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[MathJax off]

21 Lists
21.1 List Categories
21.1-1 IsList
21.1-2 IsDenseList
21.1-3 IsHomogeneousList
21.1-4 IsTable
21.1-5 IsRectangularTable
21.1-6 IsConstantTimeAccessList
21.2 Basic Operations for Lists
`21.2-1 \[\]`
21.3 List Elements
`21.3-1 \{\}`
21.4 List Assignment
`21.4-1 \{\}\:\=`
21.4-2 Add
21.4-3 Remove
21.4-4 CopyListEntries
21.4-5 Append
21.5 IsBound and Unbind for Lists
21.5-1 IsBound
21.5-2 GetWithDefault
21.5-3 Unbind
21.6 Identical Lists
21.7 Duplication of Lists
21.8 Membership Test for Lists
`21.8-1 \in`
21.9 Enlarging Internally Represented Lists
21.9-1 EmptyPlist
21.10 Comparisons of Lists
21.11 Arithmetic for Lists
21.12 Filters Controlling the Arithmetic Behaviour of Lists
21.12-1 IsGeneralizedRowVector
21.12-2 IsMultiplicativeGeneralizedRowVector
21.12-3 IsListDefault
21.12-4 NestingDepthA
21.12-5 NestingDepthM
21.13 Additive Arithmetic for Lists
21.13-1 Zero for lists
21.13-2 AdditiveInverse for lists
21.13-3 Addition of lists
21.13-4 Subtraction of lists
21.14 Multiplicative Arithmetic for Lists
21.14-1 One for lists
21.14-2 Inverse for lists
21.14-3 Multiplication of lists
21.14-4 Division of lists
21.14-5 mod for lists
21.14-6 Left quotients of lists
21.15 Mutability Status and List Arithmetic
21.15-1 ListWithIdenticalEntries
21.16 Finding Positions in Lists
21.16-1 Position
21.16-2 Positions
21.16-3 PositionCanonical
21.16-4 PositionNthOccurrence
21.16-5 PositionSorted
21.16-6 PositionSortedBy
21.16-7 PositionSet
21.16-8 PositionMaximum
21.16-9 PositionProperty
21.16-10 PositionsProperty
21.16-11 PositionBound
21.16-12 PositionsBound
21.16-13 PositionNot
21.16-14 PositionNonZero
21.16-15 PositionSublist
21.17 Properties and Attributes for Lists
21.17-1 IsMatchingSublist
21.17-2 IsDuplicateFree
21.17-3 IsSortedList
21.17-4 IsSSortedList
21.17-5 Length
21.17-6 ConstantTimeAccessList
21.18 Sorting Lists
21.18-1 Sort
21.18-2 SortParallel
21.18-3 Sortex
21.18-4 SortingPerm
21.19 Sorted Lists and Sets
`21.19-1 \in`
21.19-2 IsEqualSet
21.19-3 IsSubsetSet
21.19-4 AddSet
21.19-5 RemoveSet
21.19-6 UniteSet
21.19-7 IntersectSet
21.19-8 SubtractSet
21.20 Operations for Lists
21.20-1 Concatenation
21.20-2 Compacted
21.20-3 Collected
21.20-4 DuplicateFreeList
21.20-5 AsDuplicateFreeList
21.20-6 Flat
21.20-7 Reversed
21.20-8 Shuffle
21.20-9 Apply
21.20-10 Perform
21.20-11 PermListList
21.20-12 Maximum
21.20-13 Minimum
21.20-14 MaximumList and MinimumList
21.20-15 Cartesian
21.20-16 IteratorOfCartesianProduct
21.20-17 Permuted
21.20-18 List
21.20-19 Filtered
21.20-20 Number
21.20-21 First
21.20-22 Last
21.20-23 ForAll
21.20-24 ForAny
21.20-25 Product
21.20-26 Sum
21.20-27 Iterated
21.20-28 ListN
21.21 Advanced List Manipulations
21.21-1 FoldLeft
21.21-2 FoldLeftX
21.21-3 ListX
21.21-4 SetX
21.21-5 SumX
21.21-6 ProductX
21.21-7 ForAllX
21.21-8 ForAnyX
21.21-9 FilteredX
21.21-10 NumberX
21.21-11 PerformX
21.22 Ranges
21.22-1 IsRange
21.22-2 IsRangeRep
21.22-3 ConvertToRangeRep
21.23 Enumerators
21.23-1 IsQuickPositionList
21.24 Plain Lists
21.24-1 PlainListCopy
21.24-2 IsPlistRep

### 21 Lists

Lists are the most important way to treat objects together. A list arranges objects in a definite order. So each list implies a partial mapping from the integers to the elements of the list. I.e., there is a first element of a list, a second, a third, and so on. Lists can occur in mutable or immutable form, see 12.6 for the concept of mutability, and 21.7 for the case of lists.

This chapter deals mainly with the aspect of lists in GAP as data structures. Chapter 30 tells more about the collection aspect of certain lists, and more about lists as arithmetic objects can be found in the chapters 23 and 24.

Lists are used to implement ranges (see 21.22), sets (see 21.19), strings (see 27), row vectors and matrices (see 23 and 24, but note that GAP supports also linear algebra for objects which are not lists, see 26); boolean lists (see 22) are a further special kind of lists.

Several operations for lists, such as `Intersection` (30.5-2) and `Random` (30.7-1), will be described in Chapter 30, in particular see 30.3.

#### 21.1 List Categories

A list can be written by writing down the elements in order between square brackets `[`, `]`, and separating them with commas `,`. An empty list, i.e., a list with no elements, is written as `[]`.

    gap> [ 1, 2, 3 ];              # a list with three elements
    [ 1, 2, 3 ]
    gap> [ [], [ 1 ], [ 1, 2 ] ];  # a list may contain other lists
    [ [  ], [ 1 ], [ 1, 2 ] ]

Each list constructed this way is mutable (see 12.6).

##### 21.1-1 IsList

`‣ IsList`( obj ) | ( category )

tests whether obj is a list.

    gap> IsList( [ 1, 3, 5, 7 ] );  IsList( 1 );
    true
    false

##### 21.1-2 IsDenseList

`‣ IsDenseList`( obj ) | ( category )

A list is dense if it has no holes, i.e., contains an element at every position up to the length. It is absolutely legal to have lists with holes. They are created by leaving the entry between the commas empty. Holes at the end of a list are ignored. Lists with holes are sometimes convenient when the list represents a mapping from a finite, but not consecutive, subset of the positive integers.

    gap> IsDenseList( [ 1, 2, 3 ] );
    true
    gap> l := [ , 4, 9,, 25,, 49,,,, 121 ];;  IsDenseList( l );
    false
    gap> l[3];
    9
    gap> l[4];
    List Element: <list>[4] must have an assigned value
    not in any function
    Entering break read-eval-print loop ...
    you can 'quit;' to quit to outer loop, or
    you can 'return;' after assigning a value to continue
    brk> l[4] := 16;;  # assigning a value
    brk> return;       # to escape the break-loop
    16
    gap>

Observe that requesting the value of `l[4]`, which was not assigned, caused the entry of a `break`-loop (see Section 6.4). After assigning a value and typing `return;`, GAP is finally able to comply with our request (by responding with `16`).

##### 21.1-3 IsHomogeneousList

`‣ IsHomogeneousList`( obj ) | ( category )

returns `true` if obj is a list and it is homogeneous, and `false` otherwise.

A homogeneous list is a dense list whose elements lie in the same family (see 13.1). The empty list is homogeneous but not a collection (see 30), a nonempty homogeneous list is also a collection.

    gap> IsHomogeneousList( [ 1, 2, 3 ] );  IsHomogeneousList( [] );
    true
    true
    gap> IsHomogeneousList( [ 1, false, () ] );
    false

##### 21.1-4 IsTable

`‣ IsTable`( obj ) | ( category )

A table is a nonempty list of homogeneous lists which lie in the same family. Typical examples of tables are matrices (see 24).

    gap> IsTable( [ [ 1, 2 ], [ 3, 4 ] ] );    # in fact a matrix
    true
    gap> IsTable( [ [ 1 ], [ 2, 3 ] ] );       # not rectangular but a table
    true
    gap> IsTable( [ [ 1, 2 ], [ () , (1,2) ] ] );  # not homogeneous
    false

##### 21.1-5 IsRectangularTable

`‣ IsRectangularTable`( list ) | ( property )

A list lies in `IsRectangularTable` when it is nonempty and its elements are all homogeneous lists of the same family and the same length.

##### 21.1-6 IsConstantTimeAccessList

`‣ IsConstantTimeAccessList`( list ) | ( category )

This category indicates whether the access to each element of the list list will take roughly the same time. This is implied for example by `IsList and IsInternalRep`, so all strings, Boolean lists, ranges, and internally represented plain lists are in this category.

But also other enumerators (see 21.23) can lie in this category if they guarantee constant time access to their elements.

#### 21.2 Basic Operations for Lists

The basic operations for lists are element access (see 21.3), assignment of elements to a list (see 21.4), fetching the length of a list (see `Length` (21.17-5)), the test for a hole at a given position, and unbinding an element at a given position (see 21.5).

The term basic operation means that each other list operation can be formulated in terms of the basic operations. (But note that often a more efficient method than this one is implemented.)

Any GAP object list in the category `IsList` (21.1-1) is regarded as a list, and if methods for the basic list operations are installed for list then list can be used also for the other list operations.

For internally represented lists, kernel methods are provided for the basic list operations with positive integer indices. For other lists or other indices, it is possible to install appropriate methods for these operations. This permits the implementation of lists that do not need to store all list elements (see also 21.23); for example, the elements might be described by an algorithm, such as the elements list of a group. For this reduction of space requirements, however, a price in access time may have to be paid (see `ConstantTimeAccessList` (21.17-6)).

##### `21.2-1 \[\]`

`‣ \[\]`( list, ix ) | ( operation )

`‣ IsBound\[\]`( list, ix ) | ( operation )

`‣ \[\]\:\=`( list, pos, ix ) | ( operation )

`‣ Unbind\[\]`( list, ix ) | ( operation )

These operations implement element access, test for element boundedness, list element assignment, and removal of the element with index ix.

Note that the special characters `[`, `]`, `:`, and `=` must be escaped with a backslash `\` (see 4.3); so `\[\]` denotes the operation for element access in a list, whereas `[]` denotes an empty list. (Maybe the variable names involving special characters look strange, but nevertheless they are quite suggestive.)

`\[\]( list, ix )` is equivalent to `list[ ix ]`, which clearly will usually be preferred; the former is useful mainly if one wants to access the operation itself, for example if one wants to install a method for element access in a special kind of lists.

Similarly, `IsBound\[\]` is used explicitly mainly in method installations. In other situations, one can simply call `IsBound` (21.5-1), which then delegates to `IsBound\[\]` if the first argument is a list, and to `IsBound\.` (29.7-3) if the first argument is a record.

Analogous statements hold for `\[\]\:\=` and `Unbind\[\]`.

#### 21.3 List Elements

`list[ ix ]`

The above construct evaluates to the element of the list list with index ix. For built-in list types and collections, indexing is done with origin 1, i.e., the first element of the list is the element with index 1.

    gap> l := [ 2, 3, 5, 7, 11, 13 ];;  l[1];  l[2];  l[6];
    2
    3
    13

If list is not a built-in list, or ix does not evaluate to a positive integer, method selection is invoked to try and find a way of indexing list with index ix. If this fails, or the selected method finds that `list[ix]` is unbound, an error is signalled.

`list{ poss }`

The above construct evaluates to a new list new whose first element is `list[poss[1]]`, whose second element is `list[poss[2]]`, and so on. However, it does not need to be sorted and may contain duplicate elements. If for any \(i\), `list[ poss[`\(i\)`] ]` is unbound, an error is signalled.

    gap> l := [ 2, 3, 5, 7, 11, 13, 17, 19 ];;
    gap> l{[4..6]};  l{[1,7,1,8]};
    [ 7, 11, 13 ]
    [ 2, 17, 2, 19 ]

The result is a new list, that is not identical to any other list. The elements of that list, however, are identical to the corresponding elements of the left operand (see 21.6).

It is possible to nest such sublist extractions, as can be seen in the example below.

    gap> m := [ [1,2,3], [4,5,6], [7,8,9], [10,11,12] ];;  m{[1,2,3]}{[3,2]};
    [ [ 3, 2 ], [ 6, 5 ], [ 9, 8 ] ]
    gap> l := m{[1,2,3]};; l{[3,2]};
    [ [ 7, 8, 9 ], [ 4, 5, 6 ] ]

Note the difference between the two examples. The latter extracts elements 1, 2, and 3 from m and then extracts the elements 3 and 2 from this list. The former extracts elements 1, 2, and 3 from m and then extracts the elements 3 and 2 from each of those element lists.

To be precise: With each selector `[pos]` or `{poss}` we associate a level that is defined as the number of selectors of the form `{poss}` to its left in the same expression. For example

        l[pos1]{poss2}{poss3}[pos4]{poss5}[pos6]
    level   0      0      1     2      2     3

Then a selector `list[pos]` of level level is computed as `ListElement(list,pos,level)`, where `ListElement` is defined as follows. (Note that `ListElement` is not a GAP function.)

    ListElement := function ( list, pos, level )
     if level = 0 then
      return list[pos];
     else
      return List( list, elm -> ListElement(elm,pos,level-1) );
     fi;
    end;

and a selector `list{poss}` of level level is computed as `ListElements(list,poss,level)`, where `ListElements` is defined as follows. (Note that `ListElements` is not a GAP function.)

    ListElements := function ( list, poss, level )
     if level = 0 then
      return list{poss};
      else
       return List( list, elm -> ListElements(elm,poss,level-1) );
      fi;
    end;

##### `21.3-1 \{\}`

`‣ \{\}`( list, poss ) | ( operation )

This operation implements sublist access. For any list, the default method is to loop over the entries in the list poss, and to delegate to the element access operation. (For nested sublist extractions, cf. 21.3. For the somewhat strange variable name, cf. 21.2.)

#### 21.4 List Assignment

`list[ ix ] := object;`

The list element assignment assigns the object object, which can be of any type, to the list with index ix, in the mutable (see 12.6) list list. That means that accessing the ix-th element of the list list will return object after this assignment.

    gap> l := [ 1, 2, 3 ];;
    gap> l[1] := 3;; l;             # assign a new object
    [ 3, 2, 3 ]
    gap> l[2] := [ 4, 5, 6 ];; l;   # <object> may be of any type
    [ 3, [ 4, 5, 6 ], 3 ]
    gap> l[ l[1] ] := 10;; l;       # <index> may be an expression
    [ 3, [ 4, 5, 6 ], 10 ]

If the index ix is an integer larger than the length of the list list (see `Length` (21.17-5)), the list is automatically enlarged to make room for the new element. Note that it is possible to generate lists with holes that way.

    gap> l[4] := "another entry";; l;  # <list> is enlarged
    [ 3, [ 4, 5, 6 ], 10, "another entry" ]
    gap> l[ 10 ] := 1;; l;             # now <list> has a hole
    [ 3, [ 4, 5, 6 ], 10, "another entry",,,,,, 1 ]

The function `Add` (21.4-2) should be used if you want to add an element to the end of the list.

Note that assigning to a list changes the list, thus this list must be mutable (see 12.6). See 21.6 for subtleties of changing lists.

