or `FOREVER`, starting from 1. If no `REPEAT` or `FOREVER` is active, outputs –1.

The abbreviation `#` can be used for `REPCOUNT` unless the `REPEAT` is inside the template input to a higher order procedure such as `FOREACH`, in which case `#` has a different meaning.

#### if ¶

    IF tf instructionlist
    (IF tf instructionlist1 instructionlist2)

command. If the first input has the value `TRUE`, then `IF` runs the second input. If the first input has the value `FALSE`, then `IF` does nothing. (If given a third input, IF acts like `IFELSE`, as described below.) It is an error if the first input is not either `TRUE` or `FALSE`.

For compatibility with earlier versions of Logo, if an `IF` instruction is not enclosed in parentheses, but the first thing on the instruction line after the second input expression is a literal list (i.e., a list in square brackets), the `IF` is treated as if it were `IFELSE`, but a warning message is given. If this aberrant `IF` appears in a procedure body, the warning is given only the first time the procedure is invoked in each Logo session.

#### ifelse ¶

    IFELSE tf instructionlist1 instructionlist2

command or operation. If the first input has the value `TRUE`, then `IFELSE` runs the second input. If the first input has the value `FALSE`, then `IFELSE` runs the third input. `IFELSE` outputs a value if the instructionlist contains an expression that outputs a value.

#### test ¶

    TEST tf

command. Remembers its input, which must be `TRUE` or `FALSE`, for use by later `IFTRUE` or `IFFALSE` instructions. The effect of `TEST` is local to the procedure in which it is used; any corresponding `IFTRUE` or `IFFALSE` must be in the same procedure or a subprocedure.

See iffalse .

#### iftrue ¶

    IFTRUE instructionlist
    IFT instructionlist

command. Runs its input if the most recent `TEST` instruction had a `TRUE` input. The `TEST` must have been in the same procedure or a superprocedure.

#### iffalse ¶

    IFFALSE instructionlist
    IFF instructionlist

command. Runs its input if the most recent `TEST` instruction had a `FALSE` input. The `TEST` must have been in the same procedure or a superprocedure.

See test .

#### stop ¶

    STOP

command. Ends the running of the procedure in which it appears. Control is returned to the context in which that procedure was invoked. The stopped procedure does not output a value.

#### output ¶

    OUTPUT value
    OP value

command. Ends the running of the procedure in which it appears. That procedure outputs the value value to the context in which it was invoked. Don’t be confused: `OUTPUT` itself is a command, but the procedure that invokes `OUTPUT` is an operation.

#### catch ¶

    CATCH tag instructionlist

command or operation. Runs its second input. Outputs if that instructionlist outputs. If, while running the instructionlist, a `THROW` instruction is executed with a tag equal to the first input (case-insensitive comparison), then the running of the instructionlist is terminated immediately. In this case the `CATCH` outputs if a value input is given to `THROW`. The tag must be a word.

If the tag is the word `ERROR`, then any error condition that arises during the running of the instructionlist has the effect of `THROW "ERROR` instead of printing an error message and returning to toplevel. The `CATCH` does not output if an error is caught. Also, during the running of the instructionlist, the variable `ERRACT` is temporarily unbound. (If there is an error while `ERRACT` has a value, that value is taken as an instructionlist to be run after printing the error message. Typically the value of `ERRACT`, if any, is the list `[PAUSE]`.)

See error , erract , pause .

#### throw ¶

    THROW tag
    (THROW tag value)

command. Must be used within the scope of a `CATCH` with an equal tag. Ends the running of the instructionlist of the `CATCH`. If `THROW` is used with only one input, the corresponding `CATCH` does not output a value. If `THROW` is used with two inputs, the second provides an output for the `CATCH`.

`THROW "TOPLEVEL` can be used to terminate all running procedures and interactive pauses, and return to the toplevel instruction prompt. Typing the system interrupt character (`alt-S` for wxWidgets; otherwise normally `control-C` for Unix, `control-Q` for DOS, or `command-period` for Mac) has the same effect.

`THROW "ERROR` can be used to generate an error condition. If the error is not caught, it prints a message (`THROW "ERROR`) with the usual indication of where the error (in this case the `THROW`) occurred. If a second input is used along with a tag of `ERROR`, that second input is used as the text of the error message instead of the standard message. Also, in this case, the location indicated for the error will be, not the location of the `THROW`, but the location where the procedure containing the `THROW` was invoked. This allows user-defined procedures to generate error messages as if they were primitives. Note: in this case the corresponding `CATCH "ERROR`, if any, does not output, since the second input to `THROW` is not considered a return value.

`THROW "SYSTEM` immediately leaves Logo, returning to the operating system, without printing the usual parting message and without deleting any editor temporary file written by EDIT.

#### error ¶

    ERROR

outputs a list describing the error just caught, if any. If there was not an error caught since the last use of `ERROR`, the empty list will be output. The error list contains four members: an integer code corresponding to the type of error, the text of the error message (as a single word including spaces), the name of the procedure in which the error occurred, and the instruction line on which the error occurred.

#### pause ¶

    PAUSE

command or operation. Enters an interactive pause. The user is prompted for instructions, as at toplevel, but with a prompt that includes the name of the procedure in which `PAUSE` was invoked. Local variables of that procedure are available during the pause. `PAUSE` outputs if the pause is ended by a `CONTINUE` with an input.

If the variable `ERRACT` exists, and an error condition occurs, the contents of that variable are run as an instructionlist. Typically `ERRACT` is given the value `[PAUSE]` so that an interactive pause will be entered in the event of an error. This allows the user to check values of local variables at the time of the error.

Typing the system quit character (`alt-S` for wxWidgets; otherwise normally `control-\` for Unix, `control-W` for DOS, or `command-comma` for Mac) will also enter a pause.

See erract .

#### continue ¶

    CONTINUE value
    CO value
    (CONTINUE)
    (CO)

command. Ends the current interactive pause, returning to the context of the `PAUSE` invocation that began it. If `CONTINUE` is given an input, that value is used as the output from the `PAUSE`. If not, the `PAUSE` does not output.

Exceptionally, the `CONTINUE` command can be used without its default input and without parentheses provided that nothing follows it on the instruction line.

#### wait ¶

    WAIT time

command. Delays further execution for time 60ths of a second. Also causes any buffered characters destined for the terminal to be printed immediately. `WAIT 0` can be used to achieve this buffer flushing without actually waiting.

#### bye ¶

    BYE

command. Exits from Logo; returns to the operating system.

#### .maybeoutput ¶

    .MAYBEOUTPUT value				(special form)

works like `OUTPUT` except that the expression that provides the input value might not, in fact, output a value, in which case the effect is like `STOP`. This is intended for use in control structure definitions, for cases in which you don’t know whether or not some expression produces a value. Example:

    to invoke :function [:inputs] 2
    .maybeoutput apply :function :inputs
    end

    ? (invoke "print "a "b "c)
    a b c
    ? print (invoke "word "a "b "c)
    abc

This is an alternative to `RUNRESULT`. It’s fast and easy to use, at the cost of being an exception to Logo’s evaluation rules. (Ordinarily, it should be an error if the expression that’s supposed to provide an input to something doesn’t have a value.)

See output , stop , runresult .

#### goto ¶

    GOTO word

command. Looks for a `TAG` command with the same input in the same procedure, and continues running the procedure from the location of that `TAG`. It is meaningless to use `GOTO` outside of a procedure.

