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<h1 class="menu-title">The Rust Programming Language</h1>
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<main>
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<h2 id="improving-our-io-project"><a class="header" href="#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::build</code> function and the <code>search</code> function.</p>
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<h3 id="removing-a-clone-using-an-iterator"><a class="header" href="#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
|
||
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 13-17,
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we’ve reproduced the implementation of the <code>Config::build</code> function as it was
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in Listing 12-23.</p>
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||
<figure class="listing" id="listing-13-17">
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<span class="file-name">Filename: src/main.rs</span>
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<pre><code class="language-rust ignore"><span class="boring">use std::env;
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</span><span class="boring">use std::error::Error;
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</span><span class="boring">use std::fs;
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</span><span class="boring">use std::process;
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</span><span class="boring">
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</span><span class="boring">use minigrep::{search, search_case_insensitive};
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</span><span class="boring">
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</span><span class="boring">fn main() {
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</span><span class="boring"> let args: Vec<String> = env::args().collect();
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</span><span class="boring">
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</span><span class="boring"> let config = Config::build(&args).unwrap_or_else(|err| {
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</span><span class="boring"> println!("Problem parsing arguments: {err}");
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</span><span class="boring"> process::exit(1);
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</span><span class="boring"> });
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</span><span class="boring">
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</span><span class="boring"> if let Err(e) = run(config) {
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</span><span class="boring"> println!("Application error: {e}");
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</span><span class="boring"> process::exit(1);
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</span><span class="boring"> }
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</span><span class="boring">}
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</span><span class="boring">
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</span><span class="boring">pub struct Config {
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</span><span class="boring"> pub query: String,
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</span><span class="boring"> pub file_path: String,
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</span><span class="boring"> pub ignore_case: bool,
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</span><span class="boring">}
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</span><span class="boring">
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</span>impl Config {
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fn build(args: &[String]) -> Result<Config, &'static str> {
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if args.len() < 3 {
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return Err("not enough arguments");
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}
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let query = args[1].clone();
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let file_path = args[2].clone();
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let ignore_case = env::var("IGNORE_CASE").is_ok();
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Ok(Config {
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query,
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file_path,
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ignore_case,
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})
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}
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}
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<span class="boring">
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</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
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</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
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||
</span><span class="boring">
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</span><span class="boring"> let results = if config.ignore_case {
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</span><span class="boring"> search_case_insensitive(&config.query, &contents)
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</span><span class="boring"> } else {
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</span><span class="boring"> search(&config.query, &contents)
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</span><span class="boring"> };
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</span><span class="boring">
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</span><span class="boring"> for line in results {
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||
</span><span class="boring"> println!("{line}");
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</span><span class="boring"> }
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</span><span class="boring">
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</span><span class="boring"> Ok(())
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</span><span class="boring">}</span></code></pre>
|
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<figcaption><a href="#listing-13-17">Listing 13-17</a>: Reproduction of the <code>Config::build</code> function from Listing 12-23</figcaption>
|
||
</figure>
|
||
<p>At the time, we said not to worry about the inefficient <code>clone</code> calls because
|
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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>build</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>file_path</code> fields of <code>Config</code> so that the <code>Config</code> instance can own its
|
||
values.</p>
|
||
<p>With our new knowledge about iterators, we can change the <code>build</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::build</code> function is doing because the iterator will access the values.</p>
|
||
<p>Once <code>Config::build</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 id="using-the-returned-iterator-directly"><a class="header" href="#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"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span>fn main() {
|
||
let args: Vec<String> = env::args().collect();
|
||
|
||
let config = Config::build(&args).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {err}");
|
||
process::exit(1);
|
||
});
|
||
|
||
// --snip--
|
||
