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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html" class="active"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> Paths for Referring to an Item in the Module Tree</a></li><li class="expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> Bringing Paths Into Scope with the use Keyword</a></li><li class="expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> Separating Modules into Different Files</a></li></ol></li><li class="expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li class="expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Storing Lists of Values with Vectors</a></li><li class="expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Storing UTF-8 Encoded Text with Strings</a></li><li class="expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Storing Keys with Associated Values in Hash Maps</a></li></ol></li><li class="expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li class="expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li class="expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li class="expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li class="expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li class="expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li class="expanded "><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li class="expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li class="expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Writing Automated Tests</a></li><li><ol class="section"><li class="expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> How to Write Tests</a></li><li class="expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Controlling How Tests Are Run</a></li><li class="expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li class="expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li class="expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li class="expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li class="expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li class="expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li class="expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li class="expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li class="expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li class="expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li class="expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li class="expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li class="expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li class="expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li class="expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li class="expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li class="expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li class="expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li class="expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li class="expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li class="expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Using Box<T> to Point to Data on the Heap</a></li><li class="expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> Treating Smart Pointers Like Regular References with the Deref Trait</a></li><li class="expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> Running Code on Cleanup with the Drop Trait</a></li><li class="expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li class="expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li class="expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Reference Cycles Can Leak Memory</a></li></ol></li><li class="expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li class="expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Using Threads to Run Code Simultaneously</a></li><li class="expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Using Message Passing to Transfer Data Between Threads</a></li><li class="expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared-State Concurrency</a></li><li class="expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency with the Sync and Send Traits</a></li></ol></li><li class="expanded "><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li class="expanded "><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li class="expanded "><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects That Allow for Values of Different Types</a></li><li class="expanded "><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li class="expanded "><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns and Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns Can Be Used</a></li><li class="expanded "><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li class="expanded "><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> Pattern Syntax</a></li></ol></li><li class="expanded "><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li class="expanded "><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li class="expanded "><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.2.</strong> Advanced Traits</a></li><li class="expanded "><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.3.</strong> Advanced Types</a></li><li class="expanded "><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.4.</strong> Advanced Functions and Closures</a></li><li class="expanded "><a href="ch19-06-macros.html"><strong aria-hidden="true">19.5.</strong> Macros</a></li></ol></li><li class="expanded "><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li class="expanded "><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> Building a Single-Threaded Web Server</a></li><li class="expanded "><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning Our Single-Threaded Server into a Multithreaded Server</a></li><li class="expanded "><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li class="expanded "><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li class="expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li class="expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li class="expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li class="expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">21.4.</strong> D - Useful Development Tools</a></li><li class="expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">21.5.</strong> E - Editions</a></li><li class="expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">21.6.</strong> F - Translations of the Book</a></li><li class="expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">21.7.</strong> G - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<h2><a class="header" href="#variables-and-mutability" id="variables-and-mutability">Variables and Mutability</a></h2>
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<p>As mentioned in Chapter 2, by default variables are immutable. This is one of
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many nudges Rust gives you to write your code in a way that takes advantage of
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the safety and easy concurrency that Rust offers. However, you still have the
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option to make your variables mutable. Let’s explore how and why Rust
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encourages you to favor immutability and why sometimes you might want to opt
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out.</p>
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<p>When a variable is immutable, once a value is bound to a name, you can’t change
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that value. To illustrate this, let’s generate a new project called <em>variables</em>
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in your <em>projects</em> directory by using <code>cargo new variables</code>.</p>
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<p>Then, in your new <em>variables</em> directory, open <em>src/main.rs</em> and replace its
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||
code with the following code that won’t compile just yet:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile">fn main() {
|
||
let x = 5;
|
||
println!("The value of x is: {}", x);
|
||
x = 6;
|
||
println!("The value of x is: {}", x);
|
||
}
|
||
</code></pre>
|
||
<p>Save and run the program using <code>cargo run</code>. You should receive an error
|
||
message, as shown in this output:</p>
|
||
<pre><code class="language-text">error[E0384]: cannot assign twice to immutable variable `x`
|
||
--> src/main.rs:4:5
|
||
|
|
||
2 | let x = 5;
|
||
| - first assignment to `x`
|
||
3 | println!("The value of x is: {}", x);
|
||
4 | x = 6;
|
||
| ^^^^^ cannot assign twice to immutable variable
|
||
</code></pre>
|
||
<p>This example shows how the compiler helps you find errors in your programs.