#### tag ¶

    TAG quoted.word

command. Does nothing. The input must be a literal word following a quotation mark (`"`), not the result of a computation. Tags are used by the `GOTO` command.

#### ignore ¶

    IGNORE value					(library procedure)

command. Does nothing. Used when an expression is evaluated for a side effect and its actual value is unimportant.

#### ‘ ¶

    ` list						(library procedure)

outputs a list equal to its input but with certain substitutions. If a member of the input list is the word ‘`,`’ (comma) then the following member should be an instructionlist that produces an output when run. That output value replaces the comma and the instructionlist. If a member of the input list is the word ‘`,@`’ (comma atsign) then the following member should be an instructionlist that outputs a list when run. The members of that list replace the ‘`,@`’ and the instructionlist. Example:

    show `[foo baz ,[bf [a b c]] garply ,@[bf [a b c]]]

will print

    [foo baz [b c] garply b c]

A word starting with ‘`,`’ or ‘`,@`’ is treated as if the rest of the word were a one-word list, e.g., ‘`,:foo`’ is equivalent to ‘`,[:Foo]`’.

A word starting with ‘`",`’ (quote comma) or ‘`:,`’ (colon comma) becomes a word starting with ‘`"`’ or ‘`:`’ but with the result of running the substitution (or its first word, if the result is a list) replacing what comes after the comma.

Backquotes can be nested. Substitution is done only for commas at the same depth as the backquote in which they are found:

    ? show `[a `[b ,[1+2] ,[foo ,[1+3] d] e] f]
    [a ` [b , [1+2] , [foo 4 d] e] f]

    ?make "name1 "x
    ?make "name2 "y
    ? show `[a `[b ,:,:name1 ,",:name2 d] e]
    [a ` [b , [:x] , ["y] d] e]

#### for ¶

    FOR forcontrol instructionlist			(library procedure)

command. The first input must be a list containing three or four members: (1) a word, which will be used as the name of a local variable; (2) a word or list that will be evaluated as by `RUN` to determine a number, the starting value of the variable; (3) a word or list that will be evaluated to determine a number, the limit value of the variable; (4) an optional word or list that will be evaluated to determine the step size. If the fourth member is missing, the step size will be 1 or –1 depending on whether the limit value is greater than or less than the starting value, respectively.

The second input is an instructionlist. The effect of `FOR` is to run that instructionlist repeatedly, assigning a new value to the control variable (the one named by the first member of the forcontrol list) each time. First the starting value is assigned to the control variable. Then the value is compared to the limit value. `FOR` is complete when the sign of `(current - limit)` is the same as the sign of the step size. (If no explicit step size is provided, the instructionlist is always run at least once. An explicit step size can lead to a zero-trip `FOR`, e.g., `FOR [I 1 0 1] ...`). Otherwise, the instructionlist is run, then the step is added to the current value of the control variable and FOR returns to the comparison step.

    ? for [i 2 7 1.5] [print :i]
    2
    3.5
    5
    6.5
    ?

See run .

#### do.while ¶

    DO.WHILE instructionlist tfexpression		(library procedure)

command. Repeatedly evaluates the instructionlist as long as the evaluated remains `TRUE`. Evaluates the first input first, so the instructionlist is always run at least once. The tfexpression must be an expressionlist whose value when evaluated is `TRUE` or `FALSE`.

#### while ¶

    WHILE tfexpression instructionlist		(library procedure)

command. Repeatedly evaluates the instructionlist as long as the evaluated remains `TRUE`. Evaluates the first input first, so the instructionlist may never be run at all. The tfexpression must be an expressionlist whose value when evaluated is `TRUE` or `FALSE`.

#### do.until ¶

    DO.UNTIL instructionlist tfexpression		(library procedure)

command. Repeatedly evaluates the instructionlist as long as the evaluated remains `FALSE`. Evaluates the first input first, so the instructionlist is always run at least once. The tfexpression must be an expressionlist whose value when evaluated is `TRUE` or `FALSE`.

#### until ¶

    UNTIL tfexpression instructionlist		(library procedure)

command. Repeatedly evaluates the instructionlist as long as the evaluated remains `FALSE`. Evaluates the first input first, so the instructionlist may never be run at all. The tfexpression must be an expressionlist whose value when evaluated is `TRUE` or `FALSE`.

#### case ¶

    CASE value clauses					(library procedure)

command or operation. The second input is a list of lists (clauses); each clause is a list whose first element is either a list of values or the word `ELSE` and whose butfirst is a Logo expression or instruction. `CASE` examines the clauses in order. If a clause begins with the word `ELSE` (upper or lower case), then the butfirst of that clause is evaluated and `CASE` outputs its value, if any. If the first input to CASE is a member of the first element of a clause, then the butfirst of that clause is evaluated and `CASE` outputs its value, if any. If neither of these conditions is met, then `CASE` goes on to the next clause. If no clause is satisfied, `CASE` does nothing. Example:

    to vowelp :letter
    output case :letter [ [[a e i o u] "true] [else "false] ]
    end

#### cond ¶

    COND clauses						(library procedure)

command or operation. The input is a list of lists (clauses); each clause is a list whose first element is either an expression whose value is `TRUE` or `FALSE`, or the word `ELSE`, and whose butfirst is a Logo expression or instruction. `COND` examines the clauses in order. If a clause begins with the word `ELSE` (upper or lower case), then the butfirst of that clause is evaluated and `CASE` outputs its value, if any. Otherwise, the first element of the clause is evaluated; the resulting value must be `TRUE` or `FALSE`. If it’s `TRUE`, then the butfirst of that clause is evaluated and `COND` outputs its value, if any. If the value is `FALSE`, then `COND` goes on to the next clause. If no clause is satisfied, `COND` does nothing. Example:

    to evens :numbers	; select even numbers from a list
    op cond [ [[emptyp :numbers] []]
              [[evenp first :numbers]	; assuming EVENP is defined
               fput first :numbers evens butfirst :numbers]
              [else evens butfirst :numbers] ]
    end

### 9.2 Template-based Iteration ¶

The procedures in this section are iteration tools based on the idea of a template. This is a generalization of an instruction list or an expression list in which slots are provided for the tool to insert varying data. Four different forms of template can be used.

The most commonly used form for a template is ‘`explicit-slot`’ form, or ‘`question mark`’ form. Example:

    ? show map [? * ?] [2 3 4 5]
    [4 9 16 25]
    ?

In this example, the `MAP` tool evaluated the template `[? * ?]` repeatedly, with each of the members of the data list `[2 3 4 5]` substituted in turn for the question marks. The same value was used for every question mark in a given evaluation. Some tools allow for more than one datum to be substituted in parallel; in these cases the slots are indicated by `?1` for the first datum, `?2` for the second, and so on:

    ? show (map [(word ?1 ?2 ?1)] [a b c] [d e f])
    [ada beb cfc]
    ?

If the template wishes to compute the datum number, the form `(? 1)` is equivalent to `?1`, so `(? ?1)` means the datum whose number is given in datum number 1. Some tools allow additional slot designations, as shown in the individual descriptions.

The second form of template is the ‘`named-procedure`’ form. If the template is a word rather than a list, it is taken as the name of a procedure. That procedure must accept a number of inputs equal to the number of parallel data slots provided by the tool; the procedure is applied to all of the available data in order. That is, if data `?1` through `?3` are available, the template `"PROC` is equivalent to `[PROC ?1 ?2 ?3]`.