If list does not evaluate to a list, pos does not evaluate to a positive integer, method selection is invoked to try and find a way of indexing list with index pos. If this fails, or the selected method finds that `list[pos]` is unbound, or if object is a call to a function which does not return a value (for example `Print`) an error is signalled.

`list{ poss } := objects;`

The sublist assignment assigns the object `objects[1]`, which can be of any type, to the list list at the position `poss[1]`, the object `objects[2]` to `list[poss[2]]`, and so on. poss must be a dense list of positive integers, it need, however, not be sorted and may contain duplicate elements. objects must be a dense list and must have the same length as poss.

    gap> l := [ 2, 3, 5, 7, 11, 13, 17, 19 ];;
    gap> l{[1..4]} := [10..13];; l;
    [ 10, 11, 12, 13, 11, 13, 17, 19 ]
    gap> l{[1,7,1,10]} := [ 1, 2, 3, 4 ];; l;
    [ 3, 11, 12, 13, 11, 13, 2, 19,, 4 ]

The next example shows that it is possible to nest such sublist assignments.

    gap> m := [ [1,2,3], [4,5,6], [7,8,9], [10,11,12] ];;
    gap> m{[1,2,3]}{[3,2]} := [ [11,12], [13,14], [15,16] ];; m;
    [ [ 1, 12, 11 ], [ 4, 14, 13 ], [ 7, 16, 15 ], [ 10, 11, 12 ] ]

The exact behaviour is defined in the same way as for list extractions (see 21.3). Namely, with each selector `[pos]` or `{poss}` we associate a level that is defined as the number of selectors of the form `{poss}` to its left in the same expression. For example

        l[pos1]{poss2}{poss3}[pos4]{poss5}[pos6]
    level   0      0      1     1      1     2

Then a list assignment `list[pos] := vals;` of level level is computed as `ListAssignment( list, pos, vals, level )`, where `ListAssignment` is defined as follows. (Note that `ListAssignment` is not a GAP function.)

    ListAssignment := function ( list, pos, vals, level )
     local i;
     if level = 0 then
      list[pos] := vals;
     else
      for i in [1..Length(list)] do
       ListAssignment( list[i], pos, vals[i], level-1 );
      od;
     fi;
    end;

and a list assignment `list{poss} := vals` of level level is computed as `ListAssignments( list, poss, vals, level )`, where `ListAssignments` is defined as follows. (Note that `ListAssignments` is not a GAP function.)

    ListAssignments := function ( list, poss, vals, level )
     local i;
     if level = 0 then
      list{poss} := vals;
     else
      for i in [1..Length(list)] do
       ListAssignments( list[i], poss, vals[i], level-1 );
      od;
     fi;
    end;

##### `21.4-1 \{\}\:\=`

`‣ \{\}\:\=`( list, poss, val ) | ( operation )

This operation implements sublist assignment. For any list, the default method is to loop over the entries in the list poss, and to delegate to the element assignment operation. (For the somewhat strange variable name, cf. 21.2.)

##### 21.4-2 Add

`‣ Add`( list, obj[, pos] ) | ( operation )

adds the element obj to the mutable list list. The two argument version adds obj at the end of list, i.e., it is equivalent to the assignment `list[ Length(list) + 1 ] := obj`, see 21.4.

The three argument version adds obj in position pos, moving all later elements of the list (if any) up by one position. Any holes at or after position pos are also moved up by one position, and new holes are created before pos if they are needed.

Nothing is returned by `Add`, the function is only called for its side effect.

##### 21.4-3 Remove

`‣ Remove`( list[, pos] ) | ( operation )

removes an element from list. The one argument form removes the last element. The two argument form removes the element in position pos, moving all subsequent elements down one position. Any holes after position pos are also moved down by one position.

The one argument form always returns the removed element. In this case list must be non-empty.

The two argument form returns the old value of list[pos] if it was bound, and nothing if it was not. Note that accessing or assigning the return value of this form of the `Remove` operation is only safe when you know that there will be a value, otherwise it will cause an error.

    gap> l := [ 2, 3, 5 ];; Add( l, 7 ); l;
    [ 2, 3, 5, 7 ]
    gap> Add(l,4,2); l;
    [ 2, 4, 3, 5, 7 ]
    gap> Remove(l,2); l;
    4
    [ 2, 3, 5, 7 ]
    gap> Remove(l); l;
    7
    [ 2, 3, 5 ]
    gap> Remove(l,5); l;
    [ 2, 3, 5 ]

##### 21.4-4 CopyListEntries

`‣ CopyListEntries`( fromlst, fromind, fromstep, tolst, toind, tostep, n ) | ( function )

This function copies n elements from fromlst, starting at position fromind and incrementing the position by fromstep each time, into tolst starting at position toind and incrementing the position by tostep each time. fromlst and tolst must be plain lists. fromstep and/or tostep can be negative. Unbound positions of fromlst are simply copied to tolst.

`CopyListEntries` is used in methods for the operations `Add` (21.4-2) and `Remove` (21.4-3).

##### 21.4-5 Append

`‣ Append`( list1, list2 ) | ( operation )

adds the elements of the list list2 to the end of the mutable list list1, see 21.4. list2 may contain holes, in which case the corresponding entries in list1 will be left unbound. `Append` returns nothing, it is only called for its side effect.

Note that `Append` changes its first argument, while `Concatenation` (21.20-1) creates a new list and leaves its arguments unchanged.

    gap> l := [ 2, 3, 5 ];; Append( l, [ 7, 11, 13 ] ); l;
    [ 2, 3, 5, 7, 11, 13 ]
    gap> Append( l, [ 17,, 23 ] ); l;
    [ 2, 3, 5, 7, 11, 13, 17,, 23 ]

#### 21.5 IsBound and Unbind for Lists

##### 21.5-1 IsBound

`‣ IsBound`( list[, n] ) | ( operation )

`IsBound` returns `true` if the list list has an element at index n, and `false` otherwise. list must evaluate to a list, or to an object for which a suitable method for `IsBound\[\]` has been installed, otherwise an error is signalled.

    gap> l := [ , 2, 3, , 5, , 7, , , , 11 ];;
    gap> IsBound( l[7] );
    true
    gap> IsBound( l[4] );
    false
    gap> IsBound( l[101] );
    false

##### 21.5-2 GetWithDefault

`‣ GetWithDefault`( list, n, default ) | ( operation )

`GetWithDefault` returns the nth element of the list list, if list has a value at index n, and default otherwise.

While this method can be used on any list, it is particularly useful for Weak Pointer lists 86.1 where the value of the list can change.

To distinguish between the nth element being unbound, or default being in list, users can create a new mutable object, such as a string. `IsIdenticalObj` (12.5-1) returns `false` for different mutable strings, even if their contents are the same.

    gap> l := [1,2,,"a"];
    [ 1, 2,, "a" ]
    gap> newobj := "a";
    "a"
    gap> GetWithDefault(l, 2, newobj);
    2
    gap> GetWithDefault(l, 3, newobj);
    "a"
    gap> GetWithDefault(l, 4, newobj);
    "a"
    gap> IsIdenticalObj(GetWithDefault(l, 3, newobj), newobj);
    true
    gap> IsIdenticalObj(GetWithDefault(l, 4, newobj), newobj);
    false

##### 21.5-3 Unbind

`‣ Unbind`( list[, n] ) | ( operation )

`Unbind` deletes the element with index n in the mutable list list. That is, after execution of `Unbind`, list no longer has an assigned value with index n. Thus `Unbind` can be used to produce holes in a list. Note that it is not an error to unbind a nonexistent list element. list must evaluate to a list, or to an object for which a suitable method for `Unbind\[\]` has been installed, otherwise an error is signalled.

    gap> l := [ , 2, 3, 5, , 7, , , , 11 ];;
    gap> Unbind( l[3] ); l;
    [ , 2,, 5,, 7,,,, 11 ]
    gap> Unbind( l[4] ); l;
    [ , 2,,,, 7,,,, 11 ]

Note that `IsBound` (21.5-1) and `Unbind` are special in that they do not evaluate their argument, otherwise `IsBound` (21.5-1) would always signal an error when it is supposed to return `false` and there would be no way to tell `Unbind` which component to remove.

#### 21.6 Identical Lists

With the list assignment (see 21.4) it is possible to change a mutable list. This section describes the semantic consequences of this fact. (See also 12.5.)

First we define what it means when we say that an object is changed. You may think that in the following example the second assignment changes the integer.

    i := 3;
    i := i + 1;

But in this example it is not the integer `3` which is changed, by adding one to it. Instead the variable `i` is changed by assigning the value of `i+1`, which happens to be `4`, to `i`. The same thing happens in the example below.

    l := [ 1, 2 ];
    l := [ 1, 2, 3 ];

The second assignment does not change the first list, instead it assigns a new list to the variable `l`. On the other hand, in the following example the list is changed by the second assignment.

    l := [ 1, 2 ];
    l[3] := 3;

To understand the difference, think of a variable as a name for an object. The important point is that a list can have several names at the same time. An assignment `var:= list;` means in this interpretation that var is a name for the object list. At the end of the following example `l2` still has the value `[ 1, 2 ]` as this list has not been changed and nothing else has been assigned to it.

    l1 := [ 1, 2 ];
    l2 := l1;
    l1 := [ 1, 2, 3 ];

But after the following example the list for which `l2` is a name has been changed and thus the value of `l2` is now `[ 1, 2, 3 ]`.

    l1 := [ 1, 2 ];
    l2 := l1;
    l1[3] := 3;

We say that two lists are identical if changing one of them by a list assignment also changes the other one. This is slightly incorrect, because if two lists are identical, there are actually only two names for one list. However, the correct usage would be very awkward and would only add to the confusion. Note that two identical lists must be equal, because there is only one list with two different names. Thus identity is an equivalence relation that is a refinement of equality. Identity of objects can be detected using `IsIdenticalObj` (12.5-1).

Let us now consider under which circumstances two lists are identical.

If you enter a list literal then the list denoted by this literal is a new list that is not identical to any other list. Thus in the following example `l1` and `l2` are not identical, though they are equal of course.

    l1 := [ 1, 2 ];
    l2 := [ 1, 2 ];

Also in the following example, no lists in the list `l` are identical.

    l := [];
    for i in [1..10] do l[i] := [ 1, 2 ]; od;

If you assign a list to a variable no new list is created. Thus the list value of the variable on the left hand side and the list on the right hand side of the assignment are identical. So in the following example `l1` and `l2` are identical lists.

    l1 := [ 1, 2 ];
    l2 := l1;

If you pass a list as an argument, the old list and the argument of the function are identical. Also if you return a list from a function, the old list and the value of the function call are identical. So in the following example `l1` and `l2` are identical lists:

    l1 := [ 1, 2 ];
    f := function ( l ) return l; end;
    l2 := f( l1 );

If you change a list it keeps its identity. Thus if two lists are identical and you change one of them, you also change the other, and they are still identical afterwards. On the other hand, two lists that are not identical will never become identical if you change one of them. So in the following example both `l1` and `l2` are changed, and are still identical.

    l1 := [ 1, 2 ];
    l2 := l1;
    l1[1] := 2;

#### 21.7 Duplication of Lists

Here we describe the meaning of `ShallowCopy` (12.7-1) and `StructuralCopy` (12.7-2) for lists. For the general definition of these functions, see 12.7.

The subobjects (see `ShallowCopy` (12.7-1)) of a list are exactly its elements.

This means that for any list list, `ShallowCopy` (12.7-1) returns a mutable new list new that is not identical to any other list (see 21.6), and whose elements are identical to the elements of list.

Analogously, for a mutable list list, `StructuralCopy` (12.7-2) returns a mutable new list scp that is not identical to any other list, and whose elements are structural copies (defined recursively) of the elements of list; an element of scp is mutable (and then a new list) if and only if the corresponding element of list is mutable.

In both cases, modifying the copy new resp. scp by assignments (see 21.4) does not modify the original object list.

`ShallowCopy` (12.7-1) basically executes the following code for lists.

    new := [];
    for i in [ 1 .. Length( list ) ] do
      if IsBound( list[i] ) then
        new[i] := list[i];
      fi;
    od;

    gap> list1 := [ [ 1, 2 ], [ 3, 4 ] ];;  list2 := ShallowCopy( list1 );;
    gap> IsIdenticalObj( list1, list2 );
    false
    gap> IsIdenticalObj( list1[1], list2[1] );
    true
    gap> list2[1] := 0;;  list1;  list2;
    [ [ 1, 2 ], [ 3, 4 ] ]
    [ 0, [ 3, 4 ] ]

`StructuralCopy` (12.7-2) basically executes the following code for lists.

    new := [];
    for i in [ 1 .. Length( list ) ] do
      if IsBound( list[i] ) then
        new[i] := StructuralCopy( list[i] );
      fi;
    od;

    gap> list1 := [ [ 1, 2 ], [ 3, 4 ] ];;  list2 := StructuralCopy( list1 );;
    gap> IsIdenticalObj( list1, list2 );
    false
    gap> IsIdenticalObj( list1[1], list2[1] );
    false
    gap> list2[1][1] := 0;;  list1;  list2;
    [ [ 1, 2 ], [ 3, 4 ] ]
    [ [ 0, 2 ], [ 3, 4 ] ]

The above code is not entirely correct. If the object list contains a mutable object twice this object is not copied twice, as would happen with the above definition, but only once. This means that the copy new and the object list have exactly the same structure when viewed as a general graph.

    gap> sub := [ 1, 2 ];; list1 := [ sub, sub ];;
    gap> list2 := StructuralCopy( list1 );
    [ [ 1, 2 ], [ 1, 2 ] ]
    gap> list2[1][1] := 0;; list2;
    [ [ 0, 2 ], [ 0, 2 ] ]
    gap> list1;
    [ [ 1, 2 ], [ 1, 2 ] ]

#### 21.8 Membership Test for Lists

##### `21.8-1 \in`

`‣ \in`( obj, list ) | ( operation )

This function call or the infix variant obj `in` list tests whether there is a positive integer \(i\) such that list\([i] =\) obj holds.

If the list list knows that it is strictly sorted (see `IsSSortedList` (21.17-4)), the membership test is much quicker, because a binary search can be used instead of the linear search used for arbitrary lists, see `\in` (21.19-1).

    gap> 1 in [ 2, 2, 1, 3 ];  1 in [ 4, -1, 0, 3 ];
    true
    false
    gap> s := SSortedList( [2,4,6,8,10,12,14,16,18,20,22,24,26,28,30,32] );;
    gap> 17 in s;  # uses binary search and only 4 comparisons
    false

For finding the position of an element in a list, see 21.16.

#### 21.9 Enlarging Internally Represented Lists

Section 21.4 told you (among other things) that it is possible to assign beyond the logical end of a mutable list, automatically enlarging the list. This section tells you how this is done for internally represented lists.

It would be extremely wasteful to make all lists large enough so that there is room for all assignments, because some lists may have more than 100000 elements, while most lists have less than 10 elements.

On the other hand suppose every assignment beyond the end of a list would be done by allocating new space for the list and copying all entries to the new space. Then creating a list of 1000 elements by assigning them in order, would take half a million copy operations and also create a lot of garbage that the garbage collector would have to reclaim.

So the following strategy is used. If a list is created it is created with exactly the correct size. If a list is enlarged, because of an assignment beyond the end of the list, it is enlarged by at least `length/8 + 4` entries. Therefore the next assignments beyond the end of the list do not need to enlarge the list. For example creating a list of 1000 elements by assigning them in order, would now take only 32 enlargements.