|
||
Even though compiler errors can be frustrating, they only mean your program
|
||
isn’t safely doing what you want it to do yet; they do <em>not</em> mean that you’re
|
||
not a good programmer! Experienced Rustaceans still get compiler errors.</p>
|
||
<p>The error message indicates that the cause of the error is that you <code>cannot assign twice to immutable variable x</code>, because you tried to assign a second
|
||
value to the immutable <code>x</code> variable.</p>
|
||
<p>It’s important that we get compile-time errors when we attempt to change a
|
||
value that we previously 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 <em>sometimes</em>.</p>
|
||
<p>In Rust, the compiler guarantees that when you state that a value won’t change,
|
||
it really won’t change. That means that when you’re reading and writing code,
|
||
you don’t have to keep track of how and where a value might change. Your code
|
||
is thus easier to reason through.</p>
|
||
<p>But mutability can be very useful. Variables are immutable only by default; as
|
||
you did in Chapter 2, you can make them mutable by adding <code>mut</code> in front of the
|
||
variable name. In addition to allowing this value to change, <code>mut</code> conveys
|
||
intent to future readers of the code by indicating that other parts of the code
|
||
will be changing this variable’s value.</p>
|
||
<p>For example, let’s change <em>src/main.rs</em> to the following:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let mut x = 5;
|
||
println!("The value of x is: {}", x);
|
||
x = 6;
|
||
println!("The value of x is: {}", x);
|
||
}
|
||
</code></pre></pre>
|
||
<p>When we run the program now, we get this:</p>
|
||
<pre><code class="language-text">$ cargo run
|
||
Compiling variables v0.1.0 (file:///projects/variables)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.30 secs
|
||
Running `target/debug/variables`
|
||
The value of x is: 5
|
||
The value of x is: 6
|
||
</code></pre>
|
||
<p>We’re allowed to change the value that <code>x</code> binds to from <code>5</code> to <code>6</code> when <code>mut</code>
|
||
is used. In some cases, you’ll want to make a variable mutable because it makes
|
||
the code more convenient to write than if it had only immutable variables.</p>
|
||
<p>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.</p>
|
||
<h3><a class="header" href="#differences-between-variables-and-constants" id="differences-between-variables-and-constants">Differences Between Variables and Constants</a></h3>
|
||
<p>Being unable to change the value of a variable might have reminded you of
|
||
another programming concept that most other languages have: <em>constants</em>. 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.</p>
|
||
<p>First, you aren’t allowed to use <code>mut</code> with constants. Constants aren’t just
|
||
immutable by default—they’re always immutable.</p>
|
||
<p>You declare constants using the <code>const</code> keyword instead of the <code>let</code> keyword,
|
||
and the type of the value <em>must</em> be annotated. We’re about to cover types and
|
||
type annotations in the next section, <a href="ch03-02-data-types.html#data-types">“Data Types,”</a><!-- ignore
|
||
--> so don’t worry about the details right now. Just know that you must always
|
||
annotate the type.</p>
|
||
<p>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.</p>
|
||
<p>The last difference is that constants may be set only to a constant expression,
|
||
not the result of a function call or any other value that could only be
|
||
computed at runtime.</p>
|
||
<p>Here’s an example of a constant declaration where the constant’s name is
|
||
<code>MAX_POINTS</code> and its value is set to 100,000. (Rust’s naming convention for
|
||
constants is to use all uppercase with underscores between words, and
|
||
underscores can be inserted in numeric literals to improve readability):</p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>const MAX_POINTS: u32 = 100_000;
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p>Constants are valid for the entire time a program runs, within the scope they
|
||
were declared in, making them a useful choice 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.</p>
|
||
<p>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.</p>
|
||
<h3><a class="header" href="#shadowing" id="shadowing">Shadowing</a></h3>
|
||
<p>As you saw in the guessing game tutorial in the <a href="ch02-00-guessing-game-tutorial.html#comparing-the-guess-to-the-secret-number">“Comparing the Guess to the
|
||
Secret Number”</a><!-- ignore -->
|
||
section in Chapter 2, you can declare a new variable with the same name as a
|
||
previous variable, and the new variable shadows the previous variable.