    ? show (map "word [a b c] [d e f])
    [ad be cf]
    ?

    to dotprod :a :b	; vector dot product
    op apply "sum (map "product :a :b)
    end

The third form of template is ‘`named-slot`’ or ‘`lambda`’ form. This form is indicated by a template list containing more than one member, whose first member is itself a list. The first member is taken as a list of names; local variables are created with those names and given the available data in order as their values. The number of names must equal the number of available data. This form is needed primarily when one iteration tool must be used within the template list of another, and the `?` notation would be ambiguous in the inner template. Example:

    to matmul :m1 :m2 [:tm2 transpose :m2]	; multiply two matrices
    output map [[row] map [[col] dotprod :row :col] :tm2] :m1
    end

The fourth form is ‘`procedure text`’ form, a variant of lambda form. In this form, the template list contains at least two members, all of which are lists. This is the form used by the `DEFINE` and `TEXT` primitives, and `APPLY` accepts it so that the text of a defined procedure can be used as a template.

Note: The fourth form of template is interpreted differently from the others, in that Logo considers it to be an independent defined procedure for the purposes of `OUTPUT` and `STOP`. For example, the following two instructions are identical:

    ? print apply [[x] :x+3] [5]
    8
    ? print apply [[x] [output :x+3]] [5]
    8

although the first instruction is in named-slot form and the second is in procedure-text form. The named-slot form can be understood as telling Logo to evaluate the expression `:x+3` in place of the entire invocation of apply, with the variable `x` temporarily given the value `5`. The procedure-text form can be understood as invoking the procedure

    to foo :x
    output :x+3
    end

with input `5`, but without actually giving the procedure a name. If the use of `OUTPUT` were interchanged in these two examples, we’d get errors:

    ? print apply [[x] output :x+3] [5]
    Can only use output inside a procedure
    ? print apply [[x] [:x+3]] [5]
    You don't say what to do with 8

The named-slot form can be used with `STOP` or `OUTPUT` inside a procedure, to stop the enclosing procedure.

The following iteration tools are extended versions of the ones in Appendix B of the book Computer Science Logo Style, Volume 3: Advanced Topics by Brian Harvey [MIT Press, 1987]. The extensions are primarily to allow for variable numbers of inputs.

#### apply ¶

    APPLY template inputlist

command or operation. Runs the template, filling its slots with the members of inputlist. The number of members in inputlist must be an acceptable number of slots for template. It is illegal to apply the primitive `TO` as a template, but anything else is okay. `APPLY` outputs what template outputs, if anything.

#### invoke ¶

    INVOKE template input				(library procedure)
    (INVOKE template input1 input2 ...)

command or operation. Exactly like `APPLY` except that the inputs are provided as separate expressions rather than in a list.

#### foreach ¶

    FOREACH data template				(library procedure)
    (FOREACH data1 data2 ... template)

command. Evaluates the template list repeatedly, once for each member of the data list. If more than one data list are given, each of them must be the same length. (The data inputs can be words, in which case the template is evaluated once for each character.)

In a template, the symbol `?REST` represents the portion of the data input to the right of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `?REST` would be replaced by `[C D E]`. If multiple parallel slots are used, then `(?REST 1)` goes with ?1, etc.

In a template, the symbol `#` represents the position in the data input of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `#` would be replaced by `2`.

#### map ¶

    MAP template data				(library procedure)
    (MAP template data1 data2 ...)

outputs a word or list, depending on the type of the data input, of the same length as that data input. (If more than one data input are given, the output is of the same type as data1.) Each member of the output is the result of evaluating the template list, filling the slots with the corresponding member(s) of the data input(s). (All data inputs must be the same length.) In the case of a word output, the results of the template evaluation must be words, and they are concatenated with `WORD`.

In a template, the symbol `?REST` represents the portion of the data input to the right of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `?REST` would be replaced by `[C D E]`. If multiple parallel slots are used, then `(?REST 1)` goes with `?1`, etc.

In a template, the symbol `#` represents the position in the data input of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `#` would be replaced by `2`.

See word .

#### map.se ¶

    MAP.SE template data				(library procedure)
    (MAP.SE template data1 data2 ...)

outputs a list formed by evaluating the template list repeatedly and concatenating the results using `SENTENCE`. That is, the members of the output are the members of the results of the evaluations. The output list might, therefore, be of a different length from that of the data input(s). (If the result of an evaluation is the empty list, it contributes nothing to the final output.) The data inputs may be words or lists.

In a template, the symbol `?REST` represents the portion of the data input to the right of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `?REST` would be replaced by `[C D E]`. If multiple parallel slots are used, then `(?REST 1)` goes with `?1`, etc.

In a template, the symbol `#` represents the position in the data input of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `#` would be replaced by `2`.

See sentence .

#### filter ¶

    FILTER tftemplate data				(library procedure)

outputs a word or list, depending on the type of the data input, containing a subset of the members (for a list) or characters (for a word) of the input. The template is evaluated once for each member or character of the data, and it must produce a `TRUE` or `FALSE` value. If the value is `TRUE`, then the corresponding input constituent is included in the output.

    ? print filter "vowelp "elephant
    eea
    ?

In a template, the symbol `?REST` represents the portion of the data input to the right of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `?REST` would be replaced by `[C D E]`.

In a template, the symbol `#` represents the position in the data input of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `#` would be replaced by `2`.

#### find ¶

    FIND tftemplate data				(library procedure)

outputs the first constituent of the data input (the first member of a list, or the first character of a word) for which the value produced by evaluating the template with that consituent in its slot is `TRUE`. If there is no such constituent, the empty list is output.

In a template, the symbol `?REST` represents the portion of the data input to the right of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `?REST` would be replaced by `[C D E]`.

In a template, the symbol `#` represents the position in the data input of the member currently being used as the `?` slot-filler. That is, if the data input is `[A B C D E]` and the template is being evaluated with `?` replaced by `B`, then `#` would be replaced by `2`.

#### reduce ¶

    REDUCE template data				(library procedure)

outputs the result of applying the template to accumulate the members of the data input. The template must be a two-slot function. Typically it is an associative function name like `SUM`. If the data input has only one constituent (member in a list or character in a word), the output is that consituent. Otherwise, the template is first applied with `?1` filled with the next-to-last consitient and `?2` with the last constituent. Then, if there are more constituents, the template is applied with `?1` filled with the next constituent to the left and `?2` with the result from the previous evaluation. This process continues until all constituents have been used. The data input may not be empty.

Note: If the template is, like `SUM`, the name of a procedure that is capable of accepting arbitrarily many inputs, it is more efficient to use `APPLY` instead of `REDUCE`. The latter is good for associative procedures that have been written to accept exactly two inputs:

    to max :a :b
    output ifelse :a > :b [:a] [:b]
    end

    print reduce "max [...]

Alternatively, `REDUCE` can be used to write `MAX` as a procedure that accepts any number of inputs, as `SUM` does:

    to max [:inputs] 2
    if emptyp :inputs ~
       [(throw "error [not enough inputs to max])]
    output reduce [ifelse ?1 > ?2 [?1] [?2]] :inputs
    end

#### crossmap ¶

    CROSSMAP template listlist			(library procedure)
    (CROSSMAP template data1 data2 ...)

outputs a list containing the results of template evaluations. Each data list contributes to a slot in the template; the number of slots is equal to the number of data list inputs. As a special case, if only one data list input is given, that list is taken as a list of data lists, and each of its members contributes values to a slot. `CROSSMAP` differs from `MAP` in that instead of taking members from the data inputs in parallel, it takes all possible combinations of members of data inputs, which need not be the same length.

