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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Updates</a></li><li class="chapter-item expanded "><a href="Chapter_1.html"><strong aria-hidden="true">2.</strong> Introduction</a></li><li class="chapter-item expanded "><a href="Chapter_2.html"><strong aria-hidden="true">3.</strong> Who am I?</a></li><li class="chapter-item expanded "><a href="Chapter_3.html"><strong aria-hidden="true">4.</strong> Writing Rust in Easy English</a></li><li class="chapter-item expanded "><a href="Chapter_4.html"><strong aria-hidden="true">5.</strong> Rust Playground</a></li><li class="chapter-item expanded "><a href="Chapter_5.html"><strong aria-hidden="true">6.</strong> 🚧 and ⚠️</a></li><li class="chapter-item expanded "><a href="Chapter_6.html"><strong aria-hidden="true">7.</strong> Comments</a></li><li class="chapter-item expanded "><a href="Chapter_7.html"><strong aria-hidden="true">8.</strong> Types</a></li><li class="chapter-item expanded "><a href="Chapter_8.html"><strong aria-hidden="true">9.</strong> Type inference</a></li><li class="chapter-item expanded "><a href="Chapter_9.html"><strong aria-hidden="true">10.</strong> Printing 'hello, world!'</a></li><li class="chapter-item expanded "><a href="Chapter_10.html"><strong aria-hidden="true">11.</strong> Display and debug</a></li><li class="chapter-item expanded "><a href="Chapter_11.html"><strong aria-hidden="true">12.</strong> Mutability (changing)</a></li><li class="chapter-item expanded "><a href="Chapter_12.html"><strong aria-hidden="true">13.</strong> The stack, the heap, and pointers</a></li><li class="chapter-item expanded "><a href="Chapter_13.html"><strong aria-hidden="true">14.</strong> More about printing</a></li><li class="chapter-item expanded "><a href="Chapter_14.html"><strong aria-hidden="true">15.</strong> Strings</a></li><li class="chapter-item expanded "><a href="Chapter_15.html"><strong aria-hidden="true">16.</strong> const and static</a></li><li class="chapter-item expanded "><a href="Chapter_16.html"><strong aria-hidden="true">17.</strong> More on references</a></li><li class="chapter-item expanded "><a href="Chapter_17.html"><strong aria-hidden="true">18.</strong> Mutable references</a></li><li class="chapter-item expanded "><a href="Chapter_18.html"><strong aria-hidden="true">19.</strong> Giving references to functions</a></li><li class="chapter-item expanded "><a href="Chapter_19.html"><strong aria-hidden="true">20.</strong> Copy types</a></li><li class="chapter-item expanded "><a href="Chapter_20.html"><strong aria-hidden="true">21.</strong> Collection types</a></li><li class="chapter-item expanded "><a href="Chapter_21.html"><strong aria-hidden="true">22.</strong> Vectors</a></li><li class="chapter-item expanded "><a href="Chapter_22.html"><strong aria-hidden="true">23.</strong> Tuples</a></li><li class="chapter-item expanded "><a href="Chapter_23.html"><strong aria-hidden="true">24.</strong> Control flow</a></li><li class="chapter-item expanded "><a href="Chapter_24.html"><strong aria-hidden="true">25.</strong> Structs</a></li><li class="chapter-item expanded "><a href="Chapter_25.html"><strong aria-hidden="true">26.</strong> Enums</a></li><li class="chapter-item expanded "><a href="Chapter_26.html"><strong aria-hidden="true">27.</strong> Loops</a></li><li class="chapter-item expanded "><a href="Chapter_27.html"><strong aria-hidden="true">28.</strong> Implementing structs and enums</a></li><li class="chapter-item expanded "><a href="Chapter_28.html"><strong aria-hidden="true">29.</strong> Destructuring</a></li><li class="chapter-item expanded "><a href="Chapter_29.html"><strong aria-hidden="true">30.</strong> References and the dot operator</a></li><li class="chapter-item expanded "><a href="Chapter_30.html"><strong aria-hidden="true">31.</strong> Generics</a></li><li class="chapter-item expanded "><a href="Chapter_31.html"><strong aria-hidden="true">32.</strong> Option and Result</a></li><li class="chapter-item expanded "><a href="Chapter_32.html"><strong aria-hidden="true">33.</strong> Other collections</a></li><li class="chapter-item expanded "><a href="Chapter_33.html"><strong aria-hidden="true">34.</strong> The ? operator</a></li><li class="chapter-item expanded "><a href="Chapter_34.html"><strong aria-hidden="true">35.</strong> Traits</a></li><li class="chapter-item expanded "><a href="Chapter_35.html"><strong aria-hidden="true">36.</strong> Chaining methods</a></li><li class="chapter-item expanded "><a href="Chapter_36.html"><strong aria-hidden="true">37.</strong> Iterators</a></li><li class="chapter-item expanded "><a href="Chapter_37.html"><strong aria-hidden="true">38.</strong> Closures</a></li><li class="chapter-item expanded "><a href="Chapter_38.html"><strong aria-hidden="true">39.</strong> The dbg! macro and .inspect</a></li><li class="chapter-item expanded "><a href="Chapter_39.html"><strong aria-hidden="true">40.</strong> Types of &str</a></li><li class="chapter-item expanded "><a href="Chapter_40.html"><strong aria-hidden="true">41.</strong> Lifetimes</a></li><li class="chapter-item expanded "><a href="Chapter_41.html"><strong aria-hidden="true">42.