[TOC]

# Common Programming Concepts

This chapter covers concepts that appear in almost every programming language
and how they work in Rust. Many programming languages have much in common at
their core. None of the concepts presented in this chapter are unique to Rust,
but we’ll discuss them in the context of Rust and explain the conventions
around using these concepts.

Specifically, you’ll learn about variables, basic types, functions, comments,
and control flow. These foundations will be in every Rust program, and learning
them early will give you a strong core to start from.

> #### Keywords
>
> The Rust language has a set of *keywords* that are reserved for use by
> the language only, much as in other languages. Keep in mind that you cannot
> use these words as names of variables or functions. Most of the keywords have
> special meanings, and you’ll be using them to do various tasks in your Rust
> programs; a few have no current functionality associated with them but have
> been reserved for functionality that might be added to Rust in the future. You
> can find a list of the keywords in Appendix A.

## Variables and Mutability

As mentioned in the “Storing Values with Variables” section, by default
variables are immutable. This is one of many nudges Rust gives you to write
your code in a way that takes advantage of the safety and easy concurrency that
Rust offers. However, you still have the option to make your variables mutable.
Let’s explore how and why Rust encourages you to favor immutability and why
sometimes you might want to opt out.

When a variable is immutable, once a value is bound to a name, you can’t change
that value. To illustrate this, let’s generate a new project called *variables*
in your *projects* directory by using `cargo new variables`.

Then, in your new *variables* directory, open *src/main.rs* and replace its
code with the following code. This code won’t compile just yet, we'll first
examine the immutability error.

Filename: src/main.rs

```
fn main() {
    let x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}
```

Save and run the program using `cargo run`. You should receive an error
message, as shown in this output:

```
$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
error[E0384]: cannot assign twice to immutable variable `x`
 --> src/main.rs:4:5
  |
2 |     let x = 5;
  |         -
  |         |
  |         first assignment to `x`
  |         help: consider making this binding mutable: `mut x`
3 |     println!("The value of x is: {}", x);
4 |     x = 6;
  |     ^^^^^ cannot assign twice to immutable variable
```

This example shows how the compiler helps you find errors in your programs.
Compiler errors can be frustrating, but really they only mean your program
isn’t safely doing what you want it to do yet; they do *not* mean that you’re
not a good programmer! Experienced Rustaceans still get compiler errors.

The error message indicates that the cause of the error is that you `` cannot
assign twice to immutable variable `x` ``, because you tried to assign a second
value to the immutable `x` variable.

It’s important that we get compile-time errors when we attempt to change a
value that’s designated as immutable because this very situation can lead to
bugs. If one part of our code operates on the assumption that a value will
never change and another part of our code changes that value, it’s possible
that the first part of the code won’t do what it was designed to do. The cause
of this kind of bug can be difficult to track down after the fact, especially
when the second piece of code changes the value only *sometimes*. The Rust
compiler guarantees that when you state a value won’t change, it really won’t
change, so you don’t have to keep track of it yourself. Your code is thus
easier to reason through.

But mutability can be very useful, and can make code more convenient to write.
Variables are immutable only by default; as you did in Chapter 2, you can make
them mutable by adding `mut` in front of the variable name. Adding `mut` also
conveys intent to future readers of the code by indicating that other parts of
the code will be changing this variable’s value.

For example, let’s change *src/main.rs* to the following:

Filename: src/main.rs

```
fn main() {
    let mut x = 5;
    println!("The value of x is: {}", x);
    x = 6;
    println!("The value of x is: {}", x);
}
```

When we run the program now, we get this:

```
$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/variables`
The value of x is: 5
The value of x is: 6
```

We’re allowed to change the value that `x` binds to from `5` to `6` when `mut`
is used. There are multiple trade-offs to consider in addition to the
prevention of bugs. For example, in cases where you’re using large data
structures, mutating an instance in place may be faster than copying and
returning newly allocated instances. With smaller data structures, creating new
instances and writing in a more functional programming style may be easier to
think through, so lower performance might be a worthwhile penalty for gaining
that clarity.

### Constants

Like immutable variables, *constants* are values that are bound to a name and
are not allowed to change, but there are a few differences between constants
and variables.

First, you aren’t allowed to use `mut` with constants. Constants aren’t just
immutable by default—they’re always immutable. You declare constants using the
`const` keyword instead of the `let` keyword, and the type of the value *must*
be annotated. We’re about to cover types and type annotations in the next
section, “Data Types,” so don’t worry about the details right now. Just know
that you must always annotate the type.

Constants can be declared in any scope, including the global scope, which makes
them useful for values that many parts of code need to know about.

The last difference is that constants may be set only to a constant expression,
not the result of a value that could only be computed at runtime.

Here’s an example of a constant declaration:

```
const THREE_HOURS_IN_SECONDS: u32 = 60 * 60 * 3;
```

The constant’s name is `THREE_HOURS_IN_SECONDS` and its value is set to the
result of multiplying 60 (the number of seconds in a minute) by 60 (the number
of minutes in an hour) by 3 (the number of hours we want to count in this
program). Rust’s naming convention for constants is to use all uppercase with
underscores between words. The compiler is able to evaluate a limited set of
operations at compile time, which lets us choose to write out this value in a
way that’s easier to understand and verify, rather than setting this constant
to the value 10,800. See the Rust Reference’s section on constant evaluation at
*https://doc.rust-lang.org/reference/const_eval.html* for more information on
what operations can be used when declaring constants.

Constants are valid for the entire time a program runs, within the scope they
were declared in. This property makes constants useful for values in your
application domain that multiple parts of the program might need to know about,
such as the maximum number of points any player of a game is allowed to earn or
the speed of light.

Naming hardcoded values used throughout your program as constants is useful in
conveying the meaning of that value to future maintainers of the code. It also
helps to have only one place in your code you would need to change if the
hardcoded value needed to be updated in the future.