    ? show (crossmap [word ?1 ?2] [a b c] [1 2 3 4])
    [a1 a2 a3 a4 b1 b2 b3 b4 c1 c2 c3 c4]
    ?

For compatibility with the version in the first edition of CSLS 1, `CROSSMAP` templates may use the notation `:1` instead of `?1` to indicate slots.

See map .

#### cascade ¶

    CASCADE endtest template startvalue		(library procedure)
    (CASCADE endtest tmp1 sv1 tmp2 sv2 ...)
    (CASCADE endtest tmp1 sv1 tmp2 sv2 ... finaltemplate)

outputs the result of applying a template (or several templates, as explained below) repeatedly, with a given value filling the slot the first time, and the result of each application filling the slot for the following application.

In the simplest case, `CASCADE` has three inputs. The second input is a one-slot expression template. That template is evaluated some number of times (perhaps zero). On the first evaluation, the slot is filled with the third input; on subsequent evaluations, the slot is filled with the result of the previous evaluation. The number of evaluations is determined by the first input. This can be either a nonnegative integer, in which case the template is evaluated that many times, or a predicate expression template, in which case it is evaluated (with the same slot filler that will be used for the evaluation of the second input) repeatedly, and the `CASCADE` evaluation continues as long as the predicate value is `FALSE`. (In other words, the predicate template indicates the condition for stopping.)

If the template is evaluated zero times, the output from `CASCADE` is the third (startvalue) input. Otherwise, the output is the value produced by the last template evaluation.

`CASCADE` templates may include the symbol `#` to represent the number of times the template has been evaluated. This slot is filled with 1 for the first evaluation, 2 for the second, and so on.

    ? show cascade 5 [lput # ?] []
    [1 2 3 4 5]
    ? show cascade [vowelp first ?] [bf ?] "spring
    ing
    ? show cascade 5 [# * ?] 1
    120
    ?

Several cascaded results can be computed in parallel by providing additional template-startvalue pairs as inputs to `CASCADE`. In this case, all templates (including the endtest template, if used) are multi-slot, with the number of slots equal to the number of pairs of inputs. In each round of evaluations, `?2`, for example, represents the result of evaluating the second template in the previous round. If the total number of inputs (including the first endtest input) is odd, then the output from CASCADE is the final value of the first template. If the total number of inputs is even, then the last input is a template that is evaluated once, after the end test is satisfied, to determine the output from `CASCADE`.

    to fibonacci :n
    output (cascade :n [?1 + ?2] 1 [?1] 0)
    end

    to piglatin :word
    output (cascade [vowelp first ?] ~
                    [word bf ? first ?] ~
                    :word ~
                    [word ? "ay])
    end

#### cascade.2 ¶

    CASCADE.2 endtest temp1 startval1 temp2 startval2  (library procedure)

outputs the result of invoking `CASCADE` with the same inputs. The only difference is that the default number of inputs is five instead of three.

#### transfer ¶

    TRANSFER endtest template inbasket		(library procedure)

outputs the result of repeated evaluation of the template. The template is evaluated once for each member of the list inbasket. `TRANSFER` maintains an outbasket that is initially the empty list. After each evaluation of the template, the resulting value becomes the new outbasket.

In the template, the symbol `?IN` represents the current member from the inbasket; the symbol `?OUT` represents the entire current outbasket. Other slot symbols should not be used.

If the first (endtest) input is an empty list, evaluation continues until all inbasket members have been used. If not, the first input must be a predicate expression template, and evaluation continues until either that template’s value is `TRUE` or the inbasket is used up.

## 10 Macros ¶

### .macro ¶

    .MACRO procname :input1 :input2 ...			(special form)
    .DEFMACRO procname text

A macro is a special kind of procedure whose output is evaluated as Logo instructions in the context of the macro’s caller. `.MACRO` is exactly like `TO` except that the new procedure becomes a macro; `.DEFMACRO` is exactly like `DEFINE` with the same exception.

Macros are useful for inventing new control structures comparable to `REPEAT`, `IF`, and so on. Such control structures can almost, but not quite, be duplicated by ordinary Logo procedures. For example, here is an ordinary procedure version of `REPEAT`:

    to my.repeat :num :instructions
    if :num=0 [stop]
    run :instructions
    my.repeat :num-1 :instructions
    end

This version works fine for most purposes, e.g.,

    my.repeat 5 [print "hello]

But it doesn’t work if the instructions to be carried out include `OUTPUT`, `STOP`, or `LOCAL`. For example, consider this procedure:

    to example
    print [Guess my secret word.  You get three guesses.]
    repeat 3 [type "|?? | ~
              if readword = "secret [pr "Right! stop]]
    print [Sorry, the word was "secret"!]
    end

This procedure works as written, but if `MY.REPEAT` is used instead of `REPEAT`, it won’t work because the `STOP` will stop `MY.REPEAT` instead of stopping `EXAMPLE` as desired.

The solution is to make `MY.REPEAT` a macro. Instead of actually carrying out the computation, a macro must return a list containing Logo instructions. The contents of that list are evaluated as if they appeared in place of the call to the macro. Here’s a macro version of `REPEAT`:

    .macro my.repeat :num :instructions
    if :num=0 [output []]
    output sentence :instructions ~
                    (list "my.repeat :num-1 :instructions)
    end

Every macro is an operation — it must always output something. Even in the base case, `MY.REPEAT` outputs an empty instruction list. To show how `MY.REPEAT` works, let’s take the example

    my.repeat 5 [print "hello]

For this example, `MY.REPEAT` will output the instruction list

    [print "hello my.repeat 4 [print "hello]]

Logo then executes these instructions in place of the original invocation of `MY.REPEAT`; this prints `hello` once and invokes another repetition.

The technique just shown, although fairly easy to understand, has the defect of slowness because each repetition has to construct an instruction list for evaluation. Another approach is to make `MY.REPEAT` a macro that works just like the non-macro version unless the instructions to be repeated include `OUTPUT` or `STOP`:

    .macro my.repeat :num :instructions
    catch "repeat.catchtag ~
          [op repeat.done runresult [repeat1 :num :instructions]]
    op []
    end

    to repeat1 :num :instructions
    if :num=0 [throw "repeat.catchtag]
    run :instructions
    .maybeoutput repeat1 :num-1 :instructions
    end

    to repeat.done :repeat.result
    if emptyp :repeat.result [op [stop]]
    op list "output quoted first :repeat.result
    end

If the instructions do not include `STOP` or `OUTPUT`, then `REPEAT1` will reach its base case and invoke `THROW`. As a result, `MY.REPEAT`’s last instruction line will output an empty list, so the evaluation of the macro result by the caller will do nothing. But if a `STOP` or `OUTPUT` happens, then `REPEAT.DONE` will output a `STOP` or `OUTPUT` instruction that will be executed in the caller’s context.

The macro-defining commands have names starting with a dot because macros are an advanced feature of Logo; it’s easy to get in trouble by defining a macro that doesn’t terminate, or by failing to construct the instruction list properly.

Lisp users should note that Logo macros are not special forms. That is, the inputs to the macro are evaluated normally, as they would be for any other Logo procedure. It’s only the output from the macro that’s handled unusually.