The result of this is of course that the physical length of a list may be larger than the logical length, which is usually called simply the length of the list. Aside from the implications for the performance you need not be aware of the physical length. In fact all you can ever observe, for example by calling `Length` (21.17-5), is the logical length.

Suppose that `Length` (21.17-5) would have to take the physical length and then test how many entries at the end of a list are unassigned, to compute the logical length of the list. That would take too much time. In order to make `Length` (21.17-5), and other functions that need to know the logical length, more efficient, the length of a list is stored along with the list.

For fine tuning code dealing with plain lists we provide the following two functions.

##### 21.9-1 EmptyPlist

`‣ EmptyPlist`( len ) | ( function )

Returns: a plain list

`‣ ShrinkAllocationPlist`( l ) | ( function )

Returns: nothing

The function `EmptyPlist` returns an empty plain list which has enough memory allocated for len entries. This can be useful for creating and filling a plain list with a known number of entries.

The function `ShrinkAllocationPlist` gives back to GAP's memory manager the physical memory which is allocated for the plain list l but not needed by the current number of entries.

Note that there are similar functions `EmptyString` (27.4-5) and `ShrinkAllocationString` (27.4-5) for strings instead of plain lists.

    gap> l:=[]; for i in [1..160] do Add(l, i^2); od;
    [  ]
    gap> m:=EmptyPlist(160); for i in [1..160] do Add(m, i^2); od;
    [  ]
    gap> # now l uses about 25% more memory than the equal list m
    gap> ShrinkAllocationPlist(l);
    gap> # now l and m use the same amount of memory

#### 21.10 Comparisons of Lists

`list1 = list2`

`list1 <> list2`

Two lists list1 and list2 are equal if and only if for every index \(i\), either both entries list1\([i]\) and list2\([i]\) are unbound, or both are bound and are equal, i.e., list1\([i] =\) list2\([i]\) is `true`.

    gap> [ 1, 2, 3 ] = [ 1, 2, 3 ];
    true
    gap> [ , 2, 3 ] = [ 1, 2, ];
    false
    gap> [ 1, 2, 3 ] = [ 3, 2, 1 ];
    false

This definition will cause problems with lists which are their own entries. Comparing two such lists for equality may lead to an infinite recursion in the kernel if the list comparison has to compare the list entries which are in fact the lists themselves, and then GAP crashes.

`list1 < list2`

`list1 <= list2`

Lists are ordered lexicographically. Unbound entries are smaller than any bound entry. That implies the following behaviour. Let \(i\) be the smallest positive integer \(i\) such that list1 and list2 at position \(i\) differ, i.e., either exactly one of list1\([i]\), list2\([i]\) is bound or both entries are bound and differ. Then list1 is less than list2 if either list1\([i]\) is unbound (and list2\([i]\) is not) or both are bound and list1\([i]\) < list2\([i]\) is `true`.

    gap> [ 1, 2, 3, 4 ] < [ 1, 2, 4, 8 ]; # <list1>[3] < <list2>[3]
    true
    gap> [ 1, 2, 3 ] < [ 1, 2, 3, 5 ];  # <list1>[4] is unbound and thus < 5
    true
    gap> [ 1, , 3, 4 ] < [ 1, -1, 3 ];  # <list1>[2] is unbound and thus < -1
    true

Note that for comparing two lists with `<` or `<=`, the (relevant) list elements must be comparable with `<`, which is usually not the case for objects in different families, see 13.1. Also for the possibility to compare lists with other objects, see 13.1.

#### 21.11 Arithmetic for Lists

It is convenient to have arithmetic operations for lists, in particular because in GAP row vectors and matrices are special kinds of lists. However, it is the wide variety of list objects because of which we prescribe arithmetic operations not for all of them. (Keep in mind that list means just an object in the category `IsList` (21.1-1).)

(Due to the intended generality and flexibility, the definitions given in the following sections are quite technical. But for not too complicated cases such as matrices (see 24.3) and row vectors (see 23.2) whose entries aren't lists, the resulting behaviour should be intuitive.)

For example, we want to deal with matrices which can be added and multiplied in the usual way, via the infix operators `+` and `*`; and we want also Lie matrices, with the same additive behaviour but with the multiplication defined by the Lie bracket. Both kinds of matrices shall be lists, with the usual access to their rows, with `Length` (21.17-5) returning the number of rows etc.

For the categories and attributes that control the arithmetic behaviour of lists, see 21.12.

For the definition of return values of additive and multiplicative operations whose arguments are lists in these filters, see 21.13 and 21.14, respectively. It should be emphasized that these sections describe only what the return values are, and not how they are computed.

For the mutability status of the return values, see 21.15. (Note that this is not dealt with in the sections about the result values.)

Further details about the special cases of row vectors and matrices can be found in 23.2 and in 24.3, the compression status is dealt with in 23.3 and 24.14.

#### 21.12 Filters Controlling the Arithmetic Behaviour of Lists

The arithmetic behaviour of lists is controlled by their types. The following categories and attributes are used for that.

Note that we distinguish additive and multiplicative behaviour. For example, Lie matrices have the usual additive behaviour but not the usual multiplicative behaviour.

##### 21.12-1 IsGeneralizedRowVector

`‣ IsGeneralizedRowVector`( list ) | ( category )

For a list list, the value `true` for `IsGeneralizedRowVector` indicates that the additive arithmetic behaviour of list is as defined in 21.13, and that the attribute `NestingDepthA` (21.12-4) will return a nonzero value when called with list.

    gap> IsList( "abc" ); IsGeneralizedRowVector( "abc" );
    true
    false
    gap> liemat:= LieObject( [ [ 1, 2 ], [ 3, 4 ] ] );
    LieObject( [ [ 1, 2 ], [ 3, 4 ] ] )
    gap> IsGeneralizedRowVector( liemat );
    true

##### 21.12-2 IsMultiplicativeGeneralizedRowVector

`‣ IsMultiplicativeGeneralizedRowVector`( list ) | ( category )

For a list list, the value `true` for `IsMultiplicativeGeneralizedRowVector` indicates that the multiplicative arithmetic behaviour of list is as defined in 21.14, and that the attribute `NestingDepthM` (21.12-5) will return a nonzero value when called with list.

    gap> IsMultiplicativeGeneralizedRowVector( liemat );
    false
    gap> bas:= CanonicalBasis( FullRowSpace( Rationals, 3 ) );
    CanonicalBasis( ( Rationals^3 ) )
    gap> IsMultiplicativeGeneralizedRowVector( bas );
    true

Note that the filters `IsGeneralizedRowVector` (21.12-1), `IsMultiplicativeGeneralizedRowVector` do not enable default methods for addition or multiplication (cf. `IsListDefault` (21.12-3)).

##### 21.12-3 IsListDefault

`‣ IsListDefault`( list ) | ( category )

For a list list, `IsListDefault` indicates that the default methods for arithmetic operations of lists, such as pointwise addition and multiplication as inner product or matrix product, shall be applicable to list.

`IsListDefault` implies `IsGeneralizedRowVector` (21.12-1) and `IsMultiplicativeGeneralizedRowVector` (21.12-2).

All internally represented lists are in this category, and also all lists in the representations `IsGF2VectorRep`, `Is8BitVectorRep`, `IsGF2MatrixRep`, and `Is8BitMatrixRep` (see 23.3 and 24.14). Note that the result of an arithmetic operation with lists in `IsListDefault` will in general be an internally represented list, so most wrapped list objects will not lie in `IsListDefault`.

    gap> v:= [ 1, 2 ];;  m:= [ v, 2*v ];;
    gap> IsListDefault( v );  IsListDefault( m );
    true
    true
    gap> IsListDefault( bas );  IsListDefault( liemat );
    true
    false

##### 21.12-4 NestingDepthA

`‣ NestingDepthA`( obj ) | ( attribute )

For a GAP object obj, `NestingDepthA` returns the additive nesting depth of obj. This is defined recursively as the integer \(0\) if obj is not in `IsGeneralizedRowVector` (21.12-1), as the integer \(1\) if obj is an empty list in `IsGeneralizedRowVector` (21.12-1), and as \(1\) plus the additive nesting depth of the first bound entry in obj otherwise.

##### 21.12-5 NestingDepthM

`‣ NestingDepthM`( obj ) | ( attribute )

For a GAP object obj, `NestingDepthM` returns the multiplicative nesting depth of obj. This is defined recursively as the integer \(0\) if obj is not in `IsMultiplicativeGeneralizedRowVector` (21.12-2), as the integer \(1\) if obj is an empty list in `IsMultiplicativeGeneralizedRowVector` (21.12-2), and as \(1\) plus the multiplicative nesting depth of the first bound entry in obj otherwise.

    gap> NestingDepthA( v );  NestingDepthM( v );
    1
    1
    gap> NestingDepthA( m );  NestingDepthM( m );
    2
    2
    gap> NestingDepthA( liemat );  NestingDepthM( liemat );
    2
    0
    gap> l1:= [ [ 1, 2 ], 3 ];;  l2:= [ 1, [ 2, 3 ] ];;
    gap> NestingDepthA( l1 );  NestingDepthM( l1 );
    2
    2
    gap> NestingDepthA( l2 );  NestingDepthM( l2 );
    1
    1

#### 21.13 Additive Arithmetic for Lists

In this general context, we define the results of additive operations only in the following situations. For unary operations (zero and additive inverse), the unique argument must be in `IsGeneralizedRowVector` (21.12-1); for binary operations (addition and subtraction), at least one argument must be in `IsGeneralizedRowVector` (21.12-1), and the other either is not a list or also in `IsGeneralizedRowVector` (21.12-1).

(For non-list GAP objects, defining the results of unary operations is not an issue here, and if at least one argument is a list not in `IsGeneralizedRowVector` (21.12-1), it shall be left to this argument whether the result in question is defined and what it is.)

##### 21.13-1 Zero for lists

The zero (see `Zero` (31.10-3)) of a list \(x\) in `IsGeneralizedRowVector` (21.12-1) is defined as the list whose entry at position \(i\) is the zero of \(x[i]\) if this entry is bound, and is unbound otherwise.

    gap> Zero( [ 1, 2, 3 ] );  Zero( [ [ 1, 2 ], 3 ] );  Zero( liemat );
    [ 0, 0, 0 ]
    [ [ 0, 0 ], 0 ]
    LieObject( [ [ 0, 0 ], [ 0, 0 ] ] )

##### 21.13-2 AdditiveInverse for lists

The additive inverse (see `AdditiveInverse` (31.10-9)) of a list \(x\) in `IsGeneralizedRowVector` (21.12-1) is defined as the list whose entry at position \(i\) is the additive inverse of \(x[i]\) if this entry is bound, and is unbound otherwise.

    gap> AdditiveInverse( [ 1, 2, 3 ] );  AdditiveInverse( [ [ 1, 2 ], 3 ] );
    [ -1, -2, -3 ]
    [ [ -1, -2 ], -3 ]

##### 21.13-3 Addition of lists

If \(x\) and \(y\) are in `IsGeneralizedRowVector` (21.12-1) and have the same additive nesting depth (see `NestingDepthA` (21.12-4)), the sum \(x + y\) is defined pointwise, in the sense that the result is a list whose entry at position \(i\) is \(x[i] + y[i]\) if these entries are bound, is a shallow copy (see `ShallowCopy` (12.7-1)) of \(x[i]\) or \(y[i]\) if the other argument is not bound at position \(i\), and is unbound if both \(x\) and \(y\) are unbound at position \(i\).

If \(x\) is in `IsGeneralizedRowVector` (21.12-1) and \(y\) is in `IsGeneralizedRowVector` (21.12-1) and has lower additive nesting depth, or is neither a list nor a domain, the sum \(x + y\) is defined as a list whose entry at position \(i\) is \(x[i] + y\) if \(x\) is bound at position \(i\), and is unbound if not. The equivalent holds in the reversed case, where the order of the summands is kept, as addition is not always commutative.

    gap> 1 + [ 1, 2, 3 ];  [ 1, 2, 3 ] + [ 0, 2, 4 ];  [ 1, 2 ] + [ Z(2) ];
    [ 2, 3, 4 ]
    [ 1, 4, 7 ]
    [ 0*Z(2), 2 ]
    gap> l1:= [ 1, , 3, 4 ];;             l2:= [ , 2, 3, 4, 5 ];;
    gap> l3:= [ [ 1, 2 ], , [ 5, 6 ] ];;  l4:= [ , [ 3, 4 ], [ 5, 6 ] ];;
    gap> NestingDepthA( l1 );  NestingDepthA( l2 );
    1
    1
    gap> NestingDepthA( l3 );  NestingDepthA( l4 );
    2
    2
    gap> l1 + l2;
    [ 1, 2, 6, 8, 5 ]
    gap> l1 + l3;
    [ [ 2, 2, 3, 4 ],, [ 6, 6, 3, 4 ] ]
    gap> l2 + l4;
    [ , [ 3, 6, 3, 4, 5 ], [ 5, 8, 3, 4, 5 ] ]
    gap> l3 + l4;
    [ [ 1, 2 ], [ 3, 4 ], [ 10, 12 ] ]
    gap> l1 + [];
    [ 1,, 3, 4 ]

##### 21.13-4 Subtraction of lists

For two GAP objects \(x\) and \(y\) of which one is in `IsGeneralizedRowVector` (21.12-1) and the other is also in `IsGeneralizedRowVector` (21.12-1) or is neither a list nor a domain, \(x - y\) is defined as \(x + (-y)\).

    gap> l1 - l2;
    [ 1, -2, 0, 0, -5 ]
    gap> l1 - l3;
    [ [ 0, -2, 3, 4 ],, [ -4, -6, 3, 4 ] ]
    gap> l2 - l4;
    [ , [ -3, -2, 3, 4, 5 ], [ -5, -4, 3, 4, 5 ] ]
    gap> l3 - l4;
    [ [ 1, 2 ], [ -3, -4 ], [ 0, 0 ] ]
    gap> l1 - [];
    [ 1,, 3, 4 ]

#### 21.14 Multiplicative Arithmetic for Lists

In this general context, we define the results of multiplicative operations only in the following situations. For unary operations (one and inverse), the unique argument must be in `IsMultiplicativeGeneralizedRowVector` (21.12-2); for binary operations (multiplication and division), at least one argument must be in `IsMultiplicativeGeneralizedRowVector` (21.12-2), and the other either not a list or also in `IsMultiplicativeGeneralizedRowVector` (21.12-2).

(For non-list GAP objects, defining the results of unary operations is not an issue here, and if at least one argument is a list not in `IsMultiplicativeGeneralizedRowVector` (21.12-2), it shall be left to this argument whether the result in question is defined and what it is.)