<span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> eprintln!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span>}
|
||
<span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Config {
|
||
</span><span class="boring"> fn build(args: &[String]) -> Result<Config, &'static str> {
|
||
</span><span class="boring"> if args.len() < 3 {
|
||
</span><span class="boring"> return Err("not enough arguments");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let query = args[1].clone();
|
||
</span><span class="boring"> let file_path = args[2].clone();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(Config {
|
||
</span><span class="boring"> query,
|
||
</span><span class="boring"> file_path,
|
||
</span><span class="boring"> ignore_case,
|
||
</span><span class="boring"> })
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<p>We’ll first change the start of the <code>main</code> function that we had in Listing
|
||
12-24 to the code in Listing 13-18, which this time uses an iterator. This
|
||
won’t compile until we update <code>Config::build</code> as well.</p>
|
||
<figure class="listing" id="listing-13-18">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span>fn main() {
|
||
let config = Config::build(env::args()).unwrap_or_else(|err| {
|
||
eprintln!("Problem parsing arguments: {err}");
|
||
process::exit(1);
|
||
});
|
||
|
||
// --snip--
|
||
<span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> eprintln!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span>}
|
||
<span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Config {
|
||
</span><span class="boring"> fn build(args: &[String]) -> Result<Config, &'static str> {
|
||
</span><span class="boring"> if args.len() < 3 {
|
||
</span><span class="boring"> return Err("not enough arguments");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let query = args[1].clone();
|
||
</span><span class="boring"> let file_path = args[2].clone();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(Config {
|
||
</span><span class="boring"> query,
|
||
</span><span class="boring"> file_path,
|
||
</span><span class="boring"> ignore_case,
|
||
</span><span class="boring"> })
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-13-18">Listing 13-18</a>: Passing the return value of <code>env::args</code> to <code>Config::build</code></figcaption>
|
||
</figure>
|
||
<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::build</code>, now
|
||
we’re passing ownership of the iterator returned from <code>env::args</code> to
|
||
<code>Config::build</code> directly.</p>
|
||
<p>Next, we need to update the definition of <code>Config::build</code>. Let’s change the
|
||
signature of <code>Config::build</code> to look like Listing 13-19. This still won’t
|
||
compile, because we need to update the function body.</p>
|
||
<figure class="listing" id="listing-13-19">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let config = Config::build(env::args()).unwrap_or_else(|err| {
|
||
</span><span class="boring"> eprintln!("Problem parsing arguments: {err}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> });
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> eprintln!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Config {
|
||
fn build(
|
||
mut args: impl Iterator<Item = String>,
|
||
) -> Result<Config, &'static str> {
|
||
// --snip--
|
||
<span class="boring"> if args.len() < 3 {
|
||
</span><span class="boring"> return Err("not enough arguments");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let query = args[1].clone();
|
||
</span><span class="boring"> let file_path = args[2].clone();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(Config {
|
||
</span><span class="boring"> query,
|
||
</span><span class="boring"> file_path,
|
||
</span><span class="boring"> ignore_case,
|
||
</span><span class="boring"> })
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-13-19">Listing 13-19</a>: Updating the signature of <code>Config::build</code> to expect an iterator</figcaption>
|
||
</figure>
|
||
<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>, and that type implements
|
||
the <code>Iterator</code> trait and returns <code>String</code> values.</p>
|
||
<p>We’ve updated the signature of the <code>Config::build</code> function so that the
|
||
parameter <code>args</code> has a generic type with the trait bounds <code>impl Iterator<Item = String></code> instead of <code>&[String]</code>. This usage of the <code>impl Trait</code> syntax we
|
||
discussed in the <a href="ch10-02-traits.html#traits-as-parameters">“Using Traits as Parameters”</a><!-- ignore -->
|
||
section of Chapter 10 means that <code>args</code> can be any type that implements the
|
||
<code>Iterator</code> trait and returns <code>String</code> items.</p>
|
||
<p>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>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="using-iterator-trait-methods-instead-of-indexing"></a></p>
|
||
<h4 id="using-iterator-trait-methods"><a class="header" href="#using-iterator-trait-methods">Using <code>Iterator</code> Trait Methods</a></h4>
|
||
<p>Next, we’ll fix the body of <code>Config::build</code>. Because <code>args</code> implements the
|
||
<code>Iterator</code> trait, we know we can call the <code>next</code> method on it! Listing 13-20
|
||
updates the code from Listing 12-23 to use the <code>next</code> method.</p>
|
||
<figure class="listing" id="listing-13-20">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore noplayground"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let config = Config::build(env::args()).unwrap_or_else(|err| {
|
||
</span><span class="boring"> eprintln!("Problem parsing arguments: {err}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> });
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> eprintln!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Config {
|
||
fn build(
|
||
mut args: impl Iterator<Item = String>,
|
||
) -> Result<Config, &'static str> {
|
||
args.next();
|
||
|
||
let query = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a query string"),
|
||
};
|
||
|
||
let file_path = match args.next() {
|
||
Some(arg) => arg,
|
||
None => return Err("Didn't get a file path"),
|
||
};
|
||
|
||
let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
|
||
Ok(Config {
|
||
query,
|
||
file_path,
|
||
ignore_case,
|
||
})
|
||
}
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-13-20">Listing 13-20</a>: Changing the body of <code>Config::build</code> to use iterator methods</figcaption>
|
||
</figure>
|
||
<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. Then, 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
|
||
<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>file_path</code> value.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="making-code-clearer-with-iterator-adapters"></a></p>
|
||
<h3 id="clarifying-code-with-iterator-adapters"><a class="header" href="#clarifying-code-with-iterator-adapters">Clarifying Code with Iterator Adapters</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 13-21 as it was in Listing 12-19.</p>
|
||
<figure class="listing" id="listing-13-21">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<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
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">#[cfg(test)]
|
||
</span><span class="boring">mod tests {
|
||
</span><span class="boring"> use super::*;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> #[test]
|
||
</span><span class="boring"> fn one_result() {
|
||
</span><span class="boring"> let query = "duct";
|
||
</span><span class="boring"> let contents = "\
|
||
</span><span class="boring">Rust:
|
||
</span><span class="boring">safe, fast, productive.