</strong> Interior mutability</a></li><li class="chapter-item expanded "><a href="Chapter_42.html"><strong aria-hidden="true">43.</strong> Cow</a></li><li class="chapter-item expanded "><a href="Chapter_43.html"><strong aria-hidden="true">44.</strong> Type aliases</a></li><li class="chapter-item expanded "><a href="Chapter_44.html"><strong aria-hidden="true">45.</strong> The todo! macro</a></li><li class="chapter-item expanded "><a href="Chapter_45.html"><strong aria-hidden="true">46.</strong> Rc</a></li><li class="chapter-item expanded "><a href="Chapter_46.html"><strong aria-hidden="true">47.</strong> Multiple threads</a></li><li class="chapter-item expanded "><a href="Chapter_47.html"><strong aria-hidden="true">48.</strong> Closures in functions</a></li><li class="chapter-item expanded "><a href="Chapter_48.html"><strong aria-hidden="true">49.</strong> impl Trait</a></li><li class="chapter-item expanded "><a href="Chapter_49.html"><strong aria-hidden="true">50.</strong> Arc</a></li><li class="chapter-item expanded "><a href="Chapter_50.html"><strong aria-hidden="true">51.</strong> Channels</a></li><li class="chapter-item expanded "><a href="Chapter_51.html"><strong aria-hidden="true">52.</strong> Reading Rust documentation</a></li><li class="chapter-item expanded "><a href="Chapter_52.html"><strong aria-hidden="true">53.</strong> Attributes</a></li><li class="chapter-item expanded "><a href="Chapter_53.html"><strong aria-hidden="true">54.</strong> Box</a></li><li class="chapter-item expanded "><a href="Chapter_54.html"><strong aria-hidden="true">55.</strong> Box around traits</a></li><li class="chapter-item expanded "><a href="Chapter_55.html"><strong aria-hidden="true">56.</strong> Default and the builder pattern</a></li><li class="chapter-item expanded "><a href="Chapter_56.html"><strong aria-hidden="true">57.</strong> Deref and DerefMut</a></li><li class="chapter-item expanded "><a href="Chapter_57.html"><strong aria-hidden="true">58.</strong> Crates and modules</a></li><li class="chapter-item expanded "><a href="Chapter_58.html"><strong aria-hidden="true">59.</strong> Testing</a></li><li class="chapter-item expanded "><a href="Chapter_59.html"><strong aria-hidden="true">60.</strong> External crates</a></li><li class="chapter-item expanded "><a href="Chapter_60.html"><strong aria-hidden="true">61.</strong> A tour of the standard library</a></li><li class="chapter-item expanded "><a href="Chapter_61.html"><strong aria-hidden="true">62.</strong> Writing macros</a></li><li class="chapter-item expanded "><a href="Chapter_62.html"><strong aria-hidden="true">63.</strong> cargo</a></li><li class="chapter-item expanded "><a href="Chapter_63.html" class="active"><strong aria-hidden="true">64.</strong> Taking user input</a></li><li class="chapter-item expanded "><a href="Chapter_64.html"><strong aria-hidden="true">65.</strong> Using files</a></li><li class="chapter-item expanded "><a href="Chapter_65.html"><strong aria-hidden="true">66.</strong> cargo doc</a></li><li class="chapter-item expanded "><a href="Chapter_66.html"><strong aria-hidden="true">67.</strong> The end?</a></li></ol>
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<h1 class="menu-title">Easy Rust</h1>
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<h2 id="taking-user-input"><a class="header" href="#taking-user-input">Taking user input</a></h2>
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<p>One easy way to take input from the user is with <code>std::io::stdin</code>. This means "standard in", which is the input from the keyboard. With <code>stdin()</code> you can get user input, but then you will want to put it in a <code>&mut String</code> with <code>.read_line()</code>. Here is a simple example of that, but it both works and doesn't work:</p>
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<pre><pre class="playground"><code class="language-rust">use std::io;
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fn main() {
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println!("Please type something, or x to escape:");
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let mut input_string = String::new();
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while input_string != "x" { // This is the part that doesn't work right
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input_string.clear(); // First clear the String. Otherwise it will keep adding to it
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io::stdin().read_line(&mut input_string).unwrap(); // Get the stdin from the user, and put it in read_string
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println!("You wrote {}", input_string);
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}
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println!("See you later!");
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}
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</code></pre></pre>
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<p>Here is what an output looks like:</p>