Here’s another example:

    .macro localmake :name :value
    output (list "local          ~
                 word "" :name   ~
                 "apply          ~
                 ""make          ~
                 (list :name :value))
    end

It’s used this way:

    to try
    localmake "garply "hello
    print :garply
    end

`LOCALMAKE` outputs the list

    [local "garply apply "make [garply hello]]

The reason for the use of `APPLY` is to avoid having to decide whether or not the second input to `MAKE` requires a quotation mark before it. (In this case it would — `MAKE "GARPLY "HELLO` — but the quotation mark would be wrong if the value were a list.)

It’s often convenient to use the ``` function to construct the instruction list:

    .macro localmake :name :value
    op `[local ,[word "" :name] apply "make [,[:name] ,[:value]]]
    end

On the other hand, ``` is pretty slow, since it’s tree recursive and written in Logo.

See to , define , apply , stop , output .

#### .defmacro ¶

See .macro .

#### macrop ¶

    MACROP name
    MACRO? name

outputs `TRUE` if its input is the name of a macro.

#### macroexpand ¶

    MACROEXPAND expr				(library procedure)

takes as its input a Logo expression that invokes a macro (that is, one that begins with the name of a macro) and outputs the the Logo expression into which the macro would translate the input expression.

    .macro localmake :name :value
    op `[local ,[word "" :name] apply "make [,[:name] ,[:value]]]
    end

    ? show macroexpand [localmake "pi 3.14159]
    [local "pi apply "make [pi 3.14159]]

## 11 Error Processing ¶

If an error occurs, Logo takes the following steps. First, if there is an available variable named `ERRACT`, Logo takes its value as an instructionlist and runs the instructions. The operation `ERROR` may be used within the instructions (once) to examine the error condition. If the instructionlist invokes `PAUSE`, the error message is printed before the pause happens. Certain errors are recoverable; for one of those errors, if the instructionlist outputs a value, that value is used in place of the expression that caused the error. (If `ERRACT` invokes `PAUSE` and the user then invokes `CONTINUE` with an input, that input becomes the output from `PAUSE` and therefore the output from the `ERRACT` instructionlist.)

It is possible for an `ERRACT` instructionlist to produce an inappropriate value or no value where one is needed. As a result, the same error condition could recur forever because of this mechanism. To avoid that danger, if the same error condition occurs twice in a row from an `ERRACT` instructionlist without user interaction, the message ‘`Erract loop`’ is printed and control returns to toplevel. "Without user interaction" means that if `ERRACT` invokes `PAUSE` and the user provides an incorrect value, this loop prevention mechanism does not take effect and the user gets to try again.

During the running of the `ERRACT` instructionlist, `ERRACT` is locally unbound, so an error in the `ERRACT` instructions themselves will not cause a loop. In particular, an error during a pause will not cause a pause-within-a-pause unless the user reassigns the value `[PAUSE]` to `ERRACT` during the pause. But such an error will not return to toplevel; it will remain within the original pause loop.

If there is no available `ERRACT` value, Logo handles the error by generating an internal `THROW "ERROR`. (A user program can also generate an error condition deliberately by invoking `THROW`.) If this throw is not caught by a `CATCH "ERROR` in the user program, it is eventually caught either by the toplevel instruction loop or by a pause loop, which prints the error message. An invocation of `CATCH "ERROR` in a user program locally unbinds `ERRACT`, so the effect is that whichever of `ERRACT` and `CATCH "ERROR` is more local will take precedence.

If a floating point overflow occurs during an arithmetic operation, or a two-input mathematical function (like `POWER`) is invoked with an illegal combination of inputs, the ‘`doesn't like`’ message refers to the second operand, but should be taken as meaning the combination.

See erract , throw , error , catch , pause , continue .

### 11.1 Error Codes ¶

Here are the numeric codes that appear as the first member of the list output by `ERROR` when an error is caught, with the corresponding messages. Some messages may have two different codes depending on whether or not the error is recoverable (that is, a substitute value can be provided through the `ERRACT` mechanism) in the specific context. Some messages are warnings rather than errors; these will not be caught. Errors 0 and 32 are so bad that Logo exits immediately.

      0	Fatal internal error  (can’t be caught)
      1	Out of memory
      2	Stack overflow
      3	Turtle out of bounds
      4	proc doesn't like datum as input  (not recoverable)
      5	proc didn't output to proc
      6	Not enough inputs to proc
      7	proc doesn't like datum as input  (recoverable)
      8	Too much inside ()'s
      9 	You don't say what to do with datum
     10	')' not found
     11	var has no value
     12	Unexpected ')'
     13	I don't know how to proc  (recoverable)
     14	Can't find catch tag for throwtag
     15	proc is already defined
     16	Stopped
     17	Already dribbling
     18	File system error
     19	Assuming you mean IFELSE, not IF  (warning only)
     20	var shadowed by local in procedure call  (warning only)
     21	Throw "Error
     22	proc is a primitive
     23	Can't use TO inside a procedure
     24	I don't know how to proc  (not recoverable)
     25	IFTRUE/IFFALSE without TEST
     26	Unexpected ']'
     27	Unexpected '}'
     28	Couldn't initialize graphics
     29	Macro returned value instead of a list
     30	You don't say what to do with value
     31	Can only use STOP or OUTPUT inside a procedure
     32	APPLY doesn't like badthing as input
     33	END inside multi-line instruction
     34	Really out of memory  (can’t be caught)
     35	user-generated error message (THROW "ERROR message)
     36	END inside multi-line instruction
     37	Bad default expression for optional input: expr
     38	Can't use OUTPUT or STOP inside RUNRESULT
     39 	Assuming you meant 'FD 100', not FD100  (or similar)
     40	I can't open file filename
     41	File filename already open
     42	File filename not open
     43	Runlist [expr expr] has more than one expression.

## 12 Special Variables ¶

Logo takes special action if any of the following variable names exists. They follow the normal scoping rules, so a procedure can locally set one of them to limit the scope of its effect. Initially, no variables exist except for `ALLOWGETSET`, `CASEIGNOREDP`, and `UNBURYONEDIT`, which are `TRUE` and buried.

### allowgetset ¶

    ALLOWGETSET                           (variable)

if `TRUE`, indicates that an attempt to use a procedure that doesn’t exist should be taken as an implicit getter or setter procedure (setter if the first three letters of the name are `SET`) for a variable of the same name (without the `SET` if appropriate).

#### buttonact ¶

    BUTTONACT                           (variable)

if nonempty, should be an instruction list that will be evaluated whenever a mouse button is pressed. Note that the user may have released the button before the instructions are evaluated. `BUTTON` will still output which button was most recently pressed. `CLICKPOS` will output the position of the mouse cursor at the moment the button was pressed; this may be different from `MOUSEPOS` if the user moves the mouse after clicking.

Note that it’s possible for the user to press a button during the evaluation of the instruction list. If this would confuse your program, prevent it by temporarily setting `BUTTONACT` to the empty list. One easy way to do that is the following:

    make "buttonact [button.action]

    to button.action [:buttonact []]
    ... ; whatever you want the button to do
    end

#### caseignoredp ¶

    CASEIGNOREDP                                (variable)

if `TRUE`, indicates that lower case and upper case letters should be considered equal by `EQUALP`, `BEFOREP`, `MEMBERP`, etc. Logo initially makes this variable `TRUE`, and buries it.

See equalp , beforep , memberp .

