New upstream version 1.12.0+dfsg1

[rustc.git] / src / doc / book / loops.md

% Loops

Rust currently provides three approaches to performing some kind of iterative activity. They are: `loop`, `while` and `for`. Each approach has its own set of uses.

## loop

The infinite `loop` is the simplest form of loop available in Rust. Using the keyword `loop`, Rust provides a way to loop indefinitely until some terminating statement is reached. Rust's infinite `loop`s look like this:

```rust,ignore
loop {
    println!("Loop forever!");
}
```

## while

Rust also has a `while` loop. It looks like this:

```rust
let mut x = 5; // mut x: i32
let mut done = false; // mut done: bool

while !done {
    x += x - 3;

    println!("{}", x);

    if x % 5 == 0 {
        done = true;
    }
}
```

`while` loops are the correct choice when you’re not sure how many times
you need to loop.

If you need an infinite loop, you may be tempted to write this:

```rust,ignore
while true {
```

However, `loop` is far better suited to handle this case:

```rust,ignore
loop {
```

Rust’s control-flow analysis treats this construct differently than a `while
true`, since we know that it will always loop. In general, the more information
we can give to the compiler, the better it can do with safety and code
generation, so you should always prefer `loop` when you plan to loop
infinitely.

## for

The `for` loop is used to loop a particular number of times. Rust’s `for` loops
work a bit differently than in other systems languages, however. Rust’s `for`
loop doesn’t look like this “C-style” `for` loop:

```c
for (x = 0; x < 10; x++) {
    printf( "%d\n", x );
}
```

Instead, it looks like this:

```rust
for x in 0..10 {
    println!("{}", x); // x: i32
}
```

In slightly more abstract terms,

```rust,ignore
for var in expression {
    code
}
```

The expression is an item that can be converted into an [iterator] using
[`IntoIterator`]. The iterator gives back a series of elements. Each element is
one iteration of the loop. That value is then bound to the name `var`, which is
valid for the loop body. Once the body is over, the next value is fetched from
the iterator, and we loop another time. When there are no more values, the `for`
loop is over.

[iterator]: iterators.html
[`IntoIterator`]: ../std/iter/trait.IntoIterator.html

In our example, `0..10` is an expression that takes a start and an end position,
and gives an iterator over those values. The upper bound is exclusive, though,
so our loop will print `0` through `9`, not `10`.

Rust does not have the “C-style” `for` loop on purpose. Manually controlling
each element of the loop is complicated and error prone, even for experienced C
developers.

### Enumerate

When you need to keep track of how many times you already looped, you can use the `.enumerate()` function.

#### On ranges:

```rust
for (index, value) in (5..10).enumerate() {
    println!("index = {} and value = {}", index, value);
}
```

Outputs:

```text
index = 0 and value = 5
index = 1 and value = 6
index = 2 and value = 7
index = 3 and value = 8
index = 4 and value = 9
```

Don't forget to add the parentheses around the range.

#### On iterators:

```rust
let lines = "hello\nworld".lines();

for (linenumber, line) in lines.enumerate() {
    println!("{}: {}", linenumber, line);
}
```

Outputs:

```text
0: hello
1: world
```

## Ending iteration early

Let’s take a look at that `while` loop we had earlier:

```rust
let mut x = 5;
let mut done = false;

while !done {
    x += x - 3;

    println!("{}", x);

    if x % 5 == 0 {
        done = true;
    }
}
```

We had to keep a dedicated `mut` boolean variable binding, `done`, to know
when we should exit out of the loop. Rust has two keywords to help us with
modifying iteration: `break` and `continue`.

In this case, we can write the loop in a better way with `break`:

```rust
let mut x = 5;

loop {
    x += x - 3;

    println!("{}", x);

    if x % 5 == 0 { break; }
}
```

We now loop forever with `loop` and use `break` to break out early. Issuing an explicit `return` statement will also serve to terminate the loop early.

`continue` is similar, but instead of ending the loop, it goes to the next
iteration. This will only print the odd numbers:

```rust
for x in 0..10 {
    if x % 2 == 0 { continue; }

    println!("{}", x);
}
```

## Loop labels

You may also encounter situations where you have nested loops and need to
specify which one your `break` or `continue` statement is for. Like most
other languages, by default a `break` or `continue` will apply to innermost
loop. In a situation where you would like to `break` or `continue` for one
of the outer loops, you can use labels to specify which loop the `break` or
 `continue` statement applies to. This will only print when both `x` and `y` are
 odd:

```rust
'outer: for x in 0..10 {
    'inner: for y in 0..10 {
        if x % 2 == 0 { continue 'outer; } // continues the loop over x
        if y % 2 == 0 { continue 'inner; } // continues the loop over y
        println!("x: {}, y: {}", x, y);
    }
}
```