|
||
</span><span class="boring">Pick three.";
|
||
</span><span class="boring">
|
||
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-13-21">Listing 13-21</a>: The implementation of the <code>search</code> function from Listing 12-19</figcaption>
|
||
</figure>
|
||
<p>We can write this code in a more concise way using iterator adapter 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 13-22 shows this change.</p>
|
||
<figure class="listing" id="listing-13-22">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<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()
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">pub fn search_case_insensitive<'a>(
|
||
</span><span class="boring"> query: &str,
|
||
</span><span class="boring"> contents: &'a str,
|
||
</span><span class="boring">) -> Vec<&'a str> {
|
||
</span><span class="boring"> let query = query.to_lowercase();
|
||
</span><span class="boring"> let mut results = Vec::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in contents.lines() {
|
||
</span><span class="boring"> if line.to_lowercase().contains(&query) {
|
||
</span><span class="boring"> results.push(line);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> results
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">#[cfg(test)]
|
||
</span><span class="boring">mod tests {
|
||
</span><span class="boring"> use super::*;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> #[test]
|
||
</span><span class="boring"> fn case_sensitive() {
|
||
</span><span class="boring"> let query = "duct";
|
||
</span><span class="boring"> let contents = "\
|
||
</span><span class="boring">Rust:
|
||
</span><span class="boring">safe, fast, productive.
|
||
</span><span class="boring">Pick three.
|
||
</span><span class="boring">Duct tape.";
|
||
</span><span class="boring">
|
||
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> #[test]
|
||
</span><span class="boring"> fn case_insensitive() {
|
||
</span><span class="boring"> let query = "rUsT";
|
||
</span><span class="boring"> let contents = "\
|
||
</span><span class="boring">Rust:
|
||
</span><span class="boring">safe, fast, productive.
|
||
</span><span class="boring">Pick three.
|
||
</span><span class="boring">Trust me.";
|
||
</span><span class="boring">
|
||
</span><span class="boring"> assert_eq!(
|
||
</span><span class="boring"> vec!["Rust:", "Trust me."],
|
||
</span><span class="boring"> search_case_insensitive(query, contents)
|
||
</span><span class="boring"> );
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-13-22">Listing 13-22</a>: Using iterator adapter methods in the implementation of the <code>search</code> function</figcaption>
|
||
</figure>
|
||
<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
|
||
13-16, this code uses the <code>filter</code> adapter to keep only the lines for which
|
||
<code>line.contains(query)</code> returns <code>true</code>. 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>For a further improvement, return an iterator from the <code>search</code> function by
|
||
removing the call to <code>collect</code> and changing the return type to <code>impl Iterator<Item = &'a str></code> so that the function becomes an iterator adapter.
|
||
Note that you’ll also need to update the tests! Search through a large file
|
||
using your <code>minigrep</code> tool before and after making this change to observe the
|
||
difference in behavior. Before this change, the program won’t print any results
|
||
until it has collected all of the results, but after the change, the results
|
||
will be printed as each matching line is found because the <code>for</code> loop in the
|
||
<code>run</code> function is able to take advantage of the laziness of the iterator.</p>
|
||
<!-- Old headings. Do not remove or links may break. -->
|
||
<p><a id="choosing-between-loops-or-iterators"></a></p>
|
||
<h3 id="choosing-between-loops-and-iterators"><a class="header" href="#choosing-between-loops-and-iterators">Choosing Between Loops and Iterators</a></h3>
|
||
<p>The next logical question is which style you should choose in your own code and
|
||
why: the original implementation in Listing 13-21 or the version using
|
||
iterators in Listing 13-22 (assuming we’re collecting all the results before
|
||
returning them rather than returning the iterator). 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 adapters 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 that
|
||
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 lower-level loop will be faster. Let’s talk about performance.</p>
|
||
|
||
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