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<ol class="chapter"><li class="chapter-item affix "><a href="../about-book.html">关于本书</a></li><li class="chapter-item affix "><a href="../into-rust.html">进入 Rust 编程世界</a></li><li class="chapter-item affix "><a href="../first-try/sth-you-should-not-do.html">避免从入门到放弃</a></li><li class="chapter-item affix "><a href="../community.html">社区和锈书</a></li><li class="spacer"></li><li class="chapter-item affix "><a href="../some-thoughts.html">Datav: 可编程的数据可视化平台和可观测性平台</a></li><li class="chapter-item affix "><li class="part-title">Rust 语言基础学习</li><li class="spacer"></li><li class="chapter-item "><a href="../first-try/intro.html"><strong aria-hidden="true">1.</strong> 寻找牛刀，以便小试</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../first-try/installation.html"><strong aria-hidden="true">1.1.</strong> 安装 Rust 环境</a></li><li class="chapter-item "><a href="../first-try/editor.html"><strong aria-hidden="true">1.2.</strong> 墙推 VSCode!</a></li><li class="chapter-item "><a href="../first-try/cargo.html"><strong aria-hidden="true">1.3.</strong> 认识 Cargo</a></li><li class="chapter-item "><a href="../first-try/hello-world.html"><strong aria-hidden="true">1.4.</strong> 不仅仅是 Hello world</a></li><li class="chapter-item "><a href="../first-try/slowly-downloading.html"><strong aria-hidden="true">1.5.</strong> 下载依赖太慢了？</a></li></ol></li><li class="chapter-item expanded "><a href="../basic/intro.html"><strong aria-hidden="true">2.</strong> Rust 基础入门</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/variable.html"><strong aria-hidden="true">2.1.</strong> 变量绑定与解构</a></li><li class="chapter-item "><a href="../basic/base-type/index.html"><strong aria-hidden="true">2.2.</strong> 基本类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/base-type/numbers.html"><strong aria-hidden="true">2.2.1.</strong> 数值类型</a></li><li class="chapter-item "><a href="../basic/base-type/char-bool.html"><strong aria-hidden="true">2.2.2.</strong> 字符、布尔、单元类型</a></li><li class="chapter-item "><a href="../basic/base-type/statement-expression.html"><strong aria-hidden="true">2.2.3.</strong> 语句与表达式</a></li><li class="chapter-item "><a href="../basic/base-type/function.html"><strong aria-hidden="true">2.2.4.</strong> 函数</a></li></ol></li><li class="chapter-item "><a href="../basic/ownership/index.html"><strong aria-hidden="true">2.3.</strong> 所有权和借用</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/ownership/ownership.html"><strong aria-hidden="true">2.3.1.</strong> 所有权</a></li><li class="chapter-item "><a href="../basic/ownership/borrowing.html"><strong aria-hidden="true">2.3.2.</strong> 引用与借用</a></li></ol></li><li class="chapter-item "><a href="../basic/compound-type/intro.html"><strong aria-hidden="true">2.4.</strong> 复合类型</a><a class="toggle"><div>❱</div></a></li><li><ol class="section"><li class="chapter-item "><a href="../basic/compound-type/string-slice.html"><strong aria-hidden="true">2.4.1.</strong> 字符串与切片</a></li><li class="chapter-item "><a href="../basic/compound-type/tuple.html"><strong aria-hidden="true">2.4.2.</strong> 元组</a></li><li class="chapter-item "><a href="../basic/compound-type/struct.html"><strong aria-hidden="true">2.4.3.</strong> 结构体</a></li><li class="chapter-item "><a href="../basic/compound-type/enum.html"><strong aria-hidden="true">2.4.4.</strong> 枚举</a></li><li class="chapter-item "><a href="../basic/compound-type/array.html"><strong aria-hidden="true">2.4.5.</strong> 数组</a></li></ol></li><li class="chapter-item "><a href="../basic/flow-control.html"><strong aria-hidden="true">2.5.</strong> 流程控制</a></li><li c
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<main>
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<h1 id="格式化输出"><a class="header" href="#格式化输出">格式化输出</a></h1>
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<p>提到格式化输出，可能很多人立刻就想到 <code>"{}"</code>，但是 Rust 能做到的远比这个多的多，本章节我们将深入讲解格式化输出的各个方面。</p>
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<h2 id="满分初印象"><a class="header" href="#满分初印象">满分初印象</a></h2>
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<p>先来一段代码，看看格式化输出的初印象：</p>
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<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>println!("Hello"); // => "Hello"
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println!("Hello, {}!", "world"); // => "Hello, world!"