##### 21.14-1 One for lists

The one (see `One` (31.10-2)) of a dense list x in `IsMultiplicativeGeneralizedRowVector` (21.12-2) such that x has even multiplicative nesting depth and has the same length as each of its rows is defined as the usual identity matrix on the outer two levels, that is, an identity matrix of the same dimensions, with diagonal entries `One( x[1][1] )` and off-diagonal entries `Zero( x[1][1] )`.

    gap> One( [ [ 1, 2 ], [ 3, 4 ] ] );
    [ [ 1, 0 ], [ 0, 1 ] ]
    gap> One( [ [ [ [ 1 ] ], [ [ 2 ] ] ], [ [ [ 3 ] ], [ [ 4 ] ] ] ] );
    [ [ [ [ 1 ] ], [ [ 0 ] ] ], [ [ [ 0 ] ], [ [ 1 ] ] ] ]

##### 21.14-2 Inverse for lists

The inverse (see `Inverse` (31.10-8)) of an invertible square table x in `IsMultiplicativeGeneralizedRowVector` (21.12-2) whose entries lie in a common field is defined as the usual inverse \(y\), i.e., a square matrix over the same field such that \(\textit{x} y\) and \(y \textit{x}\) is equal to `One( x )`.

    gap> Inverse( [ [ 1, 2 ], [ 3, 4 ] ] );
    [ [ -2, 1 ], [ 3/2, -1/2 ] ]

##### 21.14-3 Multiplication of lists

There are three possible computations that might be triggered by a multiplication involving a list in `IsMultiplicativeGeneralizedRowVector` (21.12-2). Namely, \(x * y\) might be

(I)

  the inner product \(x[1] * y[1] + x[2] * y[2] + \cdots + x[n] * y[n]\), where summands are omitted for which the entry in \(x\) or \(y\) is unbound (if this leaves no summand then the multiplication is an error), or

(L)

  the left scalar multiple, i.e., a list whose entry at position \(i\) is \(x * y[i]\) if \(y\) is bound at position \(i\), and is unbound if not, or

(R)

  the right scalar multiple, i.e., a list whose entry at position \(i\) is \(x[i] * y\) if \(x\) is bound at position \(i\), and is unbound if not.

Our aim is to generalize the basic arithmetic of simple row vectors and matrices, so we first summarize the situations that shall be covered.

| scl | vec | mat
scl |  | (L) | (L)
vec | (R) | (I) | (I)
mat | (R) | (R) | (R)

This means for example that the product of a scalar (scl) with a vector (vec) or a matrix (mat) is computed according to (L). Note that this is asymmetric.

Now we can state the general multiplication rules.

If exactly one argument is in `IsMultiplicativeGeneralizedRowVector` (21.12-2) then we regard the other argument (which is then neither a list nor a domain) as a scalar, and specify result (L) or (R), depending on ordering.

In the remaining cases, both \(x\) and \(y\) are in `IsMultiplicativeGeneralizedRowVector` (21.12-2), and we distinguish the possibilities by their multiplicative nesting depths. An argument with odd multiplicative nesting depth is regarded as a vector, and an argument with even multiplicative nesting depth is regarded as a scalar or a matrix.

So if both arguments have odd multiplicative nesting depth, we specify result (I).

If exactly one argument has odd nesting depth, the other is treated as a scalar if it has lower multiplicative nesting depth, and as a matrix otherwise. In the former case, we specify result (L) or (R), depending on ordering; in the latter case, we specify result (L) or (I), depending on ordering.

We are left with the case that each argument has even multiplicative nesting depth. If the two depths are equal, we treat the computation as a matrix product, and specify result (R). Otherwise, we treat the less deeply nested argument as a scalar and the other as a matrix, and specify result (L) or (R), depending on ordering.

    gap> [ (), (2,3), (1,2), (1,2,3), (1,3,2), (1,3) ] * (1,4);
    [ (1,4), (1,4)(2,3), (1,2,4), (1,2,3,4), (1,3,2,4), (1,3,4) ]
    gap> [ 1, 2, , 4 ] * 2;
    [ 2, 4,, 8 ]
    gap> [ 1, 2, 3 ] * [ 1, 3, 5, 7 ];
    22
    gap> m:= [ [ 1, 2 ], 3 ];;  m * m;
    [ [ 7, 8 ], [ [ 3, 6 ], 9 ] ]
    gap> m * m = [ m[1] * m, m[2] * m ];
    true
    gap> n:= [ 1, [ 2, 3 ] ];;  n * n;
    14
    gap> n * n = n[1] * n[1] + n[2] * n[2];
    true

##### 21.14-4 Division of lists

For two GAP objects \(x\) and \(y\) of which one is in `IsMultiplicativeGeneralizedRowVector` (21.12-2) and the other is also in `IsMultiplicativeGeneralizedRowVector` (21.12-2) or is neither a list nor a domain, \(x / y\) is defined as \(x * y^{{-1}}\).

    gap> [ 1, 2, 3 ] / 2;  [ 1, 2 ] / [ [ 1, 2 ], [ 3, 4 ] ];
    [ 1/2, 1, 3/2 ]
    [ 1, 0 ]

##### 21.14-5 mod for lists

If x and y are in `IsMultiplicativeGeneralizedRowVector` (21.12-2) and have the same multiplicative nesting depth (see `NestingDepthM` (21.12-5)), `x mod y` is defined pointwise, in the sense that the result is a list whose entry at position \(i\) is `x[i] mod y[i]` if these entries are bound, is a shallow copy (see `ShallowCopy` (12.7-1)) of \(x[i]\) or \(y[i]\) if the other argument is not bound at position \(i\), and is unbound if both \(x\) and \(y\) are unbound at position \(i\).

If \(x\) is in `IsMultiplicativeGeneralizedRowVector` (21.12-2) and \(y\) is in `IsMultiplicativeGeneralizedRowVector` (21.12-2) and has lower multiplicative nesting depth or is neither a list nor a domain, `x mod y` is defined as a list whose entry at position \(i\) is `x[i] mod y` if x is bound at position \(i\), and is unbound if not. The equivalent holds in the reversed case, where the order of the arguments is kept.

    gap> 4711 mod [ 2, 3,, 5, 7 ];
    [ 1, 1,, 1, 0 ]
    gap> [ 2, 3, 4, 5, 6 ] mod 3;
    [ 2, 0, 1, 2, 0 ]
    gap> [ 10, 12, 14, 16 ] mod [ 3, 5, 7 ];
    [ 1, 2, 0, 16 ]

##### 21.14-6 Left quotients of lists

For two GAP objects \(x\) and \(y\) of which one is in `IsMultiplicativeGeneralizedRowVector` (21.12-2) and the other is also in `IsMultiplicativeGeneralizedRowVector` (21.12-2) or is neither a list nor a domain, `LeftQuotient( x, y )` is defined as \(x^{{-1}} * y\).

    gap> LeftQuotient( [ [ 1, 2 ], [ 3, 4 ] ], [ 1, 2 ] );
    [ 0, 1/2 ]

#### 21.15 Mutability Status and List Arithmetic

Many results of arithmetic operations, when applied to lists, are again lists, and it is of interest whether their entries are mutable or not (if applicable). Note that the mutability status of the result itself is already defined by the general rule for any result of an arithmetic operation, not only for lists (see 12.6).

However, we do not define exactly the mutability status for each element on each level of a nested list returned by an arithmetic operation. (Of course it would be possible to define this recursively, but since the methods used are in general not recursive, in particular for efficient multiplication of compressed matrices, such a general definition would be a burden in these cases.) Instead we consider, for a list \(x\) in `IsGeneralizedRowVector` (21.12-1), the sequence \(x = x_1, x_2, \ldots x_n\) where \(x_{{i+1}}\) is the first bound entry in \(x_i\) if exists (that is, if \(x_i\) is a nonempty list), and \(n\) is the largest \(i\) such that \(x_i\) lies in `IsGeneralizedRowVector` (21.12-1). The immutability level of \(x\) is defined as infinity if \(x\) is immutable, and otherwise the number of \(x_i\) which are immutable. (So the immutability level of a mutable empty list is \(0\).)

Thus a fully mutable matrix has immutability level \(0\), and a mutable matrix with immutable first row has immutability level \(1\) (independent of the mutability of other rows).

The immutability level of the result of any of the binary operations discussed here is the minimum of the immutability levels of the arguments, provided that objects of the required mutability status exist in GAP.

Moreover, the results have a homogeneous mutability status, that is, if the first bound entry at nesting depth \(i\) is immutable (mutable) then all entries at nesting depth \(i\) are immutable (mutable, provided that a mutable version of this entry exists in GAP).

Thus the sum of two mutable matrices whose first rows are mutable is a matrix all of whose rows are mutable, and the product of two matrices whose first rows are immutable is a matrix all of whose rows are immutable, independent of the mutability status of the other rows of the arguments.

For example, the sum of a matrix (mutable or immutable, i.e., of immutability level one of \(0\), \(1\), or \(2\)) and a mutable row vector (i.e., immutability level \(0\)) is a fully mutable matrix. The product of two mutable row vectors of integers is an integer, and since GAP does not support mutable integers, the result is immutable.

For unary arithmetic operations, there are three operations available, an attribute that returns an immutable result (`Zero` (31.10-3), `AdditiveInverse` (31.10-9), `One` (31.10-2), `Inverse` (31.10-8)), an operation that returns a result that is mutable (`ZeroOp` (31.10-3), `AdditiveInverseOp` (31.10-9), `OneOp` (31.10-2), `InverseOp` (31.10-8)), and an operation whose result has the same immutability level as the argument (`ZeroSameMutability` (31.10-3), `AdditiveInverseSameMutability` (31.10-9), `OneSameMutability` (31.10-2), `InverseSameMutability` (31.10-8)). The last kind of operations is equivalent to the corresponding infix operations `0 * list`, `- list`, `list^0`, and `list^-1`. (This holds not only for lists, see 12.6.)

    gap> IsMutable( l1 );  IsMutable( 2 * Immutable( [ 1, 2, 3 ] ) );
    true
    false
    gap> IsMutable( l2 );  IsMutable( l3 );
    true
    true

An example motivating the mutability rule is the use of syntactic constructs such as `obj * list` and `- list` as an elegant and efficient way to create mutable lists needed for further manipulations from mutable lists. In particular one can construct a mutable zero vector of length \(n\) by `0 * [ 1 ..` \(n\)`]`. The latter can be done also using `ListWithIdenticalEntries` (21.15-1).

##### 21.15-1 ListWithIdenticalEntries

`‣ ListWithIdenticalEntries`( n, obj ) | ( function )

is a list list of length n that has the object obj stored at each of the positions from `1` to n. Note that all elements of lists are identical, see 21.6.

    gap> ListWithIdenticalEntries( 10, 0 );
    [ 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 ]

#### 21.16 Finding Positions in Lists

##### 21.16-1 Position

`‣ Position`( list, obj[, from] ) | ( operation )

returns the position of the first occurrence obj in list, or `fail` if obj is not contained in list. If a starting index from is given, it returns the position of the first occurrence starting the search after position from.

Each call to the two argument version is translated into a call of the three argument version, with third argument the integer zero `0`. (Methods for the two argument version must be installed as methods for the version with three arguments, the third being described by `IsZeroCyc`.)

    gap> Position( [ 2, 2, 1, 3 ], 1 );
    3
    gap> Position( [ 2, 1, 1, 3 ], 1 );
    2
    gap> Position( [ 2, 1, 1, 3 ], 1, 2 );
    3
    gap> Position( [ 2, 1, 1, 3 ], 1, 3 );
    fail

##### 21.16-2 Positions

`‣ Positions`( list, obj ) | ( function )

returns the set of positions of all occurrences of obj in list.

Developers who wish to adapt this for custom list types need to install suitable methods for the operation `PositionsOp`.

    gap> Positions([1,2,1,2,3,2,2],2);
    [ 2, 4, 6, 7 ]
    gap> Positions([1,2,1,2,3,2,2],4);
    [  ]

##### 21.16-3 PositionCanonical

`‣ PositionCanonical`( list, obj ) | ( operation )

returns the position of the canonical associate of obj in list. The definition of this associate depends on list. For internally represented lists it is defined as the element itself (and `PositionCanonical` thus defaults to `Position` (21.16-1), but for example for certain enumerators (see 21.23) other canonical associates can be defined.

For example `RightTransversal` (39.8-1) defines the canonical associate to be the element in the transversal defining the same coset of a subgroup in a group.

    gap> g:=Group((1,2,3,4),(1,2));;u:=Subgroup(g,[(1,2)(3,4),(1,3)(2,4)]);;
    gap> rt:=RightTransversal(g,u);;AsList(rt);
    [ (), (3,4), (2,3), (2,3,4), (2,4,3), (2,4) ]
    gap> Position(rt,(1,2));
    fail
    gap> PositionCanonical(rt,(1,2));
    2

##### 21.16-4 PositionNthOccurrence

`‣ PositionNthOccurrence`( list, obj, n ) | ( operation )

returns the position of the n-th occurrence of obj in list and returns `fail` if obj does not occur n times.

    gap> PositionNthOccurrence([1,2,3,2,4,2,1],1,1);
    1
    gap> PositionNthOccurrence([1,2,3,2,4,2,1],1,2);
    7
    gap> PositionNthOccurrence([1,2,3,2,4,2,1],2,3);
    6
    gap> PositionNthOccurrence([1,2,3,2,4,2,1],2,4);
    fail

##### 21.16-5 PositionSorted

`‣ PositionSorted`( list, elm[, func] ) | ( function )

Called with two arguments, `PositionSorted` returns the position of the element elm in the sorted list list.

Called with three arguments, `PositionSorted` returns the position of the element elm in the list list, which must be sorted with respect to func. func must be a function of two arguments that returns `true` if the first argument is less than the second argument, and `false` otherwise.

`PositionSorted` returns pos such that \(\textit{list}[\textit{pos}-1] < \textit{elm} \leq \textit{list}[\textit{pos}]\) holds. That means, if elm appears once in list, its position is returned. If elm appears several times in list, the position of the first occurrence is returned. If elm is not an element of list, the index where elm must be inserted to keep the list sorted is returned.

`PositionSorted` uses binary search, whereas `Position` (21.16-1) can in general use only linear search, see the remark at the beginning of 21.19. For sorting lists, see 21.18, for testing whether a list is sorted, see `IsSortedList` (21.17-3) and `IsSSortedList` (21.17-4).

Developers who wish to adapt this for custom list types need to install suitable methods for the operation `PositionSortedOp`.

    gap> PositionSorted( [1,4,5,5,6,7], 0 );
    1
    gap> PositionSorted( [1,4,5,5,6,7], 2 );
    2
    gap> PositionSorted( [1,4,5,5,6,7], 4 );
    2
    gap> PositionSorted( [1,4,5,5,6,7], 5 );
    3
    gap> PositionSorted( [1,4,5,5,6,7], 8 );
    7

##### 21.16-6 PositionSortedBy

`‣ PositionSortedBy`( list, val, func ) | ( function )

This function returns the same value that would be returned by `PositionSorted(List(list, func), val)`, but computes it in a more efficient way.

To be more precise, func must be a function on one argument which returns values that can be compared to val, and list must be a list for which `func(list[i]) <= func(list[i+1])` holds for all relevant i. This property is not verified, and if the input violates it, then the result is undefined.

`PositionSortedBy` returns pos such that \(\textit{func}(\textit{list}[\textit{pos}-1]) < \textit{val} \leq \textit{func}(\textit{list}[\textit{pos}])\) holds. That means, if there are elements `elm` in list for which \(\textit{func}(elm) = \textit{val}\) holds, then the position of the first such element is returned. If no element of list satisfies this condition, then the lowest index where an element elm satisfying \(\textit{func}(elm) = \textit{val}\) must be inserted to preserve the property `func(list[i]) <= func(list[i+1])` is returned.

`PositionSortedBy` uses binary search. Each `func(list[i])` is computed at most once.