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<pre><code class="language-text">Please type something, or x to escape:
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something
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You wrote something
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Something else
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You wrote Something else
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x
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You wrote x
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x
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You wrote x
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x
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You wrote x
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</code></pre>
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<p>It takes our input and gives it back, and it even knows that we typed <code>x</code>. But it doesn't exit the program. The only way to get out is to close the window, or type ctrl and c. Let's change the <code>{}</code> to <code>{:?}</code> in <code>println!</code> to get more information (or you could use <code>dbg!(&input_string)</code> if you like that macro). Now it says:</p>
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<pre><code class="language-text">Please type something, or x to escape:
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something
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You wrote "something\r\n"
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Something else
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You wrote "Something else\r\n"
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x
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You wrote "x\r\n"
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x
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You wrote "x\r\n"
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</code></pre>
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<p>This is because the keyboard input is actually not just <code>something</code>, it is <code>something</code> and the <code>Enter</code> key. There is an easy method to fix this called <code>.trim()</code>, which removes all the whitespace. Whitespace, by the way, is all <a href="https://doc.rust-lang.org/reference/whitespace.html">these characters</a>:</p>
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<pre><code class="language-text">U+0009 (horizontal tab, '\t')
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U+000A (line feed, '\n')
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U+000B (vertical tab)
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U+000C (form feed)
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U+000D (carriage return, '\r')
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U+0020 (space, ' ')
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U+0085 (next line)
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U+200E (left-to-right mark)
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U+200F (right-to-left mark)
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U+2028 (line separator)
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U+2029 (paragraph separator)
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</code></pre>
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<p>So that will turn <code>x\r\n</code> into just <code>x</code>. Now it works:</p>
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<pre><pre class="playground"><code class="language-rust">use std::io;
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fn main() {
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println!("Please type something, or x to escape:");
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let mut input_string = String::new();
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while input_string.trim() != "x" {
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input_string.clear();
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io::stdin().read_line(&mut input_string).unwrap();
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println!("You wrote {}", input_string);
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}
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println!("See you later!");
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}
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</code></pre></pre>
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<p>Now it will print:</p>
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<pre><code class="language-text">Please type something, or x to escape:
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something
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You wrote something
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Something
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You wrote Something
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x
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You wrote x
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See you later!