### Shadowing

As you saw in the guessing game tutorial in Chapter 2, you can declare a new
variable with the same name as a previous variable. Rustaceans say that the
first variable is *shadowed* by the second, which means that the second
variable’s value is what the program sees when the variable is used. We can
shadow a variable by using the same variable’s name and repeating the use of
the `let` keyword as follows:

Filename: src/main.rs

```
fn main() {
    let x = 5;

    let x = x + 1;

    {
        let x = x * 2;
        println!("The value of x in the inner scope is: {}", x);
    }

    println!("The value of x is: {}", x);
}
```

This program first binds `x` to a value of `5`. Then it shadows `x` by
repeating `let x =`, taking the original value and adding `1` so the value of
`x` is then `6`. Then, within an inner scope, the third `let` statement also
shadows `x`, multiplying the previous value by `2` to give `x` a value of `12`.
When that scope is over, the inner shadowing ends and `x` returns to being `6`.
When we run this program, it will output the following:

```
$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/variables`
The value of x in the inner scope is: 12
The value of x is: 6
```

Shadowing is different from marking a variable as `mut`, because we’ll get a
compile-time error if we accidentally try to reassign to this variable without
using the `let` keyword. By using `let`, we can perform a few transformations
on a value but have the variable be immutable after those transformations have
been completed.




The other difference between `mut` and shadowing is that because we’re
effectively creating a new variable when we use the `let` keyword again, we can
change the type of the value but reuse the same name. For example, say our
program asks a user to show how many spaces they want between some text by
inputting space characters, and then we want to store that input as a number:

```
    let spaces = "   ";
    let spaces = spaces.len();
```

The first `spaces` variable is a string type and the second `spaces` variable
is a number type. Shadowing thus spares us from having to come up with
different names, such as `spaces_str` and `spaces_num`; instead, we can reuse
the simpler `spaces` name. However, if we try to use `mut` for this, as shown
here, we’ll get a compile-time error:

```
    let mut spaces = "   ";
    spaces = spaces.len();
```

The error says we’re not allowed to mutate a variable’s type:

```
$ cargo run
   Compiling variables v0.1.0 (file:///projects/variables)
error[E0308]: mismatched types
 --> src/main.rs:3:14
  |
3 |     spaces = spaces.len();
  |              ^^^^^^^^^^^^ expected `&str`, found `usize`
```

Now that we’ve explored how variables work, let’s look at more data types they
can have.

## Data Types

Every value in Rust is of a certain *data type*, which tells Rust what kind of
data is being specified so it knows how to work with that data. We’ll look at
two data type subsets: scalar and compound.

Keep in mind that Rust is a *statically typed* language, which means that it
must know the types of all variables at compile time. The compiler can usually
infer what type we want to use based on the value and how we use it. In cases
when many types are possible, such as when we converted a `String` to a numeric
type using `parse` in the “Comparing the Guess to the Secret Number” section in
Chapter 2, we must add a type annotation, like this:

```
let guess: u32 = "42".parse().expect("Not a number!");
```

If we don’t add the type annotation here, Rust will display the following
error, which means the compiler needs more information from us to know which
type we want to use:

```
$ cargo build
   Compiling no_type_annotations v0.1.0 (file:///projects/no_type_annotations)
error[E0282]: type annotations needed
 --> src/main.rs:2:9
  |
2 |     let guess = "42".parse().expect("Not a number!");
  |         ^^^^^ consider giving `guess` a type
```

You’ll see different type annotations for other data types.

### Scalar Types

A *scalar* type represents a single value. Rust has four primary scalar types:
integers, floating-point numbers, Booleans, and characters. You may recognize
these from other programming languages. Let’s jump into how they work in Rust.

#### Integer Types

An *integer* is a number without a fractional component. We used one integer
type in Chapter 2, the `u32` type. This type declaration indicates that the
value it’s associated with should be an unsigned integer (signed integer types
start with `i`, instead of `u`) that takes up 32 bits of space. Table 3-1 shows
the built-in integer types in Rust. We can use any of these variants to declare
the type of an integer value.

Table 3-1: Integer Types in Rust

| Length  | Signed  | Unsigned |
|---------|---------|----------|
| 8-bit   | `i8`    | `u8`     |
| 16-bit  | `i16`   | `u16`    |
| 32-bit  | `i32`   | `u32`    |
| 64-bit  | `i64`   | `u64`    |
| 128-bit | `i128`  | `u128`   |
| arch    | `isize` | `usize`  |

Each variant can be either signed or unsigned and has an explicit size.
*Signed* and *unsigned* refer to whether it’s possible for the number to be
negative—in other words, whether the number needs to have a sign with it
(signed) or whether it will only ever be positive and can therefore be
represented without a sign (unsigned). It’s like writing numbers on paper: when
the sign matters, a number is shown with a plus sign or a minus sign; however,
when it’s safe to assume the number is positive, it’s shown with no sign.
Signed numbers are stored using two’s complement representation.

Each signed variant can store numbers from -(2n - 1) to 2n -
1 - 1 inclusive, where *n* is the number of bits that variant uses. So an
`i8` can store numbers from -(27) to 27 - 1, which equals
-128 to 127. Unsigned variants can store numbers from 0 to 2n - 1,
so a `u8` can store numbers from 0 to 28 - 1, which equals 0 to 255.

Additionally, the `isize` and `usize` types depend on the architecture of the
computer your program is running on, which is denoted in the table as "arch":
64 bits if you’re on a 64-bit architecture and 32 bits if you’re on a 32-bit
architecture.