#### commandline ¶

    COMMANDLINE                                (variable)

contains any text appearing after a hyphen on the command line used to start Logo.

#### erract ¶

    ERRACT                                      (variable)

an instructionlist that will be run in the event of an error. Typically has the value `[PAUSE]` to allow interactive debugging.

See pause .

#### fullprintp ¶

    FULLPRINTP                               (variable)

if `TRUE`, then words that were created using backslash or vertical bar (to include characters that would otherwise not be treated as part of a word) are printed with the backslashes or vertical bars shown, so that the printed result could be re-read by Logo to produce the same value. If FULLPRINTP is `TRUE` then the empty word (however it was created) prints as `||`. (Otherwise it prints as nothing at all.)

#### keyact ¶

    KEYACT                               (variable)

if nonempty, should be an instruction list that will be evaluated whenever a key is pressed on the keyboard. The instruction list can use `READCHAR` to find out what key was pressed. Note that only keys that produce characters qualify; pressing `SHIFT` or `CONTROL` alone will not cause `KEYACT` to be evaluated.

Note that it’s possible for the user to press a key during the evaluation of the instruction list. If this would confuse your program, prevent it by temporarily setting `KEYACT` to the empty list. One easy way to do that is the following:

    make "keyact [key.action]

    to key.action [:keyact []]
    ... ; whatever you want the key to do
    end

#### loadnoisily ¶

    LOADNOISILY                                 (variable)

if `TRUE`, prints the names of procedures defined when loading from a file (including the temporary file made by `EDIT`).

See edit .

#### printdepthlimit ¶

    PRINTDEPTHLIMIT                             (variable)

if a nonnegative integer, indicates the maximum depth of sublist structure that will be printed by `PRINT`, etc.

See print .

#### printwidthlimit ¶

    PRINTWIDTHLIMIT                             (variable)

if a nonnegative integer, indicates the maximum number of members in any one list that will be printed by `PRINT`, etc.

See print .

#### redefp ¶

    REDEFP                                      (variable)

if `TRUE`, allows primitives to be erased (`ERASE`) or redefined (`COPYDEF`).

See erase , copydef .

#### startup ¶

    STARTUP                                     (variable)

if assigned a list value in a file loaded by `LOAD`, that value is run as an instructionlist after the loading.

See load .

#### unburyonedit ¶

    UNBURYONEDIT                            (variable)

if `TRUE`, causes any procedure defined during `EDIT` or `LOAD` to be unburied, so that it will be saved by a later `SAVE`. Files that want to define and bury procedures must do it in that order.

See edit , See load , See save .

#### usealternatenames ¶

    USEALTERNATENAMES					(variable)

if `TRUE`, causes Logo to generate non-English words (from the `Messages` file) instead of `TRUE`, `FALSE`, `END`, etc.

Logo provides the following buried variables that can be used by programs:

#### logoversion ¶

    LOGOVERSION                                   (variable)

a real number indicating the Logo version number, e.g., 5.5

#### logoplatform ¶

    LOGOPLATFORM                                   (variable)

one of the following words: `wxWidgets`, `X11`, `Windows`, or `Unix-nographics`.

## 13 Internationalization ¶

Berkeley Logo has limited support for non-English-speaking users. Alas, there is no Unicode support, and high-bit-on ASCII codes work in some contexts but not others.

If you want to translate Berkeley Logo for use with another language, there are three main things you have to do:

    1. Primitive names
    2. Error (and other) messages
    3. Documentation

For primitive names, the easiest thing is to provide a startup file that defines aliases for the English primitive names, using `COPYDEF`:

    COPYDEF "AVANT "FORWARD

This should take care of it, unless your language’s name for one primitive is spelled like the English name of a different primitive. In that case you have to turn `REDEFP` on and be sure to copy the non-conflicting name before overwriting the conflicting one!

"Primitives" that are actually in the Logo library, of course, can just be replaced or augmented with native-language-named Logo procedures and filenames.

Of course Logo programs will still not look like your native language if the word order is dramatically different, especially if you don’t put verbs before their objects.

For error messages, there is a file named `Messages` in the `logolib` directory with texts of messages, one per line. You can replace this with a file for your own language. Do not add, delete, or reorder lines; Logo finds messages by line number. The sequences `%p`, `%s`, and `%t` in these messages represent variable parts of the message and should not be translated. (%p `PRINT`s the variable part, while %s `SHOW`s it – that is, the difference is about whether or not brackets are shown surrounding a list. %t means that the variable part is a C text string rather than a Logo object.) If you want to change the order of two variable parts (no reorderable message has more than two), you would for example replace the line

    %p doesn't like %s as input

with

    %+s is a lousy input to %p

The plus sign tells the message printer to reverse the order; you must reverse the order of %p and %s, if both are used, to match. The plus sign goes just after the first percent sign in the message, which might not be at the beginning of the line. The sequence `\n` in a message represents a newline; don’t be fooled into thinking that the `n` is part of the following word.

Some messages appear twice in the file; this isn’t a mistake. The two spaces before `to` in `I don't know how\ \ to` aren’t a mistake either. The message containing just `%p` is for user-provided error messages in `THROW "ERROR`. The message "`\ \ in %s\n%s`" is the part of all error messages that indicates where the error occurred if it was inside a procedure; you might want to change the word `in` to your language. `%s defined\n` is what `LOAD` prints for each procedure defined if the variable `LOADNOISILY` is `TRUE`. "`to %p\nend\n\n`" is what `EDIT` puts in the temporary file if you ask to edit a procedure that isn’t already defined.

Also in the `Messages` file are lines containing only one word each; the first of these is the word `true`. Some of these words are recognized by Logo in user input; some are generated by Logo; some are both. For example, the words `TRUE` and `FALSE` are recognized as Boolean values by `IF` and `IFELSE`, and are also generated by Logo as outputs from the primitive predicates such as `EQUALP`. The word `END` is recognized as the end of a procedure definition, and may be generated when Logo reconstructs a procedure body for `PO` or `EDIT`. I’ve used capital letters in this paragraph for easier reading, but the words in the `Messages` file should be in lower case.

If you replace these with non-English words, Logo will recognize both the English names and your alternate names. For example, if you replace the word `true` with `vrai` then Logo will understand both of these:

    IF "TRUE [PRINT "YES]
    IF "VRAI [PRINT "YES]

The variable `UseAlternateNames` determines whether Logo will generate other-language names – for example, whether predicate functions return the other-language alternates for `TRUE` and `FALSE`. This variable is `FALSE` by default, meaning that the English words will be generated.

You might wish to have English-named predicate functions generate English `TRUE` and `FALSE`, while other-language-named predicates generate the alternate words. This can be done by leaving `UseAlternateNames` false, and instead of defining the other-language predicates with `COPYDEF`, do it this way:

    to french.boolean :bool
    if equalp :bool "true [output "vrai]
    if equalp :bool "false [output "faux]
    output :bool	; shouldn't happen
    end

    to make.french.predicate :french :english :arity
    define :french `[[[inputs] ,[:arity]]
                     [output french.boolean
    			    apply ,[word "" :english] :inputs]]
    end

    ? make.french.predicate "egal? "equal? 2
    ? pr egal? 3 4
    faux
    ? pr egal? 4 4
    vrai
    ? pr equal? 3 4
    false
    ? pr equal? 4 4
    true

The third input to `make.french.predicate` is the number of inputs that the predicate expects. This solution isn’t quite perfect because the infix predicates (`=`, `<`, `>`) will still output in English. If you want them to generate alternate-language words, set `UseAlternateNames` to `TRUE` instead.

