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<title>Match - The Rust Programming Language</title>
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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"><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" class="active"><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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<h1 class="menu-title">The Rust Programming Language</h1>
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<a class="header" href="match.html#match" id="match"><h1>Match</h1></a>
<p>Often, a simple <a href="if.html"><code>if</code></a>/<code>else</code> isn’t enough, because you have more than two
possible options. Also, conditions can get quite complex. Rust
has a keyword, <code>match</code>, that allows you to replace complicated <code>if</code>/<code>else</code>
groupings with something more powerful. Check it out:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let x = 5;
match x {
1 => println!("one"),
2 => println!("two"),
3 => println!("three"),
4 => println!("four"),
5 => println!("five"),
_ => println!("something else"),
}
#}</code></pre></pre>
<p><code>match</code> takes an expression and then branches based on its value. Each ‘arm’ of
the branch is of the form <code>val => expression</code>. When the value matches, that arm’s
expression will be evaluated. It’s called <code>match</code> because of the term ‘pattern
matching’, which <code>match</code> is an implementation of. There’s a <a href="patterns.html">separate section on
patterns</a> that covers all the patterns that are possible here.</p>
<p>One of the many advantages of <code>match</code> is it enforces ‘exhaustiveness checking’.
For example if we remove the last arm with the underscore <code>_</code>, the compiler will
give us an error:</p>
<pre><code class="language-text">error: non-exhaustive patterns: `_` not covered
</code></pre>
<p>Rust is telling us that we forgot some value. The compiler infers from <code>x</code> that it
can have any 32bit integer value; for example -2,147,483,648 to 2,147,483,647. The <code>_</code> acts
as a 'catch-all', and will catch all possible values that <em>aren't</em> specified in
an arm of <code>match</code>. As you can see in the previous example, we provide <code>match</code>
arms for integers 1-5, if <code>x</code> is 6 or any other value, then it is caught by <code>_</code>.</p>
<p><code>match</code> is also an expression, which means we can use it on the right-hand
side of a <code>let</code> binding or directly where an expression is used:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
let x = 5;
let number = match x {
1 => "one",
2 => "two",
3 => "three",
4 => "four",
5 => "five",
_ => "something else",
};
#}</code></pre></pre>
<p>Sometimes it’s a nice way of converting something from one type to another; in
this example the integers are converted to <code>String</code>.</p>
<a class="header" href="match.html#matching-on-enums" id="matching-on-enums"><h1>Matching on enums</h1></a>
<p>Another important use of the <code>match</code> keyword is to process the possible
variants of an enum:</p>
<pre><pre class="playpen"><code class="language-rust">
# #![allow(unused_variables)]
#fn main() {
enum Message {
Quit,
ChangeColor(i32, i32, i32),
Move { x: i32, y: i32 },
Write(String),
}
fn quit() { /* ... */ }
fn change_color(r: i32, g: i32, b: i32) { /* ... */ }
fn move_cursor(x: i32, y: i32) { /* ... */ }
fn process_message(msg: Message) {
match msg {
Message::Quit => quit(),
Message::ChangeColor(r, g, b) => change_color(r, g, b),
Message::Move { x, y: new_name_for_y } => move_cursor(x, new_name_for_y),
Message::Write(s) => println!("{}", s),
};
}
#}</code></pre></pre>
<p>Again, the Rust compiler checks exhaustiveness, so it demands that you
have a match arm for every variant of the enum. If you leave one off, it
will give you a compile-time error unless you use <code>_</code> or provide all possible
arms.</p>
<p>Unlike the previous uses of <code>match</code>, you can’t use the normal <code>if</code>
statement to do this. You can use the <a href="if-let.html"><code>if let</code></a> statement,
which can be seen as an abbreviated form of <code>match</code>.</p>
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