Developers who wish to adapt this for custom list types need to install suitable methods for the operation `PositionSortedByOp`.

    gap> PositionSortedBy( [ "", "ab", ], -1, Length );
    1
    gap> PositionSortedBy( [ "", "ab", ], 0, Length );
    1
    gap> PositionSortedBy( [ "", "ab", ], 1, Length );
    2
    gap> PositionSortedBy( [ "", "ab", ], 2, Length );
    2
    gap> PositionSortedBy( [ "", "ab", ], 3, Length );
    3

##### 21.16-7 PositionSet

`‣ PositionSet`( list, obj[, func] ) | ( function )

`PositionSet` is a slight variation of `PositionSorted` (21.16-5). The only difference to `PositionSorted` (21.16-5) is that `PositionSet` returns `fail` if obj is not in list.

    gap> PositionSet( [1,4,5,5,6,7], 0 );
    fail
    gap> PositionSet( [1,4,5,5,6,7], 2 );
    fail
    gap> PositionSet( [1,4,5,5,6,7], 4 );
    2
    gap> PositionSet( [1,4,5,5,6,7], 5 );
    3
    gap> PositionSet( [1,4,5,5,6,7], 8 );
    fail

##### 21.16-8 PositionMaximum

`‣ PositionMaximum`( list[, func] ) | ( function )

`‣ PositionMinimum`( list[, func] ) | ( function )

returns the position of maximum (with `PositionMaximum`) or minimum (with `PositionMinimum`) entry in the list list. If a second argument func is passed, then return instead the position of the largest/smallest entry in `List( list , func )`. If several entries of the list are equal to the maximum/minimum, the first such position is returned.

    gap> PositionMaximum( [2,4,-6,2,4] );
    2
    gap> PositionMaximum( [2,4,-6,2,4], x -> -x);
    3
    gap> PositionMinimum( [2,4,-6,2,4] );
    3
    gap> PositionMinimum( [2,4,-6,2,4], x -> -x);
    2

`Maximum` (21.20-12) and `Minimum` (21.20-13) allow you to find the maximum or minimum element of a list directly.

##### 21.16-9 PositionProperty

`‣ PositionProperty`( list, func[, from] ) | ( operation )

returns the position of the first entry in the list list for which the property tester function func returns `true`, or `fail` if no such entry exists. If a starting index from is given, it returns the position of the first entry satisfying func, starting the search after position from.

    gap> PositionProperty( [10^7..10^8], IsPrime );
    20
    gap> PositionProperty( [10^5..10^6],
    >        n -> not IsPrime(n) and IsPrimePowerInt(n) );
    490

`First` (21.20-21) allows you to extract the first element of a list that satisfies a certain property.

##### 21.16-10 PositionsProperty

`‣ PositionsProperty`( list, func ) | ( operation )

returns the set of all those positions in the list list which are bound and for which the property tester function func returns `true`.

    gap> l:= [ -5 .. 5 ];;
    gap> PositionsProperty( l, IsPosInt );
    [ 7, 8, 9, 10, 11 ]
    gap> PositionsProperty( l, IsPrimeInt );
    [ 1, 3, 4, 8, 9, 11 ]

`PositionProperty` (21.16-9) allows you to extract the position of the first element in a list that satisfies a certain property.

##### 21.16-11 PositionBound

`‣ PositionBound`( list ) | ( operation )

returns the first bound position of the list list. For the empty list it returns `fail`.

    gap> PositionBound([1,2,3]);
    1
    gap> PositionBound([,1,2,3]);
    2

##### 21.16-12 PositionsBound

`‣ PositionsBound`( list ) | ( function )

returns the set of all bound positions in the list list.

    gap> PositionsBound([1,2,3]);
    [ 1 .. 3 ]
    gap> PositionsBound([,1,,3]);
    [ 2, 4 ]
    gap> PositionsBound([]);
    []

##### 21.16-13 PositionNot

`‣ PositionNot`( list, val[, from] ) | ( operation )

For a list list and an object val, `PositionNot` returns the smallest nonnegative integer \(n\) such that \(\textit{list}[n]\) is either unbound or not equal to val. If a starting index from is given, it returns the first position with this property starting the search after position from.

    gap> l:= [ 1, 1, 2, 3, 2 ];;  PositionNot( l, 1 );
    3
    gap> PositionNot( l, 1, 4 );  PositionNot( l, 2, 4 );
    5
    6

##### 21.16-14 PositionNonZero

`‣ PositionNonZero`( vec[, from] ) | ( operation )

For a row vector vec, `PositionNonZero` returns the position of the first non-zero element of vec, or `Length(` vec `)+1` if all entries of vec are zero.

If a starting index from is given, it returns the position of the first occurrence starting the search after position from.

`PositionNonZero` implements a special case of `PositionNot` (21.16-13). Namely, the element to be avoided is the zero element, and the list must be (at least) homogeneous because otherwise the zero element cannot be specified implicitly.

    gap> PositionNonZero( [ 1, 1, 2, 3, 2 ] );
    1
    gap> PositionNonZero( [ 2, 3, 4, 5 ] * Z(2) );
    2

##### 21.16-15 PositionSublist

`‣ PositionSublist`( list, sub[, from] ) | ( operation )

returns the smallest index in the list list at which a sublist equal to sub starts. If sub does not occur the operation returns `fail`. The version with given from starts searching after position from.

To determine whether sub matches list at a particular position, use `IsMatchingSublist` (21.17-1) instead.

#### 21.17 Properties and Attributes for Lists

A list that contains mutable objects (like lists or records) cannot store attribute values that depend on the values of its entries, such as whether it is homogeneous, sorted, or strictly sorted, as changes in any of its entries could change such property values, like the following example shows.

    gap> l:=[[1],[2]];
    [ [ 1 ], [ 2 ] ]
    gap> IsSSortedList(l);
    true
    gap> l[1][1]:=3;
    3
    gap> IsSSortedList(l);
    false

For such lists these property values must be computed anew each time the property is asked for. For example, if list is a list of mutable row vectors then the call of `Position` (21.16-1) with list as first argument cannot take advantage of the fact that list is in fact sorted. One solution is to call explicitly `PositionSorted` (21.16-5) in such a situation, another solution is to replace list by an immutable copy using `Immutable` (12.6-3).

##### 21.17-1 IsMatchingSublist

`‣ IsMatchingSublist`( list, sub[, at] ) | ( operation )

returns `true` if sub matches a sublist of list from position `1` (or position at, in the case of three arguments), or `false`, otherwise. If sub is empty `true` is returned. If list is empty but sub is non-empty `false` is returned.

If you actually want to know whether there is an at for which `IsMatchingSublist( list, sub, at )` is true, use a construction like `PositionSublist( list, sub ) <> fail` instead (see `PositionSublist` (21.16-15)); it's more efficient.

##### 21.17-2 IsDuplicateFree

`‣ IsDuplicateFree`( obj ) | ( property )

`‣ IsDuplicateFreeList`( obj ) | ( filter )

`IsDuplicateFree` returns `true` if obj is both a list or collection, and it is duplicate free; otherwise it returns `false`. `IsDuplicateFreeList` is a synonym for `IsDuplicateFree and IsList`.

A list is duplicate free if it is dense and does not contain equal entries in different positions. Every domain (see 12.4) is duplicate free.

Note that GAP cannot compare arbitrary objects (by equality). This can cause that `IsDuplicateFree` runs into an error, if obj is a list with some non-comparable entries.

##### 21.17-3 IsSortedList

`‣ IsSortedList`( obj ) | ( property )

returns `true` if obj is a list and it is sorted, and `false` otherwise.

A list list is sorted if it is dense (see `IsDenseList` (21.1-2)) and satisfies the relation \(\textit{list}[i] \leq \textit{list}[j]\) whenever \(i < j\). Note that a sorted list is not necessarily duplicate free (see `IsDuplicateFree` (21.17-2) and `IsSSortedList` (21.17-4)).

Many sorted lists are in fact homogeneous (see `IsHomogeneousList` (21.1-3)), but also non-homogeneous lists may be sorted (see 31.11).

In sorted lists, membership test and computing of positions can be done by binary search, see 21.19.

Note that GAP cannot compare (by less than) arbitrary objects. This can cause that `IsSortedList` runs into an error, if obj is a list with some non-comparable entries.

##### 21.17-4 IsSSortedList

`‣ IsSSortedList`( obj ) | ( property )

`‣ IsSet`( obj ) | ( property )

returns `true` if obj is a list and it is strictly sorted, and `false` otherwise. `IsSSortedList` is short for is strictly sorted list; `IsSet` is just a synonym for `IsSSortedList`.

A list list is strictly sorted if it is sorted (see `IsSortedList` (21.17-3)) and satisfies the relation \(\textit{list}[i] < \textit{list}[j]\) whenever \(i < j\). In particular, such lists are duplicate free (see `IsDuplicateFree` (21.17-2)).

(Currently there is little special treatment of lists that are sorted but not strictly sorted. In particular, internally represented lists will not store that they are sorted but not strictly sorted.)

Note that GAP cannot compare (by less than) arbitrary objects. This can cause that `IsSSortedList` runs into an error, if obj is a list with some non-comparable entries.

##### 21.17-5 Length

`‣ Length`( list ) | ( attribute )

returns the length of the list list, which is defined to be the index of the last bound entry in list.

##### 21.17-6 ConstantTimeAccessList

`‣ ConstantTimeAccessList`( list ) | ( attribute )

`ConstantTimeAccessList` returns an immutable list containing the same elements as the list list (which may have holes) in the same order. If list is already a constant time access list, `ConstantTimeAccessList` returns an immutable copy of list directly. Otherwise it puts all elements and holes of list into a new list and makes that list immutable.

#### 21.18 Sorting Lists

GAP implements three different families of sorting algorithms. The default algorithm is pattern-defeating quicksort, a variant of quicksort which performs better on partially sorted lists and has good worst-case behaviour. The functions which begin `Stable` are stable (equal elements keep the same relative order in the sorted list) and use merge sort. Finally, the functions which begin `Shell` use the shell sort which was GAP's default search algorithm before 4.9. `Sortex` (21.18-3) and `SortingPerm` (21.18-4) are also stable.

##### 21.18-1 Sort

`‣ Sort`( list[, func] ) | ( operation )

`‣ SortBy`( list, func ) | ( operation )

`‣ StableSort`( list[, func] ) | ( operation )

`‣ StableSortBy`( list[, func] ) | ( operation )

`Sort` sorts the list list in increasing order. In the one argument form `Sort` uses the operator `<` to compare the elements. (If the list is not homogeneous it is the user's responsibility to ensure that `<` is defined for all element pairs, see 31.11) In the two argument form `Sort` uses the function func to compare elements. func must be a function taking two arguments that returns `true` if the first is regarded as strictly smaller than the second, and `false` otherwise.

`StableSort` behaves identically to `Sort`, except that `StableSort` will keep elements which compare equal in the same relative order, while `Sort` may change their relative order.

`Sort` does not return anything, it just changes the argument list. Use `ShallowCopy` (12.7-1) if you want to keep list. Use `Reversed` (21.20-7) if you want to get a new list that is sorted in decreasing order.

`SortBy` sorts the list list into an order such that `func(list[i]) <= func(list[i+1])` for all relevant i. func must thus be a function on one argument which returns values that can be compared. Each `func(list[i])` is computed just once and stored, making this more efficient than using the two-argument version of `Sort` in many cases.

`StableSortBy` behaves the same as `SortBy` except that, like `StableSort`, it keeps pairs of values which compare equal when `func` is applied to them in the same relative order.

    gap> list := [ 5, 4, 6, 1, 7, 5 ];; Sort( list ); list;
    [ 1, 4, 5, 5, 6, 7 ]
    gap> SortBy(list, x -> x mod 3);
    gap> list; # Sorted by mod 3
    [ 6, 1, 4, 7, 5, 5]
    gap> list := [ [0,6], [1,2], [1,3], [1,5], [0,4], [3,4] ];;
    gap> Sort( list, function(v,w) return v*v < w*w; end );
    gap> list;  # sorted according to the Euclidean distance from [0,0]
    [ [ 1, 2 ], [ 1, 3 ], [ 0, 4 ], [ 3, 4 ], [ 1, 5 ], [ 0, 6 ] ]
    gap> SortBy( list, function(v) return v[1] + v[2]; end );
    gap> list;  # sorted according to Manhattan distance from [0,0]
    [ [ 1, 2 ], [ 1, 3 ], [ 0, 4 ], [ 1, 5 ], [ 0, 6 ], [ 3, 4 ] ]
    gap> list := [ [0,6], [1,3], [3,4], [1,5], [1,2], [0,4], ];;
    gap> Sort( list, function(v,w) return v[1] < w[1]; end );
    gap> # note the random order of the elements with equal first component:
    gap> list;
    [ [ 0, 6 ], [ 0, 4 ], [ 1, 3 ], [ 1, 5 ], [ 1, 2 ], [ 3, 4 ] ]

##### 21.18-2 SortParallel

`‣ SortParallel`( list1, list2[, func] ) | ( operation )

`‣ StableSortParallel`( list1, list2[, func] ) | ( operation )

`SortParallel` sorts the list list1 in increasing order just as `Sort` (21.18-1) does. In parallel it applies the same exchanges that are necessary to sort list1 to the list list2, which must of course have at least as many elements as list1 does.

`StableSortParallel` behaves identically to `SortParallel`, except it keeps elements in list1 which compare equal in the same relative order.

    gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
    gap> list2 := [ 2, 3, 5, 7, 8, 9 ];;
    gap> SortParallel( list1, list2 );
    gap> list1;
    [ 1, 4, 5, 5, 6, 7 ]
    gap> list2;
    [ 7, 3, 2, 9, 5, 8 ]

Note that `[ 7, 3, 2, 9, 5, 8 ]` or `[ 7, 3, 9, 2, 5, 8 ]` are possible results. `StableSortParallel` will always return `[ 7, 3, 2, 9, 5, 8]`.

##### 21.18-3 Sortex

`‣ Sortex`( list[, func] ) | ( operation )

sorts the list list and returns a permutation that can be applied to list to obtain the sorted list. The one argument form sorts via the operator `<`, the two argument form sorts w.r.t. the function func. The permutation returned by `Sortex` will keep elements which compare equal in the same relative order. (If the list is not homogeneous it is the user's responsibility to ensure that `<` is defined for all element pairs, see 31.11)

`Permuted` (21.20-17) allows you to rearrange a list according to a given permutation.

    gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
    gap> list2 := ShallowCopy( list1 );;
    gap> perm := Sortex( list1 );
    (1,3,5,6,4)
    gap> list1;
    [ 1, 4, 5, 5, 6, 7 ]
    gap> Permuted( list2, perm );
    [ 1, 4, 5, 5, 6, 7 ]

##### 21.18-4 SortingPerm

`‣ SortingPerm`( list ) | ( attribute )

`SortingPerm` returns the same as `Sortex` (21.18-3) but does not change the argument.

    gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
    gap> list2 := ShallowCopy( list1 );;
    gap> perm := SortingPerm( list1 );
    (1,3,5,6,4)
    gap> list1;
    [ 5, 4, 6, 1, 7, 5 ]
    gap> Permuted( list2, perm );
    [ 1, 4, 5, 5, 6, 7 ]

#### 21.19 Sorted Lists and Sets

Searching objects in a list works much quicker if the list is known to be sorted. Currently GAP exploits the sortedness of a list automatically only if the list is strictly sorted, which is indicated by the property `IsSSortedList` (21.17-4).