You can write integer literals in any of the forms shown in Table 3-2. Note
that number literals that can be multiple numeric types allow a type suffix,
such as `57u8`, to designate the type. Number literals can also use `_` as a
visual separator to make the number easier to read, such as `1_000`, which will
have the same value as if you had specified `1000`.

Table 3-2: Integer Literals in Rust

| Number literals  | Example       |
|------------------|---------------|
| Decimal          | `98_222`      |
| Hex              | `0xff`        |
| Octal            | `0o77`        |
| Binary           | `0b1111_0000` |
| Byte (`u8` only) | `b'A'`        |

So how do you know which type of integer to use? If you’re unsure, Rust’s
defaults are generally good places to start: integer types default to `i32`.
The primary situation in which you’d use `isize` or `usize` is when indexing
some sort of collection.

> ##### Integer Overflow
>
> Let’s say you have a variable of type `u8` that can hold values between 0 and
> 255. If you try to change the variable to a value outside of that range, such
> as 256, *integer overflow* will occur, which can result in one of two
> behaviors. When you’re compiling in debug mode, Rust includes checks for
> integer overflow that cause your program to *panic* at runtime if this
> behavior occurs. Rust uses the term panicking when a program exits with an
> error; we’ll discuss panics in more depth in the “Unrecoverable Errors with
> `panic!`” section in Chapter 9.
>
> When you’re compiling in release mode with the `--release` flag, Rust does
> *not* include checks for integer overflow that cause panics. Instead, if
> overflow occurs, Rust performs *two’s complement wrapping*. In short, values
> greater than the maximum value the type can hold “wrap around” to the minimum
> of the values the type can hold. In the case of a `u8`, the value 256 becomes
> 0, the value 257 becomes 1, and so on. The program won’t panic, but the
> variable will have a value that probably isn’t what you were expecting it to
> have. Relying on integer overflow’s wrapping behavior is considered an error.
>
> To explicitly handle the possibility of overflow, you can use these families
> of methods provided by the standard library for primitive numeric types:
>
> - Wrap in all modes with the `wrapping_*` methods, such as `wrapping_add`
> - Return the `None` value if there is overflow with the `checked_*` methods
> - Return the value and a boolean indicating whether there was overflow with
>   the `overflowing_*` methods
> - Saturate at the value’s minimum or maximum values with `saturating_*`
>   methods

#### Floating-Point Types

Rust also has two primitive types for *floating-point numbers*, which are
numbers with decimal points. Rust’s floating-point types are `f32` and `f64`,
which are 32 bits and 64 bits in size, respectively. The default type is `f64`
because on modern CPUs it’s roughly the same speed as `f32` but is capable of
more precision. All floating-point types are signed.




Here’s an example that shows floating-point numbers in action:

Filename: src/main.rs

```
fn main() {
    let x = 2.0; // f64

    let y: f32 = 3.0; // f32
}
```

Floating-point numbers are represented according to the IEEE-754 standard. The
`f32` type is a single-precision float, and `f64` has double precision.

#### Numeric Operations

Rust supports the basic mathematical operations you’d expect for all of the
number types: addition, subtraction, multiplication, division, and remainder.
Integer division rounds down to the nearest integer. The following code shows
how you’d use each numeric operation in a `let` statement:

Filename: src/main.rs

```
fn main() {
    // addition
    let sum = 5 + 10;

    // subtraction
    let difference = 95.5 - 4.3;

    // multiplication
    let product = 4 * 30;

    // division
    let quotient = 56.7 / 32.2;
    let floored = 2 / 3; // Results in 0

    // remainder
    let remainder = 43 % 5;
}
```

Each expression in these statements uses a mathematical operator and evaluates
to a single value, which is then bound to a variable. Appendix B contains a
list of all operators that Rust provides.

#### The Boolean Type

As in most other programming languages, a Boolean type in Rust has two possible
values: `true` and `false`. Booleans are one byte in size. The Boolean type in
Rust is specified using `bool`. For example:

Filename: src/main.rs

```
fn main() {
    let t = true;

    let f: bool = false; // with explicit type annotation
}
```

The main way to use Boolean values is through conditionals, such as an `if`
expression. We’ll cover how `if` expressions work in Rust in the “Control
Flow” section.

#### The Character Type

Rust’s `char` type is the language’s most primitive alphabetic type. Here's
some examples of declaring `char` values:

Filename: src/main.rs

```
fn main() {
    let c = 'z';
    let z = 'ℤ';
    let heart_eyed_cat = '😻';
}
```

Note that we specify `char` literals with single quotes, as opposed to string
literals, which use double quotes. Rust’s `char` type is four bytes in size and
represents a Unicode Scalar Value, which means it can represent a lot more than
just ASCII. Accented letters; Chinese, Japanese, and Korean characters; emoji;
and zero-width spaces are all valid `char` values in Rust. Unicode Scalar
Values range from `U+0000` to `U+D7FF` and `U+E000` to `U+10FFFF` inclusive.
However, a “character” isn’t really a concept in Unicode, so your human
intuition for what a “character” is may not match up with what a `char` is in
Rust. We’ll discuss this topic in detail in “Storing UTF-8 Encoded Text with
Strings” in Chapter 8.

### Compound Types

*Compound types* can group multiple values into one type. Rust has two
primitive compound types: tuples and arrays.

#### The Tuple Type

A tuple is a general way of grouping together a number of values with a variety
of types into one compound type. Tuples have a fixed length: once declared,
they cannot grow or shrink in size.