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println!("The number is {}", 1); // => "The number is 1"
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println!("{:?}", (3, 4)); // => "(3, 4)"
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println!("{value}", value=4); // => "4"
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println!("{} {}", 1, 2); // => "1 2"
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println!("{:04}", 42); // => "0042" with leading zeros
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<span class="boring">}</span></code></pre></pre>
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<p>可以看到 <code>println!</code> 宏接受的是可变参数，第一个参数是一个字符串常量，它表示最终输出字符串的格式，包含其中形如 <code>{}</code> 的符号是<strong>占位符</strong>，会被 <code>println!</code> 后面的参数依次替换。</p>
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<h2 id="printprintlnformat"><a class="header" href="#printprintlnformat"><code>print!</code>，<code>println!</code>，<code>format!</code></a></h2>
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<p>它们是 Rust 中用来格式化输出的三大金刚，用途如下：</p>
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<ul>
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<li><code>print!</code> 将格式化文本输出到标准输出，不带换行符</li>
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<li><code>println!</code> 同上，但是在行的末尾添加换行符</li>
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<li><code>format!</code> 将格式化文本输出到 <code>String</code> 字符串</li>
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</ul>
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<p>在实际项目中，最常用的是 <code>println!</code> 及 <code>format!</code>，前者常用来调试输出，后者常用来生成格式化的字符串：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
let s = "hello";
|
|
|
|
|
|
println!("{}, world", s);
|
|
|
|
|
|
let s1 = format!("{}, world", s);
|
|
|
|
|
|
print!("{}", s1);
|
|
|
|
|
|
print!("{}\n", "!");
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>其中，<code>s1</code> 是通过 <code>format!</code> 生成的 <code>String</code> 字符串，最终输出如下：</p>
|
|
|
|
|
|
<pre><code class="language-console">hello, world
|
|
|
|
|
|
hello, world!
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<h4 id="eprinteprintln"><a class="header" href="#eprinteprintln"><code>eprint!</code>，<code>eprintln!</code></a></h4>
|
|
|
|
|
|
<p>除了三大金刚外，还有两大护法，使用方式跟 <code>print!</code>，<code>println!</code> 很像，但是它们输出到标准错误输出：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>eprintln!("Error: Could not complete task")
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>它们仅应该被用于输出错误信息和进度信息，其它场景都应该使用 <code>print!</code> 系列。</p>
|
|
|
|
|
|
<h2 id="-与"><a class="header" href="#-与">{} 与</a></h2>
|
|
|
|
|
|
<p>与其它语言常用的 <code>%d</code>，<code>%s</code> 不同，Rust 特立独行地选择了 <code>{}</code> 作为格式化占位符（说到这个，有点想吐槽下，Rust 中自创的概念其实还挺多的，真不知道该夸奖还是该吐槽-,-），事实证明，这种选择非常正确，它帮助用户减少了很多使用成本，你无需再为特定的类型选择特定的占位符，统一用 <code>{}</code> 来替代即可，剩下的类型推导等细节只要交给 Rust 去做。</p>
|
|
|
|
|
|
<p>与 <code>{}</code> 类似，<code>{:?}</code> 也是占位符：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code>{}</code> 适用于实现了 <code>std::fmt::Display</code> 特征的类型，用来以更优雅、更友好的方式格式化文本，例如展示给用户</li>
|
|
|
|
|
|
<li><code>{:?}</code> 适用于实现了 <code>std::fmt::Debug</code> 特征的类型，用于调试场景</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<p>其实两者的选择很简单，当你在写代码需要调试时，使用 <code>{:?}</code>，剩下的场景，选择 <code>{}</code>。</p>
|
|
|
|
|
|
<h4 id="debug-特征"><a class="header" href="#debug-特征"><code>Debug</code> 特征</a></h4>
|
|
|
|
|
|
<p>事实上，为了方便我们调试，大多数 Rust 类型都实现了 <code>Debug</code> 特征或者支持派生该特征：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">#[derive(Debug)]
|
|
|
|
|
|
struct Person {
|
|
|
|
|
|
name: String,
|
|
|
|
|
|
age: u8
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let i = 3.1415926;
|
|
|
|
|
|
let s = String::from("hello");
|
|
|
|
|
|
let v = vec![1, 2, 3];
|
|
|
|
|
|
let p = Person{name: "sunface".to_string(), age: 18};
|
|
|
|
|
|
println!("{:?}, {:?}, {:?}, {:?}", i, s, v, p);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>对于数值、字符串、数组，可以直接使用 <code>{:?}</code> 进行输出，但是对于结构体，需要<a href="https://course.rs/appendix/derive.html">派生<code>Debug</code></a>特征后，才能进行输出，总之很简单。</p>
|
|
|
|
|
|
<h4 id="display-特征"><a class="header" href="#display-特征"><code>Display</code> 特征</a></h4>
|
|
|
|
|
|