Remember that a list of mutable objects cannot store that it is strictly sorted but has to test it anew whenever it is asked whether it is sorted, see the remark in 21.17. Therefore GAP cannot take advantage of the sortedness of a list if this list has mutable entries. Moreover, if a sorted list list with mutable elements is used as an argument of a function that expects this argument to be sorted, for example `UniteSet` (21.19-6) or `RemoveSet` (21.19-5), then it is checked whether list is in fact sorted; this check can have the effect actually to slow down the computations, compared to computations with sorted lists of immutable elements or computations that do not involve functions that do automatically check sortedness.

Strictly sorted lists are used to represent sets in GAP. More precisely, a strictly sorted list is called a proper set in the following, in order to avoid confusion with domains (see 12.4) which also represent sets.

In short proper sets are represented by sorted lists without holes and duplicates in GAP. Note that we guarantee this representation, so you may make use of the fact that a set is represented by a sorted list in your functions.

In some contexts (for example see 16), we also want to talk about multisets. A multiset is like a set, except that an element may appear several times in a multiset. Such multisets are represented by sorted lists without holes that may have duplicates.

This section lists only those functions that are defined exclusively for proper sets. Set theoretic functions for general collections, such as `Intersection` (30.5-2) and `Union` (30.5-3), are described in Chapter 30. In particular, for the construction of proper sets, see `SSortedList` (30.3-7) and `AsSSortedList` (30.3-10). For finding positions in sorted lists, see `PositionSorted` (21.16-5).

There are nondestructive counterparts of the functions `UniteSet` (21.19-6), `IntersectSet` (21.19-7), and `SubtractSet` (21.19-8) available for proper sets. These are `UnionSet`, `IntersectionSet`, and `Difference` (30.5-4). The former two are methods for the more general operations `Union` (30.5-3) and `Intersection` (30.5-2), the latter is itself an operation (see `Difference` (30.5-4)).

The result of `IntersectionSet` and `UnionSet` is always a new list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the first argument set. If set is not a proper set it is not specified to which of a number of equal elements in set the element in the result is identical (see 21.6). The following functions, if not explicitly stated differently, take two arguments, set and obj, where set must be a proper set, otherwise an error is signalled; If the second argument obj is a list that is not a proper set then `Set` (30.3-7) is silently applied to it first.

##### `21.19-1 \in`

`‣ \in`( obj, list ) | ( method )

For a list list that stores that it is strictly sorted, the test with `\in` whether the object obj is an entry of list uses binary search. This test can be entered also with the infix notation obj `in` list.

##### 21.19-2 IsEqualSet

`‣ IsEqualSet`( list1, list2 ) | ( operation )

tests whether list1 and list2 are equal when viewed as sets, that is if every element of list1 is an element of list2 and vice versa. Either argument of `IsEqualSet` may also be a list that is not a proper set, in which case `Set` (30.3-7) is applied to it first.

If both lists are proper sets then they are of course equal if and only if they are also equal as lists. Thus `IsEqualSet( list1, list2 )` is equivalent to `Set( list1 ) = Set( list2 )` (see `Set` (30.3-7)), but the former is more efficient.

    gap> IsEqualSet( [2,3,5,7,11], [11,7,5,3,2] );
    true
    gap> IsEqualSet( [2,3,5,7,11], [2,3,5,7,11,13] );
    false

##### 21.19-3 IsSubsetSet

`‣ IsSubsetSet`( list1, list2 ) | ( operation )

tests whether every element of list2 is contained in list1. Either argument of `IsSubsetSet` may also be a list that is not a proper set, in which case `Set` (30.3-7) is applied to it first.

##### 21.19-4 AddSet

`‣ AddSet`( set, obj ) | ( operation )

adds the element obj to the proper set set. If obj is already contained in set then set is not changed. Otherwise obj is inserted at the correct position such that set is again a proper set afterwards.

Note that obj must be in the same family as each element of set.

    gap> s := [2,3,7,11];;
    gap> AddSet( s, 5 );  s;
    [ 2, 3, 5, 7, 11 ]
    gap> AddSet( s, 13 );  s;
    [ 2, 3, 5, 7, 11, 13 ]
    gap> AddSet( s, 3 );  s;
    [ 2, 3, 5, 7, 11, 13 ]

##### 21.19-5 RemoveSet

`‣ RemoveSet`( set, obj ) | ( operation )

removes the element obj from the proper set set. If obj is not contained in set then set is not changed. If obj is an element of set it is removed and all the following elements in the list are moved one position forward.

    gap> s := [ 2, 3, 4, 5, 6, 7 ];;
    gap> RemoveSet( s, 6 ); s;
    [ 2, 3, 4, 5, 7 ]
    gap> RemoveSet( s, 10 ); s;
    [ 2, 3, 4, 5, 7 ]

##### 21.19-6 UniteSet

`‣ UniteSet`( set, list ) | ( operation )

unites the proper set set with list. This is equivalent to adding all elements of list to set (see `AddSet` (21.19-4)).

    gap> set := [ 2, 3, 5, 7, 11 ];;
    gap> UniteSet( set, [ 4, 8, 9 ] );  set;
    [ 2, 3, 4, 5, 7, 8, 9, 11 ]
    gap> UniteSet( set, [ 16, 9, 25, 13, 16 ] );  set;
    [ 2, 3, 4, 5, 7, 8, 9, 11, 13, 16, 25 ]

##### 21.19-7 IntersectSet

`‣ IntersectSet`( set, list ) | ( operation )

intersects the proper set set with list. This is equivalent to removing from set all elements of set that are not contained in list.

    gap> set := [ 2, 3, 4, 5, 7, 8, 9, 11, 13, 16 ];;
    gap> IntersectSet( set, [ 3, 5, 7, 9, 11, 13, 15, 17 ] );  set;
    [ 3, 5, 7, 9, 11, 13 ]
    gap> IntersectSet( set, [ 9, 4, 6, 8 ] );  set;
    [ 9 ]

##### 21.19-8 SubtractSet

`‣ SubtractSet`( set, list ) | ( operation )

subtracts list from the proper set set. This is equivalent to removing from set all elements of list.

    gap> set := [ 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 ];;
    gap> SubtractSet( set, [ 6, 10 ] );  set;
    [ 2, 3, 4, 5, 7, 8, 9, 11 ]
    gap> SubtractSet( set, [ 9, 4, 6, 8 ] );  set;
    [ 2, 3, 5, 7, 11 ]

#### 21.20 Operations for Lists

Several of the following functions expect the first argument to be either a list or a collection (see 30), with possibly slightly different meaning for lists and non-list collections.

##### 21.20-1 Concatenation

`‣ Concatenation`( list1, list2, ... ) | ( function )

`‣ Concatenation`( list ) | ( function )

In the first form `Concatenation` returns the concatenation of the lists list1, list2, etc. The concatenation is the list that begins with the elements of list1, followed by the elements of list2, and so on. Each list may also contain holes, in which case the concatenation also contains holes at the corresponding positions.

In the second form list must be a dense list of lists list1, list2, etc., and `Concatenation` returns the concatenation of those lists.

The result is a new mutable list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of list1, list2, etc. (see 21.6).

Note that `Concatenation` creates a new list and leaves its arguments unchanged, while `Append` (21.4-5) changes its first argument. For computing the union of proper sets, `Union` (30.5-3) can be used, see also 21.19.

    gap> Concatenation( [ 1, 2, 3 ], [ 4, 5 ] );
    [ 1, 2, 3, 4, 5 ]
    gap> Concatenation( [2,3,,5,,7], [11,,13,,,,17,,19] );
    [ 2, 3,, 5,, 7, 11,, 13,,,, 17,, 19 ]
    gap> Concatenation( [ [1,2,3], [2,3,4], [3,4,5] ] );
    [ 1, 2, 3, 2, 3, 4, 3, 4, 5 ]

##### 21.20-2 Compacted

`‣ Compacted`( list ) | ( operation )

returns a new mutable list that contains the elements of list in the same order but omitting the holes.

    gap> l:=[,1,,,3,,,4,[5,,,6],7];;  Compacted( l );
    [ 1, 3, 4, [ 5,,, 6 ], 7 ]

##### 21.20-3 Collected

`‣ Collected`( list ) | ( operation )

returns a new list new that contains for each element elm of the list list a list of length two, the first element of this is elm itself and the second element is the number of times elm appears in list. The order of those pairs in new corresponds to the ordering of the elements elm, so that the result is sorted.

For all pairs of elements in list the comparison via `<` must be defined.

    gap> Factors( Factorial( 10 ) );
    [ 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 5, 5, 7 ]
    gap> Collected( last );
    [ [ 2, 8 ], [ 3, 4 ], [ 5, 2 ], [ 7, 1 ] ]
    gap> Collected( last );
    [ [ [ 2, 8 ], 1 ], [ [ 3, 4 ], 1 ], [ [ 5, 2 ], 1 ], [ [ 7, 1 ], 1 ] ]

##### 21.20-4 DuplicateFreeList

`‣ DuplicateFreeList`( list ) | ( operation )

`‣ Unique`( list ) | ( operation )

returns a new mutable list whose entries are the elements of the list list with duplicates removed. `DuplicateFreeList` only uses the `=` comparison and will not sort the result. Therefore `DuplicateFreeList` can be used even if the elements of list do not lie in the same family. Otherwise, if list contains objects that can be compared with `\<` (31.11-1) then it is much more efficient to use `Set` (30.3-7) instead of `DuplicateFreeList`.

`Unique` is a synonym for `DuplicateFreeList`.

    gap> l:=[1,Z(3),1,"abc",Group((1,2,3),(1,2)),Z(3),Group((1,2),(2,3))];;
    gap> DuplicateFreeList( l );
    [ 1, Z(3), "abc", Group([ (1,2,3), (1,2) ]) ]

##### 21.20-5 AsDuplicateFreeList

`‣ AsDuplicateFreeList`( list ) | ( attribute )

returns the same result as `DuplicateFreeList` (21.20-4), except that the result is immutable.

##### 21.20-6 Flat

`‣ Flat`( list ) | ( operation )

returns the list of all elements that are contained in the list list or its sublists. That is, `Flat` first makes a new empty list new. Then it loops over the elements elm of list. If elm is not a list it is added to new, otherwise `Flat` appends `Flat( elm )` to new.

    gap> Flat( [ 1, [ 2, 3 ], [ [ 1, 2 ], 3 ] ] );
    [ 1, 2, 3, 1, 2, 3 ]
    gap> Flat( [ ] );
    [  ]

To reconstruct a matrix from the list obtained by applying `Flat` to the matrix, the sublist operator can be used, as follows.

    gap> l:=[9..14];;w:=2;; # w is the length of each row
    gap> sub:=[1..w];;List([1..Length(l)/w],i->l{(i-1)*w+sub});
    [ [ 9, 10 ], [ 11, 12 ], [ 13, 14 ] ]

##### 21.20-7 Reversed

`‣ Reversed`( list ) | ( function )

returns a new mutable list, containing the elements of the dense list list in reversed order.

The argument list is unchanged. The result list is a new list, that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the argument list (see 21.6).

`Reversed` implements a special case of list assignment, which can also be formulated in terms of the `{}` operator (see 21.4).

Developers who wish to adapt this for custom list types need to install suitable methods for the operation `ReversedOp`.

    gap> Reversed( [ 1, 4, 9, 5, 6, 7 ] );
    [ 7, 6, 5, 9, 4, 1 ]

##### 21.20-8 Shuffle

`‣ Shuffle`( list ) | ( operation )

The argument list must be a dense mutable list. This operation permutes the entries of list randomly (in place), and returns list.

    gap> Reset(GlobalMersenneTwister, 12345);; # make manual tester happy
    gap> l := [1..20];
    [ 1 .. 20 ]
    gap> m := Shuffle(ShallowCopy(l));
    [ 8, 13, 1, 3, 20, 15, 4, 7, 5, 18, 6, 12, 16, 11, 2, 10, 19, 17, 9,
      14 ]
    gap> l;
    [ 1 .. 20 ]
    gap> Shuffle(l);;
    gap> l;
    [ 19, 5, 7, 20, 16, 1, 10, 15, 12, 11, 13, 2, 14, 3, 4, 17, 6, 8, 9,
      18 ]

##### 21.20-9 Apply

`‣ Apply`( list, func ) | ( function )

`Apply` applies the function func to every element of the dense and mutable list list, and replaces each element entry by the corresponding return value.

`Apply` changes its argument. The nondestructive counterpart of `Apply` is `List` (30.3-5).

    gap> l:= [ 1, 2, 3 ];;  Apply( l, i -> i^2 );  l;
    [ 1, 4, 9 ]

##### 21.20-10 Perform

`‣ Perform`( list, func ) | ( function )

`Perform` applies the function func to every element of the list list, discarding any return values. It does not return a value.

    gap> l := [1, 2, 3];; Perform(l,
    > function(x) if IsPrimeInt(x) then Print(x,"\n"); fi; end);
    2
    3

##### 21.20-11 PermListList

`‣ PermListList`( list1, list2 ) | ( function )

returns a permutation \(p\) of `[ 1 .. Length( list1 ) ]` such that list1\([i\)`^`\(p] =\) list2\([i]\). It returns `fail` if there is no such permutation.

    gap> list1 := [ 5, 4, 6, 1, 7, 5 ];;
    gap> list2 := [ 4, 1, 7, 5, 5, 6 ];;
    gap> perm := PermListList(list1, list2);
    (1,2,4)(3,5,6)
    gap> Permuted( list2, perm );
    [ 5, 4, 6, 1, 7, 5 ]

##### 21.20-12 Maximum

`‣ Maximum`( obj1, obj2, ... ) | ( function )

`‣ Maximum`( list ) | ( function )

In the first form `Maximum` returns the maximum of its arguments, i.e., one argument obj for which \(\textit{obj} \geq \textit{obj1}\), \(\textit{obj} \geq \textit{obj2}\) etc.

In the second form `Maximum` takes a homogeneous list list and returns the maximum of the elements in this list.

    gap> Maximum( -123, 700, 123, 0, -1000 );
    700
    gap> Maximum( [ -123, 700, 123, 0, -1000 ] );
    700
    gap> # lists are compared elementwise:
    gap> Maximum( [1,2], [0,15], [1,5], [2,-11] );
    [ 2, -11 ]

To get the index of the maximum element use `PositionMaximum` (21.16-8)

##### 21.20-13 Minimum

`‣ Minimum`( obj1, obj2, ... ) | ( function )

`‣ Minimum`( list ) | ( function )

In the first form `Minimum` returns the minimum of its arguments, i.e., one argument obj for which \(\textit{obj} \leq \textit{obj1}\), \(\textit{obj} \leq \textit{obj2}\) etc.

In the second form `Minimum` takes a homogeneous list list and returns the minimum of the elements in this list.

Note that for both `Maximum` (21.20-12) and `Minimum` the comparison of the objects obj1, obj2 etc. must be defined; for that, usually they must lie in the same family (see 13.1).

    gap> Minimum( -123, 700, 123, 0, -1000 );
    -1000
    gap> Minimum( [ -123, 700, 123, 0, -1000 ] );
    -1000
    gap> Minimum( [ 1, 2 ], [ 0, 15 ], [ 1, 5 ], [ 2, -11 ] );
    [ 0, 15 ]

To get the index of the minimum element use `PositionMinimum` (21.16-8)

##### 21.20-14 MaximumList and MinimumList

`‣ MaximumList`( list[, seed] ) | ( operation )

`‣ MinimumList`( list[, seed] ) | ( operation )

return the maximum resp. the minimum of the elements in the list list. They are the operations called by `Maximum` (21.20-12) resp. `Minimum` (21.20-13). Methods can be installed for special kinds of lists. For example, there are special methods to compute the maximum resp. the minimum of a range (see 21.22).