We create a tuple by writing a comma-separated list of values inside
parentheses. Each position in the tuple has a type, and the types of the
different values in the tuple don’t have to be the same. We’ve added optional
type annotations in this example:

Filename: src/main.rs

```
fn main() {
    let tup: (i32, f64, u8) = (500, 6.4, 1);
}
```

The variable `tup` binds to the entire tuple, because a tuple is considered a
single compound element. To get the individual values out of a tuple, we can
use pattern matching to destructure a tuple value, like this:

Filename: src/main.rs

```
fn main() {
    let tup = (500, 6.4, 1);

    let (x, y, z) = tup;

    println!("The value of y is: {}", y);
}
```

This program first creates a tuple and binds it to the variable `tup`. It then
uses a pattern with `let` to take `tup` and turn it into three separate
variables, `x`, `y`, and `z`. This is called *destructuring*, because it breaks
the single tuple into three parts. Finally, the program prints the value of
`y`, which is `6.4`.

We can also access a tuple element directly by using a period (`.`) followed by
the index of the value we want to access. For example:

Filename: src/main.rs

```
fn main() {
    let x: (i32, f64, u8) = (500, 6.4, 1);

    let five_hundred = x.0;

    let six_point_four = x.1;

    let one = x.2;
}
```

This program creates the tuple `x` and then makes new variables for each
element by using their respective indices. As with most programming languages,
the first index in a tuple is 0.

The tuple without any values, `()`, is a special type that has only one value,
also written `()`. The type is called the *unit type* and the value is called
the *unit value*. Expressions implicitly return the unit value if they don’t
return any other value.

#### The Array Type

Another way to have a collection of multiple values is with an *array*. Unlike
a tuple, every element of an array must have the same type. Unlike arrays in
some other languages, arrays in Rust have a fixed length.

We write the values in an array as a comma-separated list inside square
brackets:

Filename: src/main.rs

```
fn main() {
    let a = [1, 2, 3, 4, 5];
}
```

Arrays are useful when you want your data allocated on the stack rather than
the heap (we will discuss the stack and the heap more in Chapter 4) or when you
want to ensure you always have a fixed number of elements. An array isn’t as
flexible as the vector type, though. A vector is a similar collection type
provided by the standard library that *is* allowed to grow or shrink in size.
If you’re unsure whether to use an array or a vector, chances are you should
use a vector. Chapter 8 discusses vectors in more detail.

However, arrays are more useful when you know the number of elements will not
need to change. For example, if you were using the names of the month in a
program, you would probably use an array rather than a vector because you know
it will always contain 12 elements:

```
let months = ["January", "February", "March", "April", "May", "June", "July",
              "August", "September", "October", "November", "December"];
```




You write an array’s type using square brackets with the type of each element,
a semicolon, and then the number of elements in the array, like so:

```
let a: [i32; 5] = [1, 2, 3, 4, 5];
```

Here, `i32` is the type of each element. After the semicolon, the number `5`
indicates the array contains five elements.

You can also initialize an array to contain the same value for each element by
specifying the initial value, followed by a semicolon, and then the length of
the array in square brackets, as shown here:

```
let a = [3; 5];
```

The array named `a` will contain `5` elements that will all be set to the value
`3` initially. This is the same as writing `let a = [3, 3, 3, 3, 3];` but in a
more concise way.

##### Accessing Array Elements

An array is a single chunk of memory of a known, fixed size that can be
allocated on the stack. You can access elements of an array using indexing,
like this:

Filename: src/main.rs

```
fn main() {
    let a = [1, 2, 3, 4, 5];

    let first = a[0];
    let second = a[1];
}
```

In this example, the variable named `first` will get the value `1`, because
that is the value at index `[0]` in the array. The variable named `second` will
get the value `2` from index `[1]` in the array.

##### Invalid Array Element Access

Let's see what happens if you try to access an element of an array that is past
the end of the array. Say you run this code, similar to the guessing game in
Chapter 2, to get an array index from the user:

Filename: src/main.rs

```
use std::io;

fn main() {
    let a = [1, 2, 3, 4, 5];

    println!("Please enter an array index.");

    let mut index = String::new();

    io::stdin()
        .read_line(&mut index)
        .expect("Failed to read line");

    let index: usize = index
        .trim()
        .parse()
        .expect("Index entered was not a number");

    let element = a[index];

    println!(
        "The value of the element at index {} is: {}",
        index, element
    );
}
```

This code compiles successfully. If you run this code using `cargo run` and
enter 0, 1, 2, 3, or 4, the program will print out the corresponding value at
that index in the array. If you instead enter a number past the end of the
array, such as 10, you’ll see output like this:

```
thread 'main' panicked at 'index out of bounds: the len is 5 but the index is 10', src/main.rs:19:19
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
```

The program resulted in a *runtime* error at the point of using an invalid
value in the indexing operation. The program exited with an error message and
didn’t execute the final `println!` statement. When you attempt to access an
element using indexing, Rust will check that the index you’ve specified is less
than the array length. If the index is greater than or equal to the length,
Rust will panic. This check has to happen at runtime, especially in this case,
because the compiler can’t possibly know what value a user will enter when they
run the code later.

This is an example of Rust’s memory safety principles in action. In many
low-level languages, this kind of check is not done, and when you provide an
incorrect index, invalid memory can be accessed. Rust protects you against this
kind of error by immediately exiting instead of allowing the memory access and
continuing. Chapter 9 discusses more of Rust’s error handling.

## Functions

Functions are prevalent in Rust code. You’ve already seen one of the most
important functions in the language: the `main` function, which is the entry
point of many programs. You’ve also seen the `fn` keyword, which allows you to
declare new functions.

Rust code uses *snake case* as the conventional style for function and variable
names, in which all letters are lowercase and underscores separate words.
Here’s a program that contains an example function definition:

Filename: src/main.rs

```
fn main() {
    println!("Hello, world!");

    another_function();
}

fn another_function() {
    println!("Another function.");
}
```

We define a function in Rust by entering `fn` followed by a function name and a
set of parentheses. The curly brackets tell the compiler where the function
body begins and ends.

