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<ol class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong aria-hidden="true">2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong aria-hidden="true">3.</strong> Syntax and Semantics</a></li><li><ol class="section"><li><a href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html" class="active"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol>
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<a class="header" href="loops.html#loops" id="loops"><h1>Loops</h1></a>
<p>Rust currently provides three approaches to performing some kind of iterative activity. They are: <code>loop</code>, <code>while</code> and <code>for</code>. Each approach has its own set of uses.</p>
<a class="header" href="loops.html#loop" id="loop"><h2>loop</h2></a>
<p>The infinite <code>loop</code> is the simplest form of loop available in Rust. Using the keyword <code>loop</code>, Rust provides a way to loop indefinitely until some terminating statement is reached. Rust's infinite <code>loop</code>s look like this:</p>
<pre><code class="language-rust ignore">loop {
println!("Loop forever!");
}
</code></pre>
<a class="header" href="loops.html#while" id="while"><h2>while</h2></a>
<p>Rust also has a <code>while</code> loop. It looks like this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
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;
}
}
#}</code></pre></pre>
<p><code>while</code> loops are the correct choice when you’re not sure how many times
you need to loop.</p>
<p>If you need an infinite loop, you may be tempted to write this:</p>
<pre><code class="language-rust ignore">while true {
</code></pre>
<p>However, <code>loop</code> is far better suited to handle this case:</p>
<pre><code class="language-rust ignore">loop {
</code></pre>
<p>Rust’s control-flow analysis treats this construct differently than a <code>while true</code>, 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 <code>loop</code> when you plan to loop
infinitely.</p>
<a class="header" href="loops.html#for" id="for"><h2>for</h2></a>
<p>The <code>for</code> loop is used to loop a particular number of times. Rust’s <code>for</code> loops
work a bit differently than in other systems languages, however. Rust’s <code>for</code>
loop doesn’t look like this “C-style” <code>for</code> loop:</p>
<pre><code class="language-c">for (x = 0; x < 10; x++) {
printf( "%d\n", x );
}
</code></pre>
<p>Instead, it looks like this:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
for x in 0..10 {
println!("{}", x); // x: i32
}
#}</code></pre></pre>
<p>In slightly more abstract terms,</p>
<pre><code class="language-rust ignore">for var in expression {
code
}
</code></pre>
<p>The expression is an item that can be converted into an <a href="iterators.html">iterator</a> using
<a href="../../std/iter/trait.IntoIterator.html"><code>IntoIterator</code></a>. The iterator gives back a series of elements, one element per
iteration of the loop. That value is then bound to the name <code>var</code>, 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 <code>for</code>
loop is over.</p>
<p>In our example, <code>0..10</code> 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 <code>0</code> through <code>9</code>, not <code>10</code>.</p>
<p>Rust does not have the “C-style” <code>for</code> loop on purpose. Manually controlling
each element of the loop is complicated and error prone, even for experienced C
developers.</p>
<a class="header" href="loops.html#enumerate" id="enumerate"><h3>Enumerate</h3></a>
<p>When you need to keep track of how many times you have already looped, you can
use the <code>.enumerate()</code> function.</p>
<a class="header" href="loops.html#on-ranges" id="on-ranges"><h4>On ranges:</h4></a>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
for (index, value) in (5..10).enumerate() {
println!("index = {} and value = {}", index, value);
}
#}</code></pre></pre>
<p>Outputs:</p>
<pre><code class="language-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
</code></pre>
<p>Don't forget to add the parentheses around the range.</p>
<a class="header" href="loops.html#on-iterators" id="on-iterators"><h4>On iterators:</h4></a>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let lines = "hello\nworld".lines();
for (linenumber, line) in lines.enumerate() {
println!("{}: {}", linenumber, line);
}
#}</code></pre></pre>
<p>Outputs:</p>
<pre><code class="language-text">0: hello
1: world
</code></pre>
<a class="header" href="loops.html#ending-iteration-early" id="ending-iteration-early"><h2>Ending iteration early</h2></a>
<p>Let’s take a look at that <code>while</code> loop we had earlier:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = 5;
let mut done = false;
while !done {
x += x - 3;
println!("{}", x);
if x % 5 == 0 {
done = true;
}
}
#}</code></pre></pre>
<p>We had to keep a dedicated <code>mut</code> boolean variable binding, <code>done</code>, to know
when we should exit out of the loop. Rust has two keywords to help us with
modifying iteration: <code>break</code> and <code>continue</code>.</p>
<p>In this case, we can write the loop in a better way with <code>break</code>:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let mut x = 5;
loop {
x += x - 3;
println!("{}", x);
if x % 5 == 0 { break; }
}
#}</code></pre></pre>
<p>We now loop forever with <code>loop</code> and use <code>break</code> to break out early. Issuing an explicit <code>return</code> statement will also serve to terminate the loop early.</p>
<p><code>continue</code> is similar, but instead of ending the loop, it goes to the next
iteration. This will only print the odd numbers:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
for x in 0..10 {
if x % 2 == 0 { continue; }
println!("{}", x);
}
#}</code></pre></pre>
<a class="header" href="loops.html#loop-labels" id="loop-labels"><h2>Loop labels</h2></a>
<p>You may also encounter situations where you have nested loops and need to
specify which one your <code>break</code> or <code>continue</code> statement is for. Like most
other languages, Rust's <code>break</code> or <code>continue</code> apply to the innermost loop.
In a situation where you would like to <code>break</code> or <code>continue</code> for one
of the outer loops, you can use labels to specify which loop the <code>break</code> or
<code>continue</code> statement applies to.</p>
<p>In the example below, we <code>continue</code> to the next iteration of <code>outer</code> loop
when <code>x</code> is even, while we <code>continue</code> to the next iteration of <code>inner</code>
loop when y is even. So it will execute the <code>println!</code> when both <code>x</code> and
<code>y</code> are odd.</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
'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);
}
}
#}</code></pre></pre>
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