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</code></pre>
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<p>There is another kind of user input called <code>std::env::Args</code> (env means environment). <code>Args</code> is what the user types when starting the program. There is actually always at least one <code>Arg</code> in a program. Let's write a program that only prints them using <code>std::env::args()</code> to see what they are.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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println!("{:?}", std::env::args());
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}
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</code></pre></pre>
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<p>If we write <code>cargo run</code> then it prints something like this:</p>
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<pre><code class="language-text">Args { inner: ["target\\debug\\rust_book.exe"] }
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</code></pre>
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<p>Let's give it more input and see what it does. We'll type <code>cargo run but with some extra words</code>. It gives us:</p>
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<pre><code class="language-text">Args { inner: ["target\\debug\\rust_book.exe", "but", "with", "some", "extra", "words"] }
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</code></pre>
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<p>Interesting. And when we look at <a href="https://doc.rust-lang.org/std/env/struct.Args.html">the page for Args</a>, we see that it implements <code>IntoIterator</code>. That means we can do all the things we know about iterators to read and change it. Let's try this:</p>
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<pre><pre class="playground"><code class="language-rust">use std::env::args;
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fn main() {
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let input = args();
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for entry in input {
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println!("You entered: {}", entry);
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}
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}
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</code></pre></pre>
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<p>Now it says:</p>
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<pre><code class="language-text">You entered: target\debug\rust_book.exe
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You entered: but
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You entered: with
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You entered: some
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You entered: extra
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You entered: words
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</code></pre>
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<p>You can see that the first argument is always the program name, so you will often want to skip it, like this:</p>
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<pre><pre class="playground"><code class="language-rust">use std::env::args;
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fn main() {
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let input = args();
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input.skip(1).for_each(|item| {
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println!("You wrote {}, which in capital letters is {}", item, item.to_uppercase());
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})
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}
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</code></pre></pre>
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<p>That will print:</p>
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<pre><code class="language-text">You wrote but, which in capital letters is BUT
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You wrote with, which in capital letters is WITH
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You wrote some, which in capital letters is SOME
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You wrote extra, which in capital letters is EXTRA
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You wrote words, which in capital letters is WORDS
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</code></pre>
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<p>One common use for <code>Args</code> is for user settings. You can make sure that the user writes the input you need, and only run the program if it's right. Here's a small program that either makes letters big (capital) or small (lowercase):</p>
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<pre><pre class="playground"><code class="language-rust">use std::env::args;
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enum Letters {
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Capitalize,
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Lowercase,
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Nothing,
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}
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fn main() {
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let mut changes = Letters::Nothing;
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let input = args().collect::<Vec<_>>();
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if input.len() > 2 {
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match input[1].as_str() {
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"capital" => changes = Letters::Capitalize,
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"lowercase" => changes = Letters::Lowercase,
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_ => {}
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}
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}
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for word in input.iter().skip(2) {
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match changes {
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Letters::Capitalize => println!("{}", word.to_uppercase()),
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Letters::Lowercase => println!("{}", word.to_lowercase()),
|
|
_ => println!("{}", word)
|
|
}
|
|
}
|
|
|
|
}
|
|
</code></pre></pre>
|
|
<p>Here are some examples of what it gives:</p>
|
|
<p>Input: <code>cargo run please make capitals</code>:</p>
|
|
<pre><code class="language-text">make capitals
|
|
</code></pre>
|
|
<p>Input: <code>cargo run capital</code>:</p>
|
|
<pre><code class="language-text">// Nothing here...
|
|
</code></pre>
|
|
<p>Input: <code>cargo run capital I think I understand now</code>:</p>
|
|
<pre><code class="language-text">I
|
|
THINK
|
|
I
|
|
UNDERSTAND
|
|
NOW
|
|
</code></pre>
|
|
<p>Input: <code>cargo run lowercase Does this work too?</code></p>
|
|
<pre><code class="language-text">does
|
|
this
|
|
work
|
|
too?