We can call any function we’ve defined by entering its name followed by a set
of parentheses. Because `another_function` is defined in the program, it can be
called from inside the `main` function. Note that we defined `another_function`
*after* the `main` function in the source code; we could have defined it before
as well. Rust doesn’t care where you define your functions, only that they’re
defined somewhere.

Let’s start a new binary project named *functions* to explore functions
further. Place the `another_function` example in *src/main.rs* and run it. You
should see the following output:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 0.28s
     Running `target/debug/functions`
Hello, world!
Another function.
```

The lines execute in the order in which they appear in the `main` function.
First, the “Hello, world!” message prints, and then `another_function` is
called and its message is printed.

### Parameters

We can define functions to have *parameters*, which are special variables that
are part of a function’s signature. When a function has parameters, you can
provide it with concrete values for those parameters. Technically, the concrete
values are called *arguments*, but in casual conversation, people tend to use
the words *parameter* and *argument* interchangeably for either the variables
in a function’s definition or the concrete values passed in when you call a
function.

In this version of `another_function` we add a parameter:

Filename: src/main.rs

```
fn main() {
    another_function(5);
}

fn another_function(x: i32) {
    println!("The value of x is: {}", x);
}
```

Try running this program; you should get the following output:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 1.21s
     Running `target/debug/functions`
The value of x is: 5
```

The declaration of `another_function` has one parameter named `x`. The type of
`x` is specified as `i32`. When we pass `5` in to `another_function`, the
`println!` macro puts `5` where the pair of curly brackets were in the format
string.

In function signatures, you *must* declare the type of each parameter. This is
a deliberate decision in Rust’s design: requiring type annotations in function
definitions means the compiler almost never needs you to use them elsewhere in
the code to figure out what type you mean.

When defining multiple parameters, separate the parameter declarations with
commas, like this:

Filename: src/main.rs

```
fn main() {
    print_labeled_measurement(5, 'h');
}

fn print_labeled_measurement(value: i32, unit_label: char) {
    println!("The measurement is: {}{}", value, unit_label);
}
```

This example creates a function named `print_labeled_measurement` with two
parameters. The first parameter is named `value` and is an `i32`. The second is
named `unit_label` and is type `char`. The function then prints text containing
both the `value` and the `unit_label`.

Let’s try running this code. Replace the program currently in your *functions*
project’s *src/main.rs* file with the preceding example and run it using `cargo
run`:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/functions`
The measurement is: 5h
```

Because we called the function with `5` as the value for `value` and `'h'` as
the value for `unit_label`, the program output contains those values.

### Statements and Expressions

Function bodies are made up of a series of statements optionally ending in an
expression. So far, the functions we've covered haven't included an ending
expression, but you have seen an expression as part of a statement. Because
Rust is an expression-based language, this is an important distinction to
understand. Other languages don’t have the same distinctions, so let’s look at
what statements and expressions are and how their differences affect the bodies
of functions.

*Statements* are instructions that perform some action and do not return a
value. *Expressions* evaluate to a resulting value. Let’s look at some examples.

We’ve actually already used statements and expressions. Creating a variable and
assigning a value to it with the `let` keyword is a statement. In Listing 3-1,
`let y = 6;` is a statement.

Filename: src/main.rs

```
fn main() {
    let y = 6;
}
```

Listing 3-1: A `main` function declaration containing one statement

Function definitions are also statements; the entire preceding example is a
statement in itself.

Statements do not return values. Therefore, you can’t assign a `let` statement
to another variable, as the following code tries to do; you’ll get an error:

Filename: src/main.rs

```
fn main() {
    let x = (let y = 6);
}
```

When you run this program, the error you’ll get looks like this:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
error[E0658]: `let` expressions in this position are experimental
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^^^^^^^
  |
  = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
  = help: you can write `matches!(<expr>, <pattern>)` instead of `let <pattern> = <expr>`

error: expected expression, found statement (`let`)
 --> src/main.rs:2:14
  |
2 |     let x = (let y = 6);
  |              ^^^^^^^^^
  |
  = note: variable declaration using `let` is a statement

warning: unnecessary parentheses around assigned value
 --> src/main.rs:2:13
  |
2 |     let x = (let y = 6);
  |             ^^^^^^^^^^^ help: remove these parentheses
  |
  = note: `#[warn(unused_parens)]` on by default
```

The `let y = 6` statement does not return a value, so there isn’t anything for
`x` to bind to. This is different from what happens in other languages, such as
C and Ruby, where the assignment returns the value of the assignment. In those
languages, you can write `x = y = 6` and have both `x` and `y` have the value
`6`; that is not the case in Rust.

Expressions evaluate to a value and make up most of the rest of the code that
you’ll write in Rust. Consider a math operation, such as `5 + 6`, which is an
expression that evaluates to the value `11`. Expressions can be part of
statements: in Listing 3-1, the `6` in the statement `let y = 6;` is an
expression that evaluates to the value `6`. Calling a function is an
expression. Calling a macro is an expression. A new scope block created with
curly brackets is an expression, for example:

Filename: src/main.rs

```
fn main() {
    let x = 5;

    let y = {
        let x = 3;
        x + 1
    };

    println!("The value of y is: {}", y);
}
```

This expression:

```
{
    let x = 3;
    x + 1
}
```

is a block that, in this case, evaluates to `4`. That value gets bound to `y`
as part of the `let` statement. Note that the `x + 1` line doesn't have a
semicolon at the end, unlike most of the lines you’ve seen so far. Expressions
do not include ending semicolons. If you add a semicolon to the end of an
expression, you turn it into a statement, and it will then not return a value.
Keep this in mind as you explore function return values and expressions next.