Some of the words in this section of the `Messages` file are names of Logo primitives (`OUTPUT`, `STOP`, `GOTO`, `TAG`, `IF`, `IFELSE`, `TO`, `.MACRO`). To translate these names, you must use `COPYDEF` as described earlier, in addition to changing the names in `Messages`. You should be consistent in these two steps. Don’t forget the period in `.macro`!

For documentation, there are two kinds: this manual and the help files. The latter are generated automatically from this manual if you have a Unix system, so in that case you need only translate this manual, maintaining the format. (The automatic helpfile generator notices things like capital letters, tabs, hyphens, and equal signs at the beginnings of lines.) The program `makefile.c` may require modification because a few of the primitive names are special cases (e.g., `LOG10` is the only name with digits included).

If you don’t have Unix tools, you can just translate each helpfile individually. A period in a primitive name is represented as a `D` in the filename; there are no files for question marks because the `HELP` command looks for the file named after the corresponding primitive that ends in `P`.

## INDEX ¶

Jump to: | - . * / ` + < = > A B C D E F G H I K L M N O P Q R S T U V W X Y

| Index Entry | Section

-
| - | DIFFERENCE

.
| .defmacro | dMACRO
| .eq | dEQ
| .macro | dMACRO
| .maybeoutput | dMAYBEOUTPUT
| .setbf | dSETBF
| .setfirst | dSETFIRST
| .setitem | dSETITEM
| .setsegmentsize | .SETSEGMENTSIZE

*
| * | PRODUCT

/
| / | QUOTIENT

`
| ‘ | back-quote

+
| + | SUM

<
| < | LESSP
| <= | LESSEQUALP
| <> | NOTEQUALP

=
| = | EQUALP

>
| > | GREATERP
| >= | GREATEREQUALP

A
| allopen | ALLOPEN
| AllowGetSet | GETTER/SETTER VARIBLE SYNTAX
| allowgetset | ALLOWGETSET
| and | AND
| apply | APPLY
| arc | ARC
| arctan | ARCTAN
| arity | ARITY
| array | ARRAY
| array? | ARRAYP
| arrayp | ARRAYP
| arraytolist | ARRAYTOLIST
| ascii | ASCII
| ashift | ASHIFT
| ask | ASK

B
| back | BACK
| background | BACKGROUND
| before? | BEFOREP
| beforep | BEFOREP
| bf | BUTFIRST
| bfs | BUTFIRSTS
| bg | BACKGROUND
| bitand | BITAND
| bitnot | BITNOT
| bitor | BITOR
| bitxor | BITXOR
| bk | BACK
| bl | BUTLAST
| buried | BURIED
| buried? | BURIEDP
| buriedp | BURIEDP
| bury | BURY
| buryall | BURYALL
| buryname | BURYNAME
| butfirst | BUTFIRST
| butfirsts | BUTFIRSTS
| butlast | BUTLAST
| button | BUTTON
| button? | BUTTONP
| buttonact | BUTTONACT
| buttonp | BUTTONP
| bye | BYE

C
| cascade | CASCADE
| cascade.2 | CASCADEd2
| case | CASE
| case-insensitive | TOKENIZATION
| caseignoredp | CASEIGNOREDP
| catch | CATCH
| char | CHAR
| clean | CLEAN
| clearscreen | CLEARSCREEN
| cleartext | CLEARTEXT
| clickpos | CLICKPOS
| close | CLOSE
| closeall | CLOSEALL
| co | CONTINUE
| combine | COMBINE
| commandline | COMMANDLINE
| comments | TOKENIZATION
| Computer_Science_Logo_Style | OVERVIEW
| cond | COND
| contents | CONTENTS
| continue | CONTINUE
| copydef | COPYDEF
| Copyright | OVERVIEW
| cos | COS
| count | COUNT
| crossmap | CROSSMAP
| cs | CLEARSCREEN
| cslsload | CSLSLOAD
| ct | CLEARTEXT
| cursor | CURSOR

D
| decreasefont | INCREASEFONT
| define | DEFINE
| defined? | DEFINEDP
| definedp | DEFINEDP
| delimiters | TOKENIZATION
| dequeue | DEQUEUE
| difference | DIFFERENCE
| do.until | DOdUNTIL
| do.while | DOdWHILE
| dribble | DRIBBLE

E
| ed | EDIT
| edall | EDALL
| edit | EDIT
| editfile | EDITFILE
| editor | EDIT
| edn | EDN
| edns | EDNS
| edpl | EDPL
| edpls | EDPLS
| edps | EDPS
| empty? | EMPTYP
| emptyp | EMPTYP
| eof? | EOFP
| eofp | EOFP
| epspict | EPSPICT
| equal? | EQUALP
| equalp | EQUALP
| er | ERASE
| erall | ERALL
| erase | ERASE
| erasefile | ERASEFILE
| erf | ERASEFILE
| ern | ERN
| erns | ERNS
| erpl | ERPL
| erpls | ERPLS
| erps | ERPS
| erract | ERRACT
| error | ERROR
| errors | ERROR CODES
| exist | EXIST
| exp | EXP

F
| fd | FORWARD
| fence | FENCE
| file? | FILEP
| filep | FILEP
| fill | FILL
| filled | FILLED
| filter | FILTER
| find | FIND
| first | FIRST
| firsts | FIRSTS
| font | FONT
| for | FOR
| foreach | FOREACH
| forever | FOREVER
| form | FORM
| forward | FORWARD
| fput | FPUT
| fs | FULLSCREEN
| fullprintp | FULLPRINTP
| fullscreen | FULLSCREEN
| fulltext | FULLTEXT

G
| gc | GC
| gensym | GENSYM
| getter | GETTER/SETTER VARIBLE SYNTAX
| global | GLOBAL
| goto | GOTO
| gprop | GPROP
| greater? | GREATERP
| greaterequal? | GREATEREQUALP
| greaterequalp | GREATEREQUALP
| greaterp | GREATERP

H
| have | HAVE
| havemake | HAVEMAKE
| heading | HEADING
| help | HELP
| hideturtle | HIDETURTLE
| home | HOME
| ht | HIDETURTLE

I
| if | IF
| ifelse | IFELSE
| iff | IFFALSE
| iffalse | IFFALSE
| ift | IFTRUE
| iftrue | IFTRUE
| ignore | IGNORE
| increasefont | INCREASEFONT
| int | INT
| invoke | INVOKE
| iseq | ISEQ
| item | ITEM

K
| key? | KEYP
| keyact | KEYACT
| keyp | KEYP
| kindof | KINDOF

L
| label | LABEL
| labelsize | LABELSIZE
| last | LAST
| leaving `ucblogo` | ENTERING AND LEAVING LOGO
| left | LEFT
| less? | LESSP
| lessequal? | LESSEQUALP
| lessequalp | LESSEQUALP
| lessp | LESSP
| line-continuation | TOKENIZATION
| line? | LINEP
| linep | LINEP
| list | LIST
| list? | LISTP
| listp | LISTP
| listtoarray | LISTTOARRAY
| ln | LN
| load | LOAD
| loadnoisily | LOADNOISILY
| loadpict | LOADPICT
| local | LOCAL
| localmake | LOCALMAKE
| log10 | LOG10
| logohelp | HELP
| logoplatform | LOGOPLATFORM
| logoversion | LOGOVERSION
| lowercase | LOWERCASE
| lput | LPUT
| lshift | LSHIFT
| lt | LEFT

