326 lines
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326 lines
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<h1 class="menu-title">The Rust Programming Language</h1>
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<main>
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<h2 id="reading-a-file"><a class="header" href="#reading-a-file">Reading a File</a></h2>
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<p>Now we’ll add functionality to read the file specified in the <code>file_path</code>
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argument. First, we need a sample file to test it with: We’ll use a file with a
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small amount of text over multiple lines with some repeated words. Listing 12-3
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has an Emily Dickinson poem that will work well! Create a file called
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<em>poem.txt</em> at the root level of your project, and enter the poem “I’m Nobody!
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Who are you?”</p>
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<figure class="listing" id="listing-12-3">
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<span class="file-name">Filename: poem.txt</span>
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<pre><code class="language-text">I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us - don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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</code></pre>
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<figcaption><a href="#listing-12-3">Listing 12-3</a>: A poem by Emily Dickinson makes a good test case.</figcaption>
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</figure>
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<p>With the text in place, edit <em>src/main.rs</em> and add code to read the file, as
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shown in Listing 12-4.</p>
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<figure class="listing" id="listing-12-4">
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<span class="file-name">Filename: src/main.rs</span>
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<pre><code class="language-rust should_panic noplayground">use std::env;
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use std::fs;
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fn main() {
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// --snip--
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<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 query = &args[1];
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</span><span class="boring"> let file_path = &args[2];
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</span><span class="boring">
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</span><span class="boring"> println!("Searching for {query}");
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</span> println!("In file {file_path}");
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let contents = fs::read_to_string(file_path)
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.expect("Should have been able to read the file");
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println!("With text:\n{contents}");
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}</code></pre>
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<figcaption><a href="#listing-12-4">Listing 12-4</a>: Reading the contents of the file specified by the second argument</figcaption>
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</figure>
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<p>First, we bring in a relevant part of the standard library with a <code>use</code>
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statement: We need <code>std::fs</code> to handle files.</p>
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<p>In <code>main</code>, the new statement <code>fs::read_to_string</code> takes the <code>file_path</code>, opens
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that file, and returns a value of type <code>std::io::Result<String></code> that contains
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the file’s contents.</p>
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<p>After that, we again add a temporary <code>println!</code> statement that prints the value
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of <code>contents</code> after the file is read so that we can check that the program is
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working so far.</p>
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<p>Let’s run this code with any string as the first command line argument (because
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we haven’t implemented the searching part yet) and the <em>poem.txt</em> file as the
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second argument:</p>
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<pre><code class="language-console">$ cargo run -- the poem.txt
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Compiling minigrep v0.1.0 (file:///projects/minigrep)
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Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.0s
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Running `target/debug/minigrep the poem.txt`
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Searching for the
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In file poem.txt
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With text:
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I'm nobody! Who are you?
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Are you nobody, too?
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Then there's a pair of us - don't tell!
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They'd banish us, you know.
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How dreary to be somebody!
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How public, like a frog
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To tell your name the livelong day
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To an admiring bog!
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</code></pre>
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<p>Great! The code read and then printed the contents of the file. But the code
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has a few flaws. At the moment, the <code>main</code> function has multiple
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responsibilities: Generally, functions are clearer and easier to maintain if
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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 a good practice to begin refactoring early on when
|
||
developing a program because it’s much easier to refactor smaller amounts of
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code. We’ll do that next.</p>
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