### Functions with Return Values

Functions can return values to the code that calls them. We don’t name return
values, but we do declare their type after an arrow (`->`). In Rust, the return
value of the function is synonymous with the value of the final expression in
the block of the body of a function. You can return early from a function by
using the `return` keyword and specifying a value, but most functions return
the last expression implicitly. Here’s an example of a function that returns a
value:

Filename: src/main.rs

```
fn five() -> i32 {
    5
}

fn main() {
    let x = five();

    println!("The value of x is: {}", x);
}
```

There are no function calls, macros, or even `let` statements in the `five`
function—just the number `5` by itself. That’s a perfectly valid function in
Rust. Note that the function’s return type is specified too, as `-> i32`. Try
running this code; the output should look like this:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/functions`
The value of x is: 5
```

The `5` in `five` is the function’s return value, which is why the return type
is `i32`. Let’s examine this in more detail. There are two important bits:
first, the line `let x = five();` shows that we’re using the return value of a
function to initialize a variable. Because the function `five` returns a `5`,
that line is the same as the following:

```
let x = 5;
```

Second, the `five` function has no parameters and defines the type of the
return value, but the body of the function is a lonely `5` with no semicolon
because it’s an expression whose value we want to return.

Let’s look at another example:

Filename: src/main.rs

```
fn main() {
    let x = plus_one(5);

    println!("The value of x is: {}", x);
}

fn plus_one(x: i32) -> i32 {
    x + 1
}
```

Running this code will print `The value of x is: 6`. But if we place a
semicolon at the end of the line containing `x + 1`, changing it from an
expression to a statement, we’ll get an error.

Filename: src/main.rs

```
fn main() {
    let x = plus_one(5);

    println!("The value of x is: {}", x);
}

fn plus_one(x: i32) -> i32 {
    x + 1;
}
```

Compiling this code produces an error, as follows:

```
$ cargo run
   Compiling functions v0.1.0 (file:///projects/functions)
error[E0308]: mismatched types
 --> src/main.rs:7:24
  |
7 | fn plus_one(x: i32) -> i32 {
  |    --------            ^^^ expected `i32`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
8 |     x + 1;
  |          - help: consider removing this semicolon
```

The main error message, “mismatched types,” reveals the core issue with this
code. The definition of the function `plus_one` says that it will return an
`i32`, but statements don’t evaluate to a value, which is expressed by `()`,
the unit type. Therefore, nothing is returned, which contradicts the function
definition and results in an error. In this output, Rust provides a message to
possibly help rectify this issue: it suggests removing the semicolon, which
would fix the error.

## Comments

All programmers strive to make their code easy to understand, but sometimes
extra explanation is warranted. In these cases, programmers leave *comments* in
their source code that the compiler will ignore but people reading the source
code may find useful.

Here’s a simple comment:

```
// hello, world
```

In Rust, the idiomatic comment style starts a comment with two slashes, and the
comment continues until the end of the line. For comments that extend beyond a
single line, you’ll need to include `//` on each line, like this:

```
// So we’re doing something complicated here, long enough that we need
// multiple lines of comments to do it! Whew! Hopefully, this comment will
// explain what’s going on.
```

Comments can also be placed at the end of lines containing code:

Filename: src/main.rs

```
fn main() {
    let lucky_number = 7; // I’m feeling lucky today
}
```

But you’ll more often see them used in this format, with the comment on a
separate line above the code it’s annotating:

Filename: src/main.rs

```
fn main() {
    // I’m feeling lucky today
    let lucky_number = 7;
}
```

Rust also has another kind of comment, documentation comments, which we’ll
discuss in the “Publishing a Crate to Crates.io” section of Chapter 14.

## Control Flow

The ability to run some code depending on if a condition is true, or run some
code repeatedly while a condition is true, are basic building blocks in most
programming languages. The most common constructs that let you control the flow
of execution of Rust code are `if` expressions and loops.

### `if` Expressions

An `if` expression allows you to branch your code depending on conditions. You
provide a condition and then state, “If this condition is met, run this block
of code. If the condition is not met, do not run this block of code.”

Create a new project called *branches* in your *projects* directory to explore
the `if` expression. In the *src/main.rs* file, input the following:

Filename: src/main.rs

```
fn main() {
    let number = 3;

    if number < 5 {
        println!("condition was true");
    } else {
        println!("condition was false");
    }
}
```

All `if` expressions start with the keyword `if`, followed by a condition. In
this case, the condition checks whether or not the variable `number` has a
value less than 5. We place block of code to execute if the condition is true
immediately after the condition inside curly brackets. Blocks of code
associated with the conditions in `if` expressions are sometimes called *arms*,
just like the arms in `match` expressions that we discussed in the “Comparing
the Guess to the Secret Number” section of Chapter 2.

Optionally, we can also include an `else` expression, which we chose
to do here, to give the program an alternative block of code to execute should
the condition evaluate to false. If you don’t provide an `else` expression and
the condition is false, the program will just skip the `if` block and move on
to the next bit of code.

Try running this code; you should see the following output:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/branches`
condition was true
```

Let’s try changing the value of `number` to a value that makes the condition
`false` to see what happens:

```
    let number = 7;
```

Run the program again, and look at the output:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/branches`
condition was false
```

It’s also worth noting that the condition in this code *must* be a `bool`. If
the condition isn’t a `bool`, we’ll get an error. For example, try running the
following code:

Filename: src/main.rs

```
fn main() {
    let number = 3;

    if number {
        println!("number was three");
    }
}
```

The `if` condition evaluates to a value of `3` this time, and Rust throws an
error:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: mismatched types
 --> src/main.rs:4:8
  |
4 |     if number {
  |        ^^^^^^ expected `bool`, found integer
```

The error indicates that Rust expected a `bool` but got an integer. Unlike
languages such as Ruby and JavaScript, Rust will not automatically try to
convert non-Boolean types to a Boolean. You must be explicit and always provide
`if` with a Boolean as its condition. If we want the `if` code block to run
only when a number is not equal to `0`, for example, we can change the `if`
expression to the following:

Filename: src/main.rs

```
fn main() {
    let number = 3;

    if number != 0 {
        println!("number was something other than zero");
    }
}
```

Running this code will print `number was something other than zero`.

