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<ol class="chapter"><li class="chapter-item expanded "><a href="Chapter_0.html"><strong aria-hidden="true">1.</strong> Update</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" class="active"><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">
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<main>
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<h2><a class="header" href="#display-and-debug" id="display-and-debug">Display and debug</a></h2>
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<p>Simple variables in Rust can be printed with <code>{}</code> inside <code>println!</code>. But some variables can't, and you need to <strong>debug print</strong>. Debug print is printing for the programmer, because it usually shows more information. Debug sometimes doesn't look pretty, because it has extra information to help you.</p>
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<p>How do you know if you need <code>{:?}</code> and not <code>{}</code>? The compiler will tell you. For example:</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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let doesnt_print = ();
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println!("This will not print: {}", doesnt_print); // ⚠️
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}
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</code></pre></pre>
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<p>When we run this, the compiler says:</p>
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<pre><code class="language-text">error[E0277]: `()` doesn't implement `std::fmt::Display`
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--> src\main.rs:3:41
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3 | println!("This will not print: {}", doesnt_print);
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| ^^^^^^^^^^^^ `()` cannot be formatted with the default formatter
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= help: the trait `std::fmt::Display` is not implemented for `()`
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= note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
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= note: required by `std::fmt::Display::fmt`
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= note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
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</code></pre>
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<p>This is a lot of information. But the important part is: <code>you may be able to use {:?} (or {:#?} for pretty-print) instead</code>. This means that you can try <code>{:?}</code>, and also <code>{:#?}</code> <code>{:#?}</code> is called "pretty printing". It is like <code>{:?}</code> but prints with different formatting over more lines.</p>
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<p>So Display means printing with <code>{}</code>, and Debug means printing with <code>{:?}</code>.</p>
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<p>One more thing: you can also use <code>print!</code> without <code>ln</code> if you don't want a new line.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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print!("This will not print a new line");
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println!(" so this will be on the same line");
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}
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</code></pre></pre>
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<p>This prints <code>This will not print a new line so this will be on the same line</code>.</p>
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<h3><a class="header" href="#smallest-and-largest-numbers" id="smallest-and-largest-numbers">Smallest and largest numbers</a></h3>
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<p>If you want to see the smallest and biggest numbers, you can use MIN and MAX. <code>std</code> means "standard library" and has all the main functions etc. for Rust. We will learn about the standard library later. But in the meantime, you can remember that this is how you get the smallest and largest number for a type.</p>
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<pre><pre class="playground"><code class="language-rust">fn main() {
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println!("The smallest i8 is {} and the biggest i8 is {}.", std::i8::MIN, std::i8::MAX); // hint: printing std::i8::MIN means "print MIN inside of the i8 section in the standard library"
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println!("The smallest u8 is {} and the biggest u8 is {}.", std::u8::MIN, std::u8::MAX);
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println!("The smallest i16 is {} and the biggest i16 is {}.", std::i16::MIN, std::i16::MAX);
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println!("The smallest u16 is {} and the biggest u16 is {}.", std::u16::MIN, std::u16::MAX);
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println!("The smallest i32 is {} and the biggest i32 is {}.", std::i32::MIN, std::i32::MAX);
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println!("The smallest u32 is {} and the biggest u32 is {}.", std::u32::MIN, std::u32::MAX);
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println!("The smallest i64 is {} and the biggest i64 is {}.", std::i64::MIN, std::i64::MAX);
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println!("The smallest u64 is {} and the biggest u64 is {}.", std::u64::MIN, std::u64::MAX);
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println!("The smallest i128 is {} and the biggest i128 is {}.", std::i128::MIN, std::i128::MAX);
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println!("The smallest u128 is {} and the biggest u128 is {}.", std::u128::MIN, std::u128::MAX);
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}
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</code></pre></pre>
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<p>This will print:</p>
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<pre><code class="language-text">The smallest i8 is -128 and the biggest i8 is 127.
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The smallest u8 is 0 and the biggest u8 is 255.
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The smallest i16 is -32768 and the biggest i16 is 32767.
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The smallest u16 is 0 and the biggest u16 is 65535.
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The smallest i32 is -2147483648 and the biggest i32 is 2147483647.
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The smallest u32 is 0 and the biggest u32 is 4294967295.
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The smallest i64 is -9223372036854775808 and the biggest i64 is 9223372036854775807.
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The smallest u64 is 0 and the biggest u64 is 18446744073709551615.
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The smallest i128 is -170141183460469231731687303715884105728 and the biggest i128 is 170141183460469231731687303715884105727.
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The smallest u128 is 0 and the biggest u128 is 340282366920938463463374607431768211455.
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</code></pre>
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