If a second argument seed is supplied, then the result is the maximum resp. minimum of the union of list and seed. In this manner, the operations may be applied to empty lists.

##### 21.20-15 Cartesian

`‣ Cartesian`( list1, list2, ... ) | ( function )

`‣ Cartesian`( list ) | ( function )

In the first form `Cartesian` returns the cartesian product of the lists list1, list2, etc.

In the second form list must be a list of lists list1, list2, etc., and `Cartesian` returns the cartesian product of those lists.

The cartesian product is a list cart of lists tup, such that the first element of tup is an element of list1, the second element of tup is an element of list2, and so on. The total number of elements in cart is the product of the lengths of the argument lists. In particular cart is empty if and only if at least one of the argument lists is empty. Also cart contains duplicates if and only if no argument list is empty and at least one contains duplicates.

The last index runs fastest. That means that the first element tup1 of cart contains the first element from list1, from list2 and so on. The second element tup2 of cart contains the first element from list1, the first from list2, and so on, but the last element of tup2 is the second element of the last argument list. This implies that cart is a proper set if and only if all argument lists are proper sets (see 21.19).

The function `Tuples` (16.2-8) computes the k-fold cartesian product of a list.

    gap> Cartesian( [1,2], [3,4], [5,6] );
    [ [ 1, 3, 5 ], [ 1, 3, 6 ], [ 1, 4, 5 ], [ 1, 4, 6 ], [ 2, 3, 5 ],
      [ 2, 3, 6 ], [ 2, 4, 5 ], [ 2, 4, 6 ] ]
    gap> Cartesian( [1,2,2], [1,1,2] );
    [ [ 1, 1 ], [ 1, 1 ], [ 1, 2 ], [ 2, 1 ], [ 2, 1 ], [ 2, 2 ],
      [ 2, 1 ], [ 2, 1 ], [ 2, 2 ] ]

##### 21.20-16 IteratorOfCartesianProduct

`‣ IteratorOfCartesianProduct`( list1, list2, ... ) | ( function )

`‣ IteratorOfCartesianProduct`( list ) | ( function )

In the first form `IteratorOfCartesianProduct` returns an iterator (see 30.8) of all elements of the cartesian product (see `Cartesian` (21.20-15)) of the lists list1, list2, etc.

In the second form list must be a list of lists list1, list2, etc., and `IteratorOfCartesianProduct` returns an iterator of the cartesian product of those lists.

Resulting tuples will be returned in the lexicographic order. Usage of iterators of cartesian products is recommended in the case when the resulting cartesian product is big enough, so its generating and storage will require essential amount of runtime and memory. For smaller cartesian products it is faster to generate the full set of tuples using `Cartesian` (21.20-15) and then loop over its elements (with some minor overhead of needing more memory).

##### 21.20-17 Permuted

`‣ Permuted`( list, perm ) | ( operation )

returns a new list new that contains the elements of the list list permuted according to the permutation perm. That is `new[i^perm] = list[i]` whenever `list[i]` is bound.

`Sortex` (21.18-3) allows you to compute a permutation that must be applied to a list in order to get the sorted list.

    gap> Permuted( [ 5, 4, 6, 1, 7, 5 ], (1,3,5,6,4) );
    [ 1, 4, 5, 5, 6, 7 ]
    gap> Permuted( [ 5, 4, 6,, 7, 5 ], (1,3,5,6,4) );
    [ , 4, 5, 5, 6, 7 ]

##### 21.20-18 List

`‣ List`( list[, func] ) | ( function )

This function returns a new mutable list `new` of the same length as the list list (which may have holes). The entry `new[i]` is unbound if `list[i]` is unbound. Otherwise `new[i] = func(list[i])`. If the argument func is omitted, its default is `IdFunc` (5.4-6), so this function does the same as `ShallowCopy` (12.7-1) (see also 21.7).

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `ListOp`.

    gap> List( [1,2,3], i -> i^2 );
    [ 1, 4, 9 ]
    gap> List( [1..10], IsPrime );
    [ false, true, true, false, true, false, true, false, false, false ]
    gap> List([,1,,3,4], x-> x > 2);
    [ , false,, true, true ]

(See also `List` (30.3-5).)

##### 21.20-19 Filtered

`‣ Filtered`( listorcoll, func ) | ( function )

returns a new list that contains those elements of the list or collection listorcoll (see 30), respectively, for which the unary function func returns `true`.

If the first argument is a list, the order of the elements in the result is the same as the order of the corresponding elements of this list. If an element for which func returns `true` appears several times in the list it will also appear the same number of times in the result. The argument list may contain holes, they are ignored by `Filtered`.

For each element of listorcoll, func must return either `true` or `false`, otherwise an error is signalled.

The result is a new list that is not identical to any other list. The elements of that list however are identical to the corresponding elements of the argument list (see 21.6).

List assignment using the operator `\{\}` (21.3-1) (see 21.4) can be used to extract elements of a list according to indices given in another list.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `FilteredOp`.

    gap> Filtered( [1..20], IsPrime );
    [ 2, 3, 5, 7, 11, 13, 17, 19 ]
    gap> Filtered( [ 1, 3, 4, -4, 4, 7, 10, 6 ], IsPrimePowerInt );
    [ 3, 4, 4, 7 ]
    gap> Filtered( [ 1, 3, 4, -4, 4, 7, 10, 6 ],
    >              n -> IsPrimePowerInt(n) and n mod 2 <> 0 );
    [ 3, 7 ]
    gap> Filtered( Group( (1,2), (1,2,3) ), x -> Order( x ) = 2 );
    [ (2,3), (1,2), (1,3) ]

##### 21.20-20 Number

`‣ Number`( listorcoll[, func] ) | ( function )

Called with a list listorcoll, `Number` returns the number of bound entries in this list. For dense lists `Number`, `Length` (21.17-5), and `Size` (30.4-6) return the same value; for lists with holes `Number` returns the number of bound entries, `Length` (21.17-5) returns the largest index of a bound entry, and `Size` (30.4-6) signals an error.

Called with two arguments, a list or collection listorcoll and a unary function func, `Number` returns the number of elements of listorcoll for which func returns `true`. If an element for which func returns `true` appears several times in listorcoll it will also be counted the same number of times.

For each element of listorcoll, func must return either `true` or `false`, otherwise an error is signalled.

`Filtered` (21.20-19) allows you to extract the elements of a list that have a certain property.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `NumberOp`.

    gap> Number( [ 2, 3, 5, 7 ] );
    4
    gap> Number( [, 2, 3,, 5,, 7,,,, 11 ] );
    5
    gap> Number( [1..20], IsPrime );
    8
    gap> Number( [ 1, 3, 4, -4, 4, 7, 10, 6 ], IsPrimePowerInt );
    4
    gap> Number( [ 1, 3, 4, -4, 4, 7, 10, 6 ],
    >            n -> IsPrimePowerInt(n) and n mod 2 <> 0 );
    2
    gap> Number( Group( (1,2), (1,2,3) ), x -> Order( x ) = 2 );
    3

##### 21.20-21 First

`‣ First`( list[, func] ) | ( operation )

`First` returns the first element of the list list for which the unary function func returns `true`; if func is not given, the first element is returned. list may contain holes. func must return either `true` or `false` for each element of list, otherwise an error is signalled. If func returns `false` for all elements of list then `First` returns `fail`.

`PositionProperty` (21.16-9) allows you to find the position of the first element in a list that satisfies a certain property.

Before GAP 4.12, developers who wished to adapt this for custom list types needed to install suitable methods for the operation `FirstOp`. This is still possible for backwards compatibility, but `FirstOp` now is just a synonym for `First`.

    gap> First( [10^7..10^8], IsPrime );
    10000019
    gap> First( [10^5..10^6],
    >      n -> not IsPrime(n) and IsPrimePowerInt(n) );
    100489
    gap> First( [ 1 .. 20 ], x -> x < 0 );
    fail
    gap> First( [ fail ], x -> x = fail );
    fail

##### 21.20-22 Last

`‣ Last`( list[, func] ) | ( function )

`Last` returns the last element of the list list for which the unary function func returns `true`; if func is not given, the last element is returned. list may contain holes. func must return either `true` or `false` for each element of list, otherwise an error is signalled. If func returns `false` for all elements of list then `Last` returns `fail`.

Developers who wish to adapt this for custom list types need to install suitable methods for the operation `LastOp`.

    gap> Last( [10^7..10^8], IsPrime );
    99999989
    gap> Last( [10^5..10^6],
    >      n -> not IsPrime(n) and IsPrimePowerInt(n) );
    994009
    gap> Last( [ 1 .. 20 ], x -> x < 0 );
    fail
    gap> Last( [ fail ], x -> x = fail );
    fail

##### 21.20-23 ForAll

`‣ ForAll`( listorcoll, func ) | ( function )

tests whether the unary function func returns `true` for all elements in the list or collection listorcoll.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `ForAllOp`.

    gap> ForAll( [1..20], IsPrime );
    false
    gap> ForAll( [2,3,4,5,8,9], IsPrimePowerInt );
    true
    gap> ForAll( [2..14], n -> IsPrimePowerInt(n) or n mod 2 = 0 );
    true
    gap> ForAll( Group( (1,2), (1,2,3) ), i -> SignPerm(i) = 1 );
    false

##### 21.20-24 ForAny

`‣ ForAny`( listorcoll, func ) | ( function )

tests whether the unary function func returns `true` for at least one element in the list or collection listorcoll.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `ForAnyOp`.

    gap> ForAny( [1..20], IsPrime );
    true
    gap> ForAny( [2,3,4,5,8,9], IsPrimePowerInt );
    true
    gap> ForAny( [2..14],
    >    n -> IsPrimePowerInt(n) and n mod 5 = 0 and not IsPrime(n) );
    false
    gap> ForAny( Integers, i ->     i > 0
    >                           and ForAll( [0,2..4], j -> IsPrime(i+j) ) );
    true

##### 21.20-25 Product

`‣ Product`( listorcoll[, func][, init] ) | ( function )

Called with one argument, a dense list or collection listorcoll, `Product` returns the product of the elements of listorcoll (see 30).

Called with a dense list or collection listorcoll and a function func, which must be a function taking one argument, `Product` applies the function func to the elements of listorcoll, and returns the product of the results. In either case `Product` returns `1` if the first argument is empty.

The general rules for arithmetic operations apply (see 21.15), so the result is immutable if and only if all summands are immutable.

If listorcoll contains exactly one element then this element (or its image under func if applicable) itself is returned, not a shallow copy of this element.

If an additional initial value init is given, `Product` returns the product of init and the elements of the first argument resp. of their images under the function func. This is useful for example if the first argument is empty and a different identity than `1` is desired, in which case init is returned.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `ProductOp`.

    gap> Product( [ 2, 3, 5, 7, 11, 13, 17, 19 ] );
    9699690
    gap> Product( [1..10], x->x^2 );
    13168189440000
    gap> Product( [ (1,2), (1,3), (1,4), (2,3), (2,4), (3,4) ] );
    (1,4)(2,3)
    gap> Product( GF(8) );
    0*Z(2)

##### 21.20-26 Sum

`‣ Sum`( listorcoll[, func][, init] ) | ( function )

Called with one argument, a dense list or collection listorcoll, `Sum` returns the sum of the elements of listorcoll (see 30).

Called with a dense list or collection listorcoll and a function func, which must be a function taking one argument, `Sum` applies the function func to the elements of listorcoll, and returns the sum of the results. In either case `Sum` returns `0` if the first argument is empty.

The general rules for arithmetic operations apply (see 21.15), so the result is immutable if and only if all summands are immutable.

If listorcoll contains exactly one element then this element (or its image under func if applicable) itself is returned, not a shallow copy of this element.

If an additional initial value init is given, `Sum` returns the sum of init and the elements of the first argument resp. of their images under the function func. This is useful for example if the first argument is empty and a different zero than `0` is desired, in which case init is returned.

Developers who wish to adapt this for custom list or collection types need to install suitable methods for the operation `SumOp`.

    gap> Sum( [ 2, 3, 5, 7, 11, 13, 17, 19 ] );
    77
    gap> Sum( [1..10], x->x^2 );
    385
    gap> Sum( [ [1,2], [3,4], [5,6] ] );
    [ 9, 12 ]
    gap> Sum( GF(8) );
    0*Z(2)

##### 21.20-27 Iterated

`‣ Iterated`( list, f ) | ( operation )

returns the result of the iterated application of the function f, which must take two arguments, to the elements of the list list. More precisely, if list has length \(n\) then `Iterated` returns the result of the following application, \(\textit{f}( \ldots \textit{f}( \textit{f}( \textit{list}[1], \textit{list}[2] ), \textit{list}[3] ), \ldots, \textit{list}[n] )\).

    gap> Iterated( [ 126, 66, 105 ], Gcd );
    3

##### 21.20-28 ListN

`‣ ListN`( list1, list2, ..., listn, f ) | ( function )

applies the \(n\)-argument function f to the lists. That is, `ListN` returns the list whose \(i\)-th entry is \(\textit{f}(\textit{list1}[i], \textit{list2}[i], \ldots, \textit{listn}[i])\).

    gap> ListN( [1,2], [3,4], \+ );
    [ 4, 6 ]

#### 21.21 Advanced List Manipulations

The following functions are generalizations of `List` (30.3-5), `Set` (30.3-7), `Sum` (21.20-26), `Product` (21.20-25), `ForAll` (21.20-23), `ForAny` (21.20-24), `Filtered` (21.20-19), `Number` (21.20-20), and `Perform` (21.20-10).

##### 21.21-1 FoldLeft

`‣ FoldLeft`( C, func[, init] ) | ( operation )

`FoldLeft` applies the binary function func left-associatively to the elements of the list or collection C.

If no initial accumulator init is given, the first element of C is used as the initial accumulator and the remaining elements are combined with it from left to right. In that case C must not be empty.

If init is given, the accumulator is initialized with init. Then an empty list or collection is allowed, and init is returned unchanged in that case.

If C is a list with holes, these holes are ignored, just as when iterating over the list with a `for`-loop.

This operation can be used to express reductions similar to `Sum` (21.20-26) or `Product` (21.20-25), but it is more general because the accumulator is updated by an arbitrary binary function.

    gap> FoldLeft( [ 1 .. 10 ], \+ );
    55
    gap> FoldLeft( [ 1 .. 4 ], \*, 2 );
    48
    gap> FoldLeft( [ 1,, 3 ], { a, b } -> a + b );
    4

##### 21.21-2 FoldLeftX

`‣ FoldLeftX`( gens, func, init[, abortValue] ) | ( function )

`FoldLeftX` is a generalization of `FoldLeft` (21.21-1) which applies an accumulation function func to a bunch of inputs which are derived from gens.