#### Handling Multiple Conditions with `else if`

You can use multiple conditions by combining `if` and `else` in an `else if`
expression. For example:

Filename: src/main.rs

```
fn main() {
    let number = 6;

    if number % 4 == 0 {
        println!("number is divisible by 4");
    } else if number % 3 == 0 {
        println!("number is divisible by 3");
    } else if number % 2 == 0 {
        println!("number is divisible by 2");
    } else {
        println!("number is not divisible by 4, 3, or 2");
    }
}
```

This program has four possible paths it can take. After running it, you should
see the following output:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.31s
     Running `target/debug/branches`
number is divisible by 3
```

When this program executes, it checks each `if` expression in turn and executes
the first body for which the condition holds true. Note that even though 6 is
divisible by 2, we don’t see the output `number is divisible by 2`, nor do we
see the `number is not divisible by 4, 3, or 2` text from the `else` block.
That’s because Rust only executes the block for the first true condition, and
once it finds one, it doesn’t even check the rest.

Using too many `else if` expressions can clutter your code, so if you have more
than one, you might want to refactor your code. Chapter 6 describes a powerful
Rust branching construct called `match` for these cases.

#### Using `if` in a `let` Statement

Because `if` is an expression, we can use it on the right side of a `let`
statement to assign the outcome to a variable, as in Listing 3-2.

Filename: src/main.rs

```
fn main() {
    let condition = true;
    let number = if condition { 5 } else { 6 };

    println!("The value of number is: {}", number);
}
```

Listing 3-2: Assigning the result of an `if` expression
to a variable

The `number` variable will be bound to a value based on the outcome of the `if`
expression. Run this code to see what happens:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
    Finished dev [unoptimized + debuginfo] target(s) in 0.30s
     Running `target/debug/branches`
The value of number is: 5
```

Remember that blocks of code evaluate to the last expression in them, and
numbers by themselves are also expressions. In this case, the value of the
whole `if` expression depends on which block of code executes. This means the
values that have the potential to be results from each arm of the `if` must be
the same type; in Listing 3-2, the results of both the `if` arm and the `else`
arm were `i32` integers. If the types are mismatched, as in the following
example, we’ll get an error:

Filename: src/main.rs

```
fn main() {
    let condition = true;

    let number = if condition { 5 } else { "six" };

    println!("The value of number is: {}", number);
}
```

When we try to compile this code, we’ll get an error. The `if` and `else` arms
have value types that are incompatible, and Rust indicates exactly where to
find the problem in the program:

```
$ cargo run
   Compiling branches v0.1.0 (file:///projects/branches)
error[E0308]: `if` and `else` have incompatible types
 --> src/main.rs:4:44
  |
4 |     let number = if condition { 5 } else { "six" };
  |                                 -          ^^^^^ expected integer, found `&str`
  |                                 |
  |                                 expected because of this
```

The expression in the `if` block evaluates to an integer, and the expression in
the `else` block evaluates to a string. This won’t work because variables must
have a single type, and Rust needs to know at compile time what type the
`number` variable is, definitively. Knowing the type of `number` lets the
compiler verify the type is valid everywhere we use `number`. Rust wouldn’t be
able to do that if the type of `number` was only determined at runtime; the
compiler would be more complex and would make fewer guarantees about the code
if it had to keep track of multiple hypothetical types for any variable.

### Repetition with Loops

It’s often useful to execute a block of code more than once. For this task,
Rust provides several *loops*, which will run through the code inside the loop
body to the end and then start immediately back at the beginning. To
experiment with loops, let’s make a new project called *loops*.

Rust has three kinds of loops: `loop`, `while`, and `for`. Let’s try each one.

#### Repeating Code with `loop`

The `loop` keyword tells Rust to execute a block of code over and over again
forever or until you explicitly tell it to stop.

As an example, change the *src/main.rs* file in your *loops* directory to look
like this:

Filename: src/main.rs

```
fn main() {
    loop {
        println!("again!");
    }
}
```

When we run this program, we’ll see `again!` printed over and over continuously
until we stop the program manually. Most terminals support the keyboard shortcut
ctrl-c to interrupt a program that is stuck in
a continual loop. Give it a try:

```
$ cargo run
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.29s
     Running `target/debug/loops`
again!
again!
again!
again!
^Cagain!
```

The symbol `^C` represents where you pressed ctrl-c
. You may or may not see the word `again!` printed after the `^C`,
depending on where the code was in the loop when it received the interrupt
signal.

Fortunately, Rust also provides a way to break out of a loop using code. You can
place the `break` keyword within the loop to tell the program when to stop
executing the loop. Recall that we did this in the guessing game in the
“Quitting After a Correct Guess” section of Chapter 2 to exit the program when
the user won the game by guessing the correct number.

We also used `continue` in the guessing game, which in a loop tells the program
to skip over any remaining code in this iteration of the loop and go to the
next iteration.