|
||
Rustaceans say that the first variable is <em>shadowed</em> by the second, which means
|
||
that the second variable’s value is what appears when the variable is used. We
|
||
can shadow a variable by using the same variable’s name and repeating the use
|
||
of the <code>let</code> keyword as follows:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust">fn main() {
|
||
let x = 5;
|
||
|
||
let x = x + 1;
|
||
|
||
let x = x * 2;
|
||
|
||
println!("The value of x is: {}", x);
|
||
}
|
||
</code></pre></pre>
|
||
<p>This program first binds <code>x</code> to a value of <code>5</code>. Then it shadows <code>x</code> by
|
||
repeating <code>let x =</code>, taking the original value and adding <code>1</code> so the value of
|
||
<code>x</code> is then <code>6</code>. The third <code>let</code> statement also shadows <code>x</code>, multiplying the
|
||
previous value by <code>2</code> to give <code>x</code> a final value of <code>12</code>. When we run this
|
||
program, it will output the following:</p>
|
||
<pre><code class="language-text">$ cargo run
|
||
Compiling variables v0.1.0 (file:///projects/variables)
|
||
Finished dev [unoptimized + debuginfo] target(s) in 0.31 secs
|
||
Running `target/debug/variables`
|
||
The value of x is: 12
|
||
</code></pre>
|
||
<p>Shadowing is different from marking a variable as <code>mut</code>, because we’ll get a
|
||
compile-time error if we accidentally try to reassign to this variable without
|
||
using the <code>let</code> keyword. By using <code>let</code>, we can perform a few transformations
|
||
on a value but have the variable be immutable after those transformations have
|
||
been completed.</p>
|
||
<p>The other difference between <code>mut</code> and shadowing is that because we’re
|
||
effectively creating a new variable when we use the <code>let</code> 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, but we really want to store that input as a number:</p>
|
||
<pre><pre class="playpen"><code class="language-rust">
|
||
<span class="boring">#![allow(unused_variables)]
|
||
</span><span class="boring">fn main() {
|
||
</span>let spaces = " ";
|
||
let spaces = spaces.len();
|
||
<span class="boring">}
|
||
</span></code></pre></pre>
|
||
<p>This construct is allowed because the first <code>spaces</code> variable is a string type
|
||
and the second <code>spaces</code> variable, which is a brand-new variable that happens to
|
||
have the same name as the first one, is a number type. Shadowing thus spares us
|
||
from having to come up with different names, such as <code>spaces_str</code> and
|
||
<code>spaces_num</code>; instead, we can reuse the simpler <code>spaces</code> name. However, if we
|
||
try to use <code>mut</code> for this, as shown here, we’ll get a compile-time error:</p>
|
||
<pre><code class="language-rust ignore does_not_compile">let mut spaces = " ";
|
||
spaces = spaces.len();
|
||
</code></pre>
|
||
<p>The error says we’re not allowed to mutate a variable’s type:</p>
|
||
<pre><code class="language-text">error[E0308]: mismatched types
|
||
--> src/main.rs:3:14
|
||
|
|
||
3 | spaces = spaces.len();
|
||
| ^^^^^^^^^^^^ expected &str, found usize
|
||
|
|
||
= note: expected type `&str`
|
||
found type `usize`
|
||
</code></pre>
|
||
<p>Now that we’ve explored how variables work, let’s look at more data types they
|
||
can have.</p>
|
||
|
||
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|
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