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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"><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" class="active"><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="#improving-our-io-project" id="improving-our-io-project">Improving Our I/O Project</a></h2>
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<p>With this new knowledge about iterators, we can improve the I/O project in
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Chapter 12 by using iterators to make places in the code clearer and more
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concise. Let’s look at how iterators can improve our implementation of the
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<code>Config::new</code> function and the <code>search</code> function.</p>
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<h3><a class="header" href="#removing-a-clone-using-an-iterator" id="removing-a-clone-using-an-iterator">Removing a <code>clone</code> Using an Iterator</a></h3>
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<p>In Listing 12-6, we added code that took a slice of <code>String</code> values and created
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an instance of the <code>Config</code> struct by indexing into the slice and cloning the
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values, allowing the <code>Config</code> struct to own those values. In Listing 13-24,
|
||
we’ve reproduced the implementation of the <code>Config::new</code> function as it was in
|
||
Listing 12-23:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">impl Config {
|
||
pub fn new(args: &[String]) -> Result<Config, &'static str> {
|
||
if args.len() < 3 {
|
||
return Err("not enough arguments");
|
||
}
|
||
|
||
let query = args[1].clone();
|
||
let filename = args[2].clone();
|
||
|
||
let case_sensitive = env::var("CASE_INSENSITIVE").is_err();
|
||
|
||
Ok(Config { query, filename, case_sensitive })
|
||
}
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 13-24: Reproduction of the <code>Config::new</code> function
|
||
from Listing 12-23</span></p>
|
||
<p>At the time, we said not to worry about the inefficient <code>clone</code> calls because
|
||
we would remove them in the future. Well, that time is now!</p>
|
||
<p>We needed <code>clone</code> here because we have a slice with <code>String</code> elements in the
|
||
parameter <code>args</code>, but the <code>new</code> function doesn’t own <code>args</code>. To return
|
||
ownership of a <code>Config</code> instance, we had to clone the values from the <code>query</code>
|
||
and <code>filename</code> fields of <code>Config</code> so the <code>Config</code> instance can own its values.</p>
|
||
<p>With our new knowledge about iterators, we can change the <code>new</code> function to
|
||
take ownership of an iterator as its argument instead of borrowing a slice.
|
||
We’ll use the iterator functionality instead of the code that checks the length
|
||
of the slice and indexes into specific locations. This will clarify what the
|
||
<code>Config::new</code> function is doing because the iterator will access the values.</p>
|
||
<p>Once <code>Config::new</code> takes ownership of the iterator and stops using indexing
|
||
operations that borrow, we can move the <code>String</code> values from the iterator into
|
||
<code>Config</code> rather than calling <code>clone</code> and making a new allocation.</p>
|
||
<h4><a class="header" href="#using-the-returned-iterator-directly" id="using-the-returned-iterator-directly">Using the Returned Iterator Directly</a></h4>
|
||
<p>Open your I/O project’s <em>src/main.rs</em> file, which should look like this:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore">fn main() {
|
||
let args: Vec<String> = env::args().collect();
|
||
|
||
let config = Config::new(&args).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {}", err);
|
||
process::exit(1);
|
||
});
|
||
|
||
// --snip--
|
||
}
|
||
</code></pre>
|
||
<p>We’ll change the start of the <code>main</code> function that we had in Listing 12-24 to
|
||
the code in Listing 13-25. This won’t compile until we update <code>Config::new</code> as
|
||
well.</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore">fn main() {
|
||
let config = Config::new(env::args()).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {}", err);
|
||
process::exit(1);
|
||
});
|
||
|
||
// --snip--
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 13-25: Passing the return value of <code>env::args</code> to
|
||
<code>Config::new</code></span></p>
|
||
<p>The <code>env::args</code> function returns an iterator! Rather than collecting the
|
||
iterator values into a vector and then passing a slice to <code>Config::new</code>, now
|
||
we’re passing ownership of the iterator returned from <code>env::args</code> to
|
||
<code>Config::new</code> directly.</p>
|
||
<p>Next, we need to update the definition of <code>Config::new</code>. In your I/O project’s
|
||
<em>src/lib.rs</em> file, let’s change the signature of <code>Config::new</code> to look like
|
||
Listing 13-26. This still won’t compile because we need to update the function
|
||
body.</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">impl Config {
|
||
pub fn new(mut args: std::env::Args) -> Result<Config, &'static str> {
|
||
// --snip--
|
||
</code></pre>
|
||
<p><span class="caption">Listing 13-26: Updating the signature of <code>Config::new</code> to
|
||
expect an iterator</span></p>
|
||
<p>The standard library documentation for the <code>env::args</code> function shows that the
|
||
type of the iterator it returns is <code>std::env::Args</code>. We’ve updated the
|
||
signature of the <code>Config::new</code> function so the parameter <code>args</code> has the type
|
||
