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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"><
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<h2><a class="header" href="#reading-a-file" id="reading-a-file">Reading a File</a></h2>
<p>Now we’ll add functionality to read the file that is specified in the
<code>filename</code> command line argument. First, we need a sample file to test it with:
the best kind of file to use to make sure <code>minigrep</code> is working is one with a
small amount of text over multiple lines with some repeated words. Listing 12-3
has an Emily Dickinson poem that will work well! Create a file called
<em>poem.txt</em> at the root level of your project, and enter the poem “I’m Nobody!
Who are you?”</p>
<p><span class="filename">Filename: poem.txt</span></p>
<pre><code class="language-text">I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us - don't tell!
They'd banish us, you know.
How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
</code></pre>
<p><span class="caption">Listing 12-3: A poem by Emily Dickinson makes a good test
case</span></p>
<p>With the text in place, edit <em>src/main.rs</em> and add code to read the file, as
shown in Listing 12-4.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust should_panic">use std::env;
use std::fs;
fn main() {
<span class="boring"> let args: Vec&lt;String&gt; = env::args().collect();
</span><span class="boring">
</span><span class="boring"> let query = &amp;args[1];
</span><span class="boring"> let filename = &amp;args[2];
</span><span class="boring">
</span><span class="boring"> println!(&quot;Searching for {}&quot;, query);
</span> // --snip--
println!(&quot;In file {}&quot;, filename);
let contents = fs::read_to_string(filename)
.expect(&quot;Something went wrong reading the file&quot;);
println!(&quot;With text:\n{}&quot;, contents);
}
</code></pre></pre>
<p><span class="caption">Listing 12-4: Reading the contents of the file specified
by the second argument</span></p>
<p>First, we add another <code>use</code> statement to bring in a relevant part of the
standard library: we need <code>std::fs</code> to handle files.</p>
<p>In <code>main</code>, we’ve added a new statement: <code>fs::read_to_string</code> takes the
<code>filename</code>, opens that file, and returns a <code>Result&lt;String&gt;</code> of the file’s
contents.</p>
<p>After that statement, we’ve again added a temporary <code>println!</code> statement that
prints the value of <code>contents</code> after the file is read, so we can check that the
program is working so far.</p>
<p>Let’s run this code with any string as the first command line argument (because
we haven’t implemented the searching part yet) and the <em>poem.txt</em> file as the
second argument:</p>
<pre><code class="language-text">$ cargo run the poem.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target/debug/minigrep the poem.txt`
Searching for the
In file poem.txt
With text:
I'm nobody! Who are you?
Are you nobody, too?
Then there's a pair of us — don't tell!
They'd banish us, you know.
How dreary to be somebody!
How public, like a frog
To tell your name the livelong day
To an admiring bog!
</code></pre>
<p>Great! The code read and then printed the contents of the file. But the code
has a few flaws. The <code>main</code> function has multiple responsibilities: generally,
functions are clearer and easier to maintain if each function is responsible
for only one idea. The other problem is that we’re not handling errors as well
as we could. The program is still small, so these flaws aren’t a big problem,
but as the program grows, it will be harder to fix them cleanly. It’s good
practice to begin refactoring early on when developing a program, because it’s
much easier to refactor smaller amounts of code. We’ll do that next.</p>
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