The argument gens must be a plain list whose entries describe a sequence of nested loops and filters. Specifically, let `n` denote the length of gens. Then each of the entries gens`[1]`, \(\ldots\) gens`[n]` must be one of the following:

a list or collection

  this introduces a new for-loop in the sequence of nested for-loops and if-statements;

a function returning a list or collection

  this introduces a new for-loop in the sequence of nested for-loops and if-statements, where the loop-range depends on the values of the outer loop-variables; or

a function returning `true` or `false`

  this introduces a new if-statement in the sequence of nested for-loops and if-statements.

Each function occurring in gens is called with the values selected by the preceding entries of gens, in order. Thus the function in gens`[k]` receives one argument for each of gens`[1]`, \(\ldots\), gens`[k-1]` that introduced a loop variable.

The argument func must be a binary function, whose first argument is an accumulator variable, and the second argument is the tuple of values of the loop-variables.

The accumulator is initialized with init. For every tuple that is produced by the loops and filters described by gens, func is called with the current accumulator and that tuple. Its return value becomes the new accumulator.

If the optional argument abortValue is given, iteration stops as soon as the accumulator becomes identical to abortValue. This is useful for short-circuiting computations such as `ForAllX` (21.21-7) and `ForAnyX` (21.21-8).

    gap> FoldLeftX( [ [ 1 .. 3 ], i -> [ 1 .. i ],
    >                { i, j } -> i <> j ],
    >              { acc, x } -> acc + 1, 0 );
    3
    gap> FoldLeftX( [ [ 1 .. 3 ], [ 1 .. 3 ], \< ],
    >              function( acc, x )
    >                Add( acc, ShallowCopy( x ) );
    >                return acc;
    >              end, [] );
    [ [ 1, 2 ], [ 1, 3 ], [ 2, 3 ] ]

##### 21.21-3 ListX

`‣ ListX`( arg1, arg2, ..., argn, func ) | ( function )

`ListX` returns a new list constructed from the arguments. The general mechanism underlying this and the related X-functions is described in `FoldLeftX` (21.21-2).

Each of the arguments arg1, arg2, \(\ldots\) argn must be one of the following:

a list or collection

  this introduces a new for-loop in the sequence of nested for-loops and if-statements;

a function returning a list or collection

  this introduces a new for-loop in the sequence of nested for-loops and if-statements, where the loop-range depends on the values of the outer loop-variables; or

a function returning `true` or `false`

  this introduces a new if-statement in the sequence of nested for-loops and if-statements.

Each function occurring among the arguments is called with the values selected by the preceding arguments, in order. Thus the function in the k-th argument position receives one argument for each preceding argument that introduced a loop variable.

The last argument func must be a function, it is applied to the values of the loop-variables and the results are collected.

Thus `ListX( list, func )` is the same as `List( list, func )`, and `ListX( list, func, x -> x )` is the same as `Filtered( list, func )`.

As a more elaborate example, assume arg1 is a list or collection, arg2 is a function returning `true` or `false`, arg3 is a function returning a list or collection, and arg4 is another function returning `true` or `false`, then

`result := ListX( arg1, arg2, arg3, arg4, func );`

is equivalent to

    result := [];
    for v1 in arg1 do
      if arg2( v1 ) then
        for v2 in arg3( v1 ) do
          if arg4( v1, v2 ) then
            Add( result, func( v1, v2 ) );
          fi;
        od;
      fi;
    od;

The following example shows how `ListX` can be used to compute all pairs and all strictly sorted pairs of elements in a list.

    gap> l:= [ 1, 2, 3, 4 ];;
    gap> pair:= function( x, y ) return [ x, y ]; end;;
    gap> ListX( l, l, {x,y} -> [x,y] );
    [ [ 1, 1 ], [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 1 ], [ 2, 2 ],
      [ 2, 3 ], [ 2, 4 ], [ 3, 1 ], [ 3, 2 ], [ 3, 3 ], [ 3, 4 ],
      [ 4, 1 ], [ 4, 2 ], [ 4, 3 ], [ 4, 4 ] ]

In the following example, `\<` (31.11-1) is the comparison operation:

    gap> ListX( l, l, \<, pair );
    [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ]

##### 21.21-4 SetX

`‣ SetX`( arg1, arg2, ..., func ) | ( function )

The only difference between `SetX` and `ListX` (21.21-3) is that the result list of `SetX` is strictly sorted.

    gap> SetX( [ 1 .. 3 ], [ 1 .. 3 ], \+ );
    [ 2, 3, 4, 5, 6 ]

##### 21.21-5 SumX

`‣ SumX`( arg1, arg2, ..., func ) | ( function )

`SumX` returns the sum of the elements in the list obtained by `ListX` (21.21-3) when this is called with the same arguments.

    gap> SumX( [ 1 .. 3 ], [ 1 .. 3 ], \+ );
    36

##### 21.21-6 ProductX

`‣ ProductX`( arg1, arg2, ..., func ) | ( function )

`ProductX` returns the product of the elements in the list obtained by `ListX` (21.21-3) when this is called with the same arguments.

    gap> ProductX( [ 1 .. 3 ], [ 1 .. 3 ], \+ );
    172800

##### 21.21-7 ForAllX

`‣ ForAllX`( arg1, arg2, ..., func ) | ( function )

`ForAllX` returns `true` if all elements are `true` in the list obtained by calling `ListX` (21.21-3) with the same arguments. Otherwise `false` is returned. As with `ForAll` (21.20-23), the last argument must return either `true` or `false`, and evaluation stops as soon as the result is known.

    gap> ForAllX( [ 1 .. 3 ], [ 1 .. 3 ], \< );
    false

##### 21.21-8 ForAnyX

`‣ ForAnyX`( arg1, arg2, ..., func ) | ( function )

`ForAnyX` returns `true` if any element is `true` in the list obtained by calling `ListX` (21.21-3) with the same arguments. Otherwise `false` is returned. As with `ForAny` (21.20-24), the last argument must return either `true` or `false`, and evaluation stops as soon as the result is known.

    gap> ForAnyX( [ 1 .. 3 ], [ 1 .. 3 ], \< );
    true

##### 21.21-9 FilteredX

`‣ FilteredX`( arg1, arg2, ..., func ) | ( function )

`FilteredX` returns the tuples of loop-variable values for which the last argument returns `true`. In other words, it keeps precisely those tuples that would pass the filters when calling `ListX` (21.21-3) with the same arguments.

Even with only one generator, the result consists of singleton tuples. Thus `FilteredX( [ 1 .. 4 ], IsEvenInt )` returns `[ [ 2 ], [ 4 ] ]`, not `[ 2, 4 ]`. Use `Filtered` (21.20-19) if you want to filter the elements of a single list or collection directly.

    gap> FilteredX( [ 1 .. 4 ], IsEvenInt );
    [ [ 2 ], [ 4 ] ]
    gap> FilteredX( [ 1 .. 4 ], [ 1 .. 4 ], \< );
    [ [ 1, 2 ], [ 1, 3 ], [ 1, 4 ], [ 2, 3 ], [ 2, 4 ], [ 3, 4 ] ]

##### 21.21-10 NumberX

`‣ NumberX`( arg1, arg2, ..., func ) | ( function )

`NumberX` returns the number of tuples selected by the last argument, using the same arguments as `ListX` (21.21-3). Equivalently, it counts the entries of the result that `FilteredX` (21.21-9) would return with the same arguments.

    gap> NumberX( [ 1 .. 4 ], IsEvenInt );
    2
    gap> NumberX( [ 1 .. 4 ], [ 1 .. 4 ], \< );
    6

##### 21.21-11 PerformX

`‣ PerformX`( arg1, arg2, ..., func ) | ( function )

`PerformX` works like `ListX` (21.21-3) except that it returns nothing and ignores the return values of func. It is useful for iterating through the tuples described by the arguments purely for their side effects.

    gap> PerformX( [ 1 .. 2 ], [ 3 .. 4 ], Print );  Print( "\n" );
    13142324

#### 21.22 Ranges

A range is a dense list of integers in arithmetic progression (or degression). This is a list of integers such that the difference between consecutive elements is a nonzero constant. Ranges can be abbreviated with the syntactic construct

`[ first, second .. last ]`

or, if the difference between consecutive elements is 1, as

`[ first .. last ]`.

If `first > last`, `[ first .. last ]` is the empty list, which by definition is also a range; also, if `second > first > last` or `second < first < last`, then `[ first, second .. last ]` is the empty list. If `first = last`, `[ first, second .. last ]` is a singleton list, which is a range, too. Note that `last - first` must be divisible by the increment `second - first`, otherwise an error is signalled.

Currently, the integers first, second and last and the length of a range must be small integers as defined in chapter 14.

Note also that a range is just a special case of a list. Thus you can access elements in a range (see 21.3), test for membership etc. You can even assign to such a range if it is mutable (see 21.4). Of course, unless you assign `last + second - first` to the entry `range[ Length( range ) + 1 ]`, the resulting list will no longer be a range. Note that assigning to an entry of range will convert it back into a plain list.

    gap> r := [10..20];
    [ 10 .. 20 ]
    gap> Length( r );
    11
    gap> r[3];
    12
    gap> 17 in r;
    true
    gap> # r still is a range but is now represented as a plain list
    gap> r[1] := 10;; r;
    [ 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 ]
    gap> IsRange(r);
    true
    gap> # r is no longer a range
    gap> r[12] := 25;;
    gap> IsRange(r);
    false
    gap> r := [1,3..17];
    [ 1, 3 .. 17 ]
    gap> Length( r );
    9
    gap> r[4];
    7
    gap> r := [0,-1..-9];
    [ 0, -1 .. -9 ]
    gap> r[5];
    -4
    gap> r := [ 1, 4 .. 32 ];
    Error, Range: <last>-<first> (31) must be divisible by <inc> (3)

Most often ranges are used in connection with the `for`-loop see 4.15-6). Here the construct

`for var in [ first .. last ] do statements od`

replaces the

`for var from first to last do statements od`

which is more usual in other programming languages.

    gap> s := [];; for i in [10..20] do Add( s, i^2 ); od; s;
    [ 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400 ]

Note that a range with `last >= first` is at the same time also a proper set (see 21.19), because it contains no holes or duplicates and is sorted, and also a row vector (see 23), because it contains no holes and all elements are integers.

##### 21.22-1 IsRange

`‣ IsRange`( obj ) | ( category )

tests if the object obj is a range, i.e. is a dense list of integers that is also a range (see 21.22 for a definition of range).

    gap> IsRange( [1,2,3] );  IsRange( [7,5,3,1] );
    true
    true
    gap> IsRange( [1 .. 3] );
    true
    gap> IsRange( [1,2,4,5] );  IsRange( [1,,3,,5,,7] );
    false
    false
    gap> IsRange( [] );  IsRange( [1] );
    true
    true

##### 21.22-2 IsRangeRep

`‣ IsRangeRep`( obj ) | ( representation )

Tests whether obj is represented as a range, that is by internally storing only the first value, the in- or decrement, and the last value of the range.

To test whether a list is a range in the mathematical sense see `IsRange` (21.22-1).

Lists created by the syntactic construct `[ first, second .. last ]`, see 21.22, are in `IsRangeRep`.

Note that if you modify an `IsRangeRep` object by assigning to one of its entries, or by using `Add` (21.4-2) or `Append` (21.4-5), then the range may be converted into a plain list, even though the resulting list may still be a range, mathematically.

    gap> IsRangeRep( [1 .. 3] );
    true
    gap> IsRangeRep( [1, 2, 3] );
    false
    gap> l := [1..3];;
    gap> l[1] := 1;;
    gap> l;
    [ 1, 2, 3 ]

##### 21.22-3 ConvertToRangeRep

`‣ ConvertToRangeRep`( list ) | ( function )

For some lists the GAP kernel knows that they are in fact ranges. Those lists are represented internally in a compact way, namely in `IsRangeRep` (21.22-2), instead of as plain lists. A list that is represented as a plain list might still be a range but GAP may not know this.

If list is a range then `ConvertToRangeRep` changes the representation of list to `IsRangeRep` (21.22-2). A call of `ConvertToRangeRep` for a list that is not a range is ignored.

    gap> r:= [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ];
    [ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 ]
    gap> ConvertToRangeRep( r );  r;
    [ 1 .. 10 ]
    gap> l:= [ 1, 2, 4, 5 ];;  ConvertToRangeRep( l );  l;
    [ 1, 2, 4, 5 ]

#### 21.23 Enumerators

An enumerator is an immutable list that need not store its elements explicitly but knows, from a set of basic data, how to determine the \(i\)-th element and the position of a given object. A typical example of this is a vector space over a finite field with \(q\) elements for which it is very easy to enumerate all elements using \(q\)-adic expansions of integers.

Using this enumeration can be even quicker than a binary search in a sorted list of vectors, see `IsQuickPositionList` (21.23-1).

On the one hand, element access to an enumerator may take more time than element access to an internally represented list containing the same elements. On the other hand, an enumerator may save a vast amount of memory. Take for example a permutation group of size a few millions. Even for moderate degree it is unlikely that a list of all its elements will fit into memory whereas it is no problem to construct an enumerator from a stabilizer chain (see 43.6).

There are situations where one only wants to loop over the elements of a domain, without using the special facilities of an enumerator, namely the particular order of elements and the possibility to find the position of elements. For such cases, GAP provides iterators (see 30.8).

The functions `Enumerator` (30.3-2) and `EnumeratorSorted` (30.3-3) return enumerators of domains. Most of the special implementations of enumerators in the GAP library are based on the general interface that is provided by `EnumeratorByFunctions` (30.3-4); one generic example is `EnumeratorByBasis` (61.6-5), which can be used to get an enumerator of a finite dimensional free module.

Also enumerators for non-domains can be implemented via `EnumeratorByFunctions` (30.3-4); for a discussion, see 79.5.

##### 21.23-1 IsQuickPositionList

`‣ IsQuickPositionList`( list ) | ( filter )

This filter indicates that a position test in list is quicker than about 5 or 6 element comparisons for smaller. If this is the case it can be beneficial to use `Position` (21.16-1) in list and a bit list than ordered lists to represent subsets of list.

#### 21.24 Plain Lists

Plain lists are the default kind of lists in GAP, in the sense that GAP stores the list entries and does not know how to do better (as opposed to ranges or strings, which are also lists). Often it is not necessary to know how a given list is represented internally, the operations defined for lists apply to all lists.

Typical situations where the representation matters are when one wants to make sure that the given list is not a plain list and thus will be handled more efficiently, for example when one installs a method for a particular operation, where an argument is required to be a list in a particular representation.

##### 21.24-1 PlainListCopy

`‣ PlainListCopy`( list ) | ( function )

This function returns a list equal to its argument, in a plain list representation (see `IsPlistRep` (21.24-2)). This is intended for use in certain rare situations, such as before objectifying, or calling some kernel functions.

##### 21.24-2 IsPlistRep

`‣ IsPlistRep`( obj ) | ( representation )

GAP lists created by entering comma separated values in square brackets are usually represented internally as so-called plain lists. Other representations of lists are `IsBlistRep` (22.5-1), `IsRangeRep` (21.22-2), `IsStringRep` (27.4-1), or the ones that are chosen for implementing enumerators, see Section 21.23.

    gap> IsPlistRep( [ 1, 2, 3 ] );
    true
    gap> IsPlistRep( "abc" );
    false
    gap> IsPlistRep( [ 1 .. 5 ] );
    false
    gap> IsPlistRep( BlistList( [ 1 .. 5 ], [ 1 ] ) );
    false
    gap> IsPlistRep( 0 );
    false

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