|
|
</code></pre>
|
|
<p>Besides <code>Args</code> given by the user, available in <code>std::env::args()</code>, there are also <code>Vars</code> which are the system variables. Those are the basic settings for the program that the user didn't type in. You can use <code>std::env::vars()</code> to see them all as a <code>(String, String)</code>. There are very many. For example:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
for item in std::env::vars() {
|
|
println!("{:?}", item);
|
|
}
|
|
}
|
|
</code></pre></pre>
|
|
<p>Just doing this shows you all the information about your user session. It will show information like this:</p>
|
|
<pre><code class="language-text">("CARGO", "/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/cargo")
|
|
("CARGO_HOME", "/playground/.cargo")
|
|
("CARGO_MANIFEST_DIR", "/playground")
|
|
("CARGO_PKG_AUTHORS", "The Rust Playground")
|
|
("CARGO_PKG_DESCRIPTION", "")
|
|
("CARGO_PKG_HOMEPAGE", "")
|
|
("CARGO_PKG_NAME", "playground")
|
|
("CARGO_PKG_REPOSITORY", "")
|
|
("CARGO_PKG_VERSION", "0.0.1")
|
|
("CARGO_PKG_VERSION_MAJOR", "0")
|
|
("CARGO_PKG_VERSION_MINOR", "0")
|
|
("CARGO_PKG_VERSION_PATCH", "1")
|
|
("CARGO_PKG_VERSION_PRE", "")
|
|
("DEBIAN_FRONTEND", "noninteractive")
|
|
("HOME", "/playground")
|
|
("HOSTNAME", "f94c15b8134b")
|
|
("LD_LIBRARY_PATH", "/playground/target/debug/build/backtrace-sys-3ec4c973f371c302/out:/playground/target/debug/build/libsqlite3-sys-fbddfbb9b241dacb/out:/playground/target/debug/build/ring-cadba5e583648abb/out:/playground/target/debug/deps:/playground/target/debug:/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib:/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib")
|
|
("PATH", "/playground/.cargo/bin:/playground/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin")
|
|
("PLAYGROUND_EDITION", "2018")
|
|
("PLAYGROUND_TIMEOUT", "10")
|
|
("PWD", "/playground")
|
|
("RUSTUP_HOME", "/playground/.rustup")
|
|
("RUSTUP_TOOLCHAIN", "stable-x86_64-unknown-linux-gnu")
|
|
("RUST_RECURSION_COUNT", "1")
|
|
("SHLVL", "1")
|
|
("SSL_CERT_DIR", "/usr/lib/ssl/certs")
|
|
("SSL_CERT_FILE", "/usr/lib/ssl/certs/ca-certificates.crt")
|
|
("USER", "playground")
|
|
("_", "/usr/bin/timeout")
|
|
</code></pre>
|
|
<p>So if you need this information, <code>Vars</code> is what you want.</p>
|
|
<p>The easiest way to get a single <code>Var</code> is by using the <code>env!</code> macro. You just give it the name of the variable, and it will give you a <code>&str</code> with the value. It won't work if the variable is spelled wrong or does not exist, so if you aren't sure then use <code>option_env!</code> instead. If we write this on the Playground:</p>
|
|
<pre><pre class="playground"><code class="language-rust">fn main() {
|
|
println!("{}", env!("USER"));
|
|
println!("{}", option_env!("ROOT").unwrap_or("Can't find ROOT"));
|
|
println!("{}", option_env!("CARGO").unwrap_or("Can't find CARGO"));
|
|
}
|
|
</code></pre></pre>
|
|
<p>then we get the output:</p>
|
|
<pre><code class="language-text">playground
|
|
Can't find ROOT
|
|
/playground/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/bin/cargo
|
|
</code></pre>
|
|
<p>So <code>option_env!</code> is always going to be the safer macro. <code>env!</code> is better if you actually want the program to crash when you can't find the environment variable.</p>
|
|
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