M
| macro? | MACROP
| macroexpand | MACROEXPAND
| macrop | MACROP
| make | MAKE
| map | MAP
| map.se | MAPdSE
| mdarray | MDARRAY
| mditem | MDITEM
| mdsetitem | MDSETITEM
| member | MEMBER
| member? | MEMBERP
| memberp | MEMBERP
| minus | MINUS
| modulo | MODULO
| mousepos | MOUSEPOS
| myname? | MYNAMEP
| mynamep | MYNAMEP
| mynames | MYNAMES
| myproc? | MYPROCP
| myprocp | MYPROCP
| myprocs | MYPROCS

N
| name | NAME
| name? | NAMEP
| namelist | NAMELIST
| namep | NAMEP
| names | NAMES
| nodes | NODES
| nodribble | NODRIBBLE
| norefresh | NOREFRESH
| not | NOT
| notequal? | NOTEQUALP
| notequalp | NOTEQUALP
| number? | NUMBERP
| numberp | NUMBERP

O
| oneof | ONEOF
| op | OUTPUT
| openappend | OPENAPPEND
| openread | OPENREAD
| openupdate | OPENUPDATE
| openwrite | OPENWRITE
| or | OR
| output | OUTPUT

P
| palette | PALETTE
| parents | PARENTS
| parse | PARSE
| pause | PAUSE
| pc | PENCOLOR
| pd | PENDOWN
| pe | PENERASE
| pen | PEN
| pencolor | PENCOLOR
| pendown | PENDOWN
| pendown? | PENDOWNP
| pendownp | PENDOWNP
| penerase | PENERASE
| penmode | PENMODE
| penpaint | PENPAINT
| penpattern | PENSIZE
| penreverse | PENREVERSE
| pensize | PENSIZE
| penup | PENUP
| pick | PICK
| plist | PLIST
| plist? | PLISTP
| plistp | PLISTP
| plists | PLISTS
| pllist | PLLIST
| po | PO
| poall | POALL
| pon | PON
| pons | PONS
| pop | POP
| popl | POPL
| popls | POPLS
| pops | POPS
| pos | POS
| pot | POT
| pots | POTS
| power | POWER
| pprop | PPROP
| ppt | PENPAINT
| pr | PRINT
| prefix | PREFIX
| primitive? | PRIMITIVEP
| primitivep | PRIMITIVEP
| primitives | PRIMITIVES
| print | PRINT
| printdepthlimit | PRINTDEPTHLIMIT
| printout | PO
| printwidthlimit | PRINTWIDTHLIMIT
| procedure? | PROCEDUREP
| procedurep | PROCEDUREP
| procedures | PROCEDURES
| product | PRODUCT
| pu | PENUP
| push | PUSH
| px | PENREVERSE

Q
| queue | QUEUE
| quoted | QUOTED
| quotient | QUOTIENT

R
| radarctan | RADARCTAN
| radcos | RADCOS
| radsin | RADSIN
| random | RANDOM
| rawascii | RAWASCII
| rc | READCHAR
| rcs | READCHARS
| readchar | READCHAR
| readchars | READCHARS
| reader | READER
| readlist | READLIST
| readpos | READPOS
| readrawline | READRAWLINE
| readword | READWORD
| redefp | REDEFP
| reduce | REDUCE
| refresh | REFRESH
| remainder | REMAINDER
| remdup | REMDUP
| remove | REMOVE
| remprop | REMPROP
| repcount | REPCOUNT
| repeat | REPEAT
| rerandom | RERANDOM
| reverse | REVERSE
| right | RIGHT
| rl | READLIST
| round | ROUND
| rseq | RSEQ
| rt | RIGHT
| run | RUN
| runparse | RUNPARSE
| runparsing | TOKENIZATION
| runresult | RUNRESULT
| rw | READWORD

S
| save | SAVE
| savel | SAVEL
| savepict | SAVEPICT
| screenmode | SCREENMODE
| scrunch | SCRUNCH
| scrunch.dat | SETSCRUNCH
| se | SENTENCE
| self | SELF
| sentence | SENTENCE
| setbackground | SETBACKGROUND
| setbg | SETBACKGROUND
| setcslsloc | SETCSLSLOC
| setcursor | SETCURSOR
| seteditor | SETEDITOR
| setfont | SETFONT
| seth | SETHEADING
| setheading | SETHEADING
| sethelploc | SETHELPLOC
| setitem | SETITEM
| setlabelheight | SETLABELHEIGHT
| setlibloc | SETLIBLOC
| setmargins | SETMARGINS
| setpalette | SETPALETTE
| setpc | SETPENCOLOR
| setpen | SETPEN
| setpencolor | SETPENCOLOR
| setpenpattern | SETPENPATTERN
| setpensize | SETPENSIZE
| setpos | SETPOS
| setprefix | SETPREFIX
| setread | SETREAD
| setreadpos | SETREADPOS
| setscrunch | SETSCRUNCH
| settc | SETTEXTCOLOR
| settemploc | SETTEMPLOC
| setter | GETTER/SETTER VARIBLE SYNTAX
| settextcolor | SETTEXTCOLOR
| settextsize | SETTEXTSIZE
| setwrite | SETWRITE
| setwritepos | SETWRITEPOS
| setx | SETX
| setxy | SETXY
| sety | SETY
| shell | SHELL
| show | SHOW
| shown? | SHOWNP
| shownp | SHOWNP
| showturtle | SHOWTURTLE
| sin | SIN
| something | SOMETHING
| splitscreen | SPLITSCREEN
| sqrt | SQRT
| ss | SPLITSCREEN
| st | SHOWTURTLE
| standout | STANDOUT
| starting `ucblogo` | ENTERING AND LEAVING LOGO
| startup | STARTUP
| step | STEP
| stepped | STEPPED
| stepped? | STEPPEDP
| steppedp | STEPPEDP
| stop | STOP
| substring? | SUBSTRINGP
| substringp | SUBSTRINGP
| sum | SUM

T
| tag | TAG
| talkto | TALKTO
| temp | EDIT
| template | TEMPLATE-BASED ITERATION
| test | TEST
| text | TEXT
| textscreen | TEXTSCREEN
| textsize | TEXTSIZE
| thing | THING
| throw | THROW
| to | TO
| towards | TOWARDS
| trace | TRACE
| traced | TRACED
| traced? | TRACEDP
| tracedp | TRACEDP
| transfer | TRANSFER
| turtlemode | TURTLEMODE
| type | TYPE

U
| unbury | UNBURY
| unburyall | UNBURYALL
| unburyname | UNBURYNAME
| unburyonedit | UNBURYONEDIT
| unstep | UNSTEP
| until | UNTIL
| untrace | UNTRACE
| uppercase | UPPERCASE
| usealternatenames | USEALTERNATENAMES
| usual | USUAL

V
| vbarred? | VBARREDP
| vbarredp | VBARREDP

W
| wait | WAIT
| while | WHILE
| whosename | WHOSENAME
| whoseproc | WHOSEPROC
| window | WINDOW
| word | WORD
| wordp | WORDP
| wrap | WRAP
| writepos | WRITEPOS
| writer | WRITER

X
| xcor | XCOR

Y
| ycor | YCOR

#### Footnotes

##### (1)

Computer Science Logo Style