<code>std::env::Args</code> instead of <code>&[String]</code>. Because we’re taking ownership of
|
||
<code>args</code> and we’ll be mutating <code>args</code> by iterating over it, we can add the <code>mut</code>
|
||
keyword into the specification of the <code>args</code> parameter to make it mutable.</p>
|
||
<h4><a class="header" href="#using-iterator-trait-methods-instead-of-indexing" id="using-iterator-trait-methods-instead-of-indexing">Using <code>Iterator</code> Trait Methods Instead of Indexing</a></h4>
|
||
<p>Next, we’ll fix the body of <code>Config::new</code>. The standard library documentation
|
||
also mentions that <code>std::env::Args</code> implements the <code>Iterator</code> trait, so we know
|
||
we can call the <code>next</code> method on it! Listing 13-27 updates the code from
|
||
Listing 12-23 to use the <code>next</code> method:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><pre class="playpen"><code class="language-rust"><span class="boring">fn main() {}
|
||
</span><span class="boring">use std::env;
|
||
</span><span class="boring">
|
||
</span><span class="boring">struct Config {
|
||
</span><span class="boring"> query: String,
|
||
</span><span class="boring"> filename: String,
|
||
</span><span class="boring"> case_sensitive: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Config {
|
||
pub fn new(mut args: std::env::Args) -> Result<Config, &'static str> {
|
||
args.next();
|
||
|
||
let query = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a query string"),
|
||
};
|
||
|
||
let filename = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a file name"),
|
||
};
|
||
|
||
let case_sensitive = env::var("CASE_INSENSITIVE").is_err();
|
||
|
||
Ok(Config { query, filename, case_sensitive })
|
||
}
|
||
}
|
||
</code></pre></pre>
|
||
<p><span class="caption">Listing 13-27: Changing the body of <code>Config::new</code> to use
|
||
iterator methods</span></p>
|
||
<p>Remember that the first value in the return value of <code>env::args</code> is the name of
|
||
the program. We want to ignore that and get to the next value, so first we call
|
||
<code>next</code> and do nothing with the return value. Second, we call <code>next</code> to get the
|
||
value we want to put in the <code>query</code> field of <code>Config</code>. If <code>next</code> returns a
|
||
<code>Some</code>, we use a <code>match</code> to extract the value. If it returns <code>None</code>, it means
|
||
not enough arguments were given and we return early with an <code>Err</code> value. We do
|
||
the same thing for the <code>filename</code> value.</p>
|
||
<h3><a class="header" href="#making-code-clearer-with-iterator-adaptors" id="making-code-clearer-with-iterator-adaptors">Making Code Clearer with Iterator Adaptors</a></h3>
|
||
<p>We can also take advantage of iterators in the <code>search</code> function in our I/O
|
||
project, which is reproduced here in Listing 13-28 as it was in Listing 12-19:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
let mut results = Vec::new();
|
||
|
||
for line in contents.lines() {
|
||
if line.contains(query) {
|
||
results.push(line);
|
||
}
|
||
}
|
||
|
||
results
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 13-28: The implementation of the <code>search</code>
|
||
function from Listing 12-19</span></p>
|
||
<p>We can write this code in a more concise way using iterator adaptor methods.
|
||
Doing so also lets us avoid having a mutable intermediate <code>results</code> vector. The
|
||
functional programming style prefers to minimize the amount of mutable state to
|
||
make code clearer. Removing the mutable state might enable a future enhancement
|
||
to make searching happen in parallel, because we wouldn’t have to manage
|
||
concurrent access to the <code>results</code> vector. Listing 13-29 shows this change:</p>
|
||
<p><span class="filename">Filename: src/lib.rs</span></p>
|
||
<pre><code class="language-rust ignore">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
contents.lines()
|
||
.filter(|line| line.contains(query))
|
||
.collect()
|
||
}
|
||
</code></pre>
|
||
<p><span class="caption">Listing 13-29: Using iterator adaptor methods in the
|
||
implementation of the <code>search</code> function</span></p>
|
||
<p>Recall that the purpose of the <code>search</code> function is to return all lines in
|
||
<code>contents</code> that contain the <code>query</code>. Similar to the <code>filter</code> example in Listing
|
||
13-19, this code uses the <code>filter</code> adaptor to keep only the lines that
|
||
<code>line.contains(query)</code> returns <code>true</code> for. We then collect the matching lines
|
||
into another vector with <code>collect</code>. Much simpler! Feel free to make the same
|
||
change to use iterator methods in the <code>search_case_insensitive</code> function as
|
||
well.</p>
|
||
<p>The next logical question is which style you should choose in your own code and
|
||
why: the original implementation in Listing 13-28 or the version using
|
||
iterators in Listing 13-29. Most Rust programmers prefer to use the iterator
|
||
style. It’s a bit tougher to get the hang of at first, but once you get a feel
|
||
for the various iterator adaptors and what they do, iterators can be easier to
|
||
understand. Instead of fiddling with the various bits of looping and building
|
||
new vectors, the code focuses on the high-level objective of the loop. This
|
||
abstracts away some of the commonplace code so it’s easier to see the concepts
|
||
that are unique to this code, such as the filtering condition each element in
|
||
the iterator must pass.</p>
|
||
<p>But are the two implementations truly equivalent? The intuitive assumption
|
||
might be that the more low-level loop will be faster. Let’s talk about
|
||
performance.</p>
|
||
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