If you have loops within loops, `break` and `continue` apply to the innermost
loop at that point. You can optionally specify a *loop label* on a loop that we
can then use with `break` or `continue` to specify that those keywords apply to
the labeled loop instead of the innermost loop. Here’s an example with two
nested loops:

```
fn main() {
    let mut count = 0;
    'counting_up: loop {
        println!("count = {}", count);
        let mut remaining = 10;

        loop {
            println!("remaining = {}", remaining);
            if remaining == 9 {
                break;
            }
            if count == 2 {
                break 'counting_up;
            }
            remaining -= 1;
        }

        count += 1;
    }
    println!("End count = {}", count);
}
```

The outer loop has the label `'counting_up`, and it will count up from 0 to 2.
The inner loop without a label counts down from 10 to 9. The first `break` that
doesn’t specify a label will exit the inner loop only. The `break
'counting_up;` statement will exit the outer loop. This code prints:

```
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.58s
     Running `target/debug/loops`
count = 0
remaining = 10
remaining = 9
count = 1
remaining = 10
remaining = 9
count = 2
remaining = 10
End count = 2
```

#### Returning Values from Loops

One of the uses of a `loop` is to retry an operation you know might fail, such
as checking whether a thread has completed its job. You might also need to pass
the result of that operation out of the loop to the rest of your code. To do
this, you can add the value you want returned after the `break` expression you
use to stop the loop; that value will be returned out of the loop so you can
use it, as shown here:

```
fn main() {
    let mut counter = 0;

    let result = loop {
        counter += 1;

        if counter == 10 {
            break counter * 2;
        }
    };

    println!("The result is {}", result);
}
```

Before the loop, we declare a variable named `counter` and initialize it to
`0`. Then we declare a variable named `result` to hold the value returned from
the loop. On every iteration of the loop, we add `1` to the `counter` variable,
and then check whether the counter is equal to `10`. When it is, we use the
`break` keyword with the value `counter * 2`. After the loop, we use a
semicolon to end the statement that assigns the value to `result`. Finally, we
print the value in `result`, which in this case is 20.

#### Conditional Loops with `while`

A program will often need to evaluate a condition within a loop. While the
condition is true, the loop runs. When the condition ceases to be true, the
program calls `break`, stopping the loop. It's possible to implement behavior
like this using a combination of `loop`, `if`, `else`, and `break`; you could
try that now in a program, if you’d like. However, this pattern is so common
that Rust has a built-in language construct for it, called a `while` loop. In
Listing 3-3, we use `while` to loop the program three times, counting down each
time, and then, after the loop, print a message and exit.

Filename: src/main.rs

```
fn main() {
    let mut number = 3;

    while number != 0 {
        println!("{}!", number);

        number -= 1;
    }

    println!("LIFTOFF!!!");
}
```

Listing 3-3: Using a `while` loop to run code while a condition holds true

This construct eliminates a lot of nesting that would be necessary if you used
`loop`, `if`, `else`, and `break`, and it’s clearer. While a condition holds
true, the code runs; otherwise, it exits the loop.

#### Looping Through a Collection with `for`

You can choose to use the `while` construct to loop over the elements of a
collection, such as an array. For example, the loop in Listing 3-4 prints each
element in the array `a`.

Filename: src/main.rs

```
fn main() {
    let a = [10, 20, 30, 40, 50];
    let mut index = 0;

    while index < 5 {
        println!("the value is: {}", a[index]);

        index += 1;
    }
}
```

Listing 3-4: Looping through each element of a collection using a `while` loop

Here, the code counts up through the elements in the array. It starts at index
`0`, and then loops until it reaches the final index in the array (that is,
when `index < 5` is no longer true). Running this code will print every element
in the array:

```
$ cargo run
   Compiling loops v0.1.0 (file:///projects/loops)
    Finished dev [unoptimized + debuginfo] target(s) in 0.32s
     Running `target/debug/loops`
the value is: 10
the value is: 20
the value is: 30
the value is: 40
the value is: 50
```

All five array values appear in the terminal, as expected. Even though `index`
will reach a value of `5` at some point, the loop stops executing before trying
to fetch a sixth value from the array.

However, this approach is error prone; we could cause the program to panic if
the index value or test condition are incorrect. For example, if you changed
the definition of the `a` array to have four elements but forgot to update the
condition to `while index < 4`, the code would panic. It’s also slow, because
the compiler adds runtime code to perform the conditional check of whether the
index is within the bounds of the array on every iteration through the loop.

As a more concise alternative, you can use a `for` loop and execute some code
for each item in a collection. A `for` loop looks like the code in Listing 3-5.

Filename: src/main.rs

```
fn main() {
    let a = [10, 20, 30, 40, 50];

    for element in a {
        println!("the value is: {}", element);
    }
}
```

Listing 3-5: Looping through each element of a collection using a `for` loop

When we run this code, we’ll see the same output as in Listing 3-4. More
importantly, we’ve now increased the safety of the code and eliminated the
chance of bugs that might result from going beyond the end of the array or not
going far enough and missing some items.

Using the `for` loop, you wouldn’t need to remember to change any other code if
you changed the number of values in the array, as you would with the method
used in Listing 3-4.

The safety and conciseness of `for` loops make them the most commonly used loop
construct in Rust. Even in situations in which you want to run some code a
certain number of times, as in the countdown example that used a `while` loop
in Listing 3-3, most Rustaceans would use a `for` loop. The way to do that
would be to use a `Range`, provided by the standard library, which generates
all numbers in sequence starting from one number and ending before another
number.

Here’s what the countdown would look like using a `for` loop and another method
we’ve not yet talked about, `rev`, to reverse the range:

Filename: src/main.rs

```
fn main() {
    for number in (1..4).rev() {
        println!("{}!", number);
    }
    println!("LIFTOFF!!!");
}
```

This code is a bit nicer, isn’t it?

## Summary

You made it! That was a sizable chapter: you learned about variables, scalar
and compound data types, functions, comments, `if` expressions, and loops!
To practice with the concepts discussed in this chapter, try building
programs to do the following:

* Convert temperatures between Fahrenheit and Celsius.
* Generate the nth Fibonacci number.
* Print the lyrics to the Christmas carol “The Twelve Days of Christmas,”
  taking advantage of the repetition in the song.

When you’re ready to move on, we’ll talk about a concept in Rust that *doesn’t*
commonly exist in other programming languages: ownership.