<p>与大部分类型实现了 <code>Debug</code> 不同，实现了 <code>Display</code> 特征的 Rust 类型并没有那么多，往往需要我们自定义想要的格式化方式：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>let i = 3.1415926;
|
|
|
|
|
|
let s = String::from("hello");
|
|
|
|
|
|
let v = vec![1, 2, 3];
|
|
|
|
|
|
let p = Person {
|
|
|
|
|
|
name: "sunface".to_string(),
|
|
|
|
|
|
age: 18,
|
|
|
|
|
|
};
|
|
|
|
|
|
println!("{}, {}, {}, {}", i, s, v, p);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>运行后可以看到 <code>v</code> 和 <code>p</code> 都无法通过编译，因为没有实现 <code>Display</code> 特征，但是你又不能像派生 <code>Debug</code> 一般派生 <code>Display</code>，只能另寻他法：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li>使用 <code>{:?}</code> 或 <code>{:#?}</code></li>
|
|
|
|
|
|
<li>为自定义类型实现 <code>Display</code> 特征</li>
|
|
|
|
|
|
<li>使用 <code>newtype</code> 为外部类型实现 <code>Display</code> 特征</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<p>下面来一一看看这三种方式。</p>
|
|
|
|
|
|
<h4 id=""><a class="header" href="#"></a></h4>
|
|
|
|
|
|
<p><code>{:#?}</code> 与 <code>{:?}</code> 几乎一样，唯一的区别在于它能更优美地输出内容：</p>
|
|
|
|
|
|
<pre><code class="language-console">// {:?}
|
|
|
|
|
|
[1, 2, 3], Person { name: "sunface", age: 18 }
|
|
|
|
|
|
|
|
|
|
|
|
// {:#?}
|
|
|
|
|
|
[
|
|
|
|
|
|
1,
|
|
|
|
|
|
2,
|
|
|
|
|
|
3,
|
|
|
|
|
|
], Person {
|
|
|
|
|
|
name: "sunface",
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>因此对于 <code>Display</code> 不支持的类型，可以考虑使用 <code>{:#?}</code> 进行格式化，虽然理论上它更适合进行调试输出。</p>
|
|
|
|
|
|
<h4 id="为自定义类型实现-display-特征"><a class="header" href="#为自定义类型实现-display-特征">为自定义类型实现 <code>Display</code> 特征</a></h4>
|
|
|
|
|
|
<p>如果你的类型是定义在当前作用域中的，那么可以为其实现 <code>Display</code> 特征，即可用于格式化输出：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">struct Person {
|
|
|
|
|
|
name: String,
|
|
|
|
|
|
age: u8,
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
use std::fmt;
|
|
|
|
|
|
impl fmt::Display for Person {
|
|
|
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
|
|
|
write!(
|
|
|
|
|
|
f,
|
|
|
|
|
|
"大佬在上，请受我一拜，小弟姓名{}，年芳{}，家里无田又无车，生活苦哈哈",
|
|
|
|
|
|
self.name, self.age
|
|
|
|
|
|
)
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let p = Person {
|
|
|
|
|
|
name: "sunface".to_string(),
|
|
|
|
|
|
age: 18,
|
|
|
|
|
|
};
|
|
|
|
|
|
println!("{}", p);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>如上所示，只要实现 <code>Display</code> 特征中的 <code>fmt</code> 方法，即可为自定义结构体 <code>Person</code> 添加自定义输出：</p>
|
|
|
|
|
|
<pre><code class="language-console">大佬在上，请受我一拜，小弟姓名sunface，年芳18，家里无田又无车，生活苦哈哈
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<h4 id="为外部类型实现-display-特征"><a class="header" href="#为外部类型实现-display-特征">为外部类型实现 <code>Display</code> 特征</a></h4>
|
|
|
|
|
|
<p>在 Rust 中，无法直接为外部类型实现外部特征，但是可以使用<a href="https://course.rs/advance/into-types/custom-type.html#newtype"><code>newtype</code></a>解决此问题：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">struct Array(Vec<i32>);
|
|
|
|
|
|
|
|
|
|
|
|
use std::fmt;
|
|
|
|
|
|
impl fmt::Display for Array {
|
|
|
|
|
|
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
|
|
|
|
|
|
write!(f, "数组是：{:?}", self.0)
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let arr = Array(vec![1, 2, 3]);
|
|
|
|
|
|
println!("{}", arr);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p><code>Array</code> 就是我们的 <code>newtype</code>，它将想要格式化输出的 <code>Vec</code> 包裹在内，最后只要为 <code>Array</code> 实现 <code>Display</code> 特征，即可进行格式化输出：</p>
|
|
|
|
|
|
<pre><code class="language-console">数组是：[1, 2, 3]
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<p>至此，关于 <code>{}</code> 与 <code>{:?}</code> 的内容已介绍完毕，下面让我们正式开始格式化输出的旅程。</p>
|
|
|
|
|
|
<h2 id="位置参数"><a class="header" href="#位置参数">位置参数</a></h2>
|
|
|
|
|
|
<p>除了按照依次顺序使用值去替换占位符之外，还能让指定位置的参数去替换某个占位符，例如 <code>{1}</code>，表示用第二个参数替换该占位符(索引从 0 开始)：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
println!("{}{}", 1, 2); // =>"12"
|
|
|
|
|
|
println!("{1}{0}", 1, 2); // =>"21"
|
|
|
|
|
|
// => Alice, this is Bob. Bob, this is Alice
|
|
|
|
|
|
println!("{0}, this is {1}. {1}, this is {0}", "Alice", "Bob");
|
|
|
|
|
|
println!("{1}{}{0}{}", 1, 2); // => 2112
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h2 id="具名参数"><a class="header" href="#具名参数">具名参数</a></h2>
|
|
|
|
|
|
<p>除了像上面那样指定位置外，我们还可以为参数指定名称：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
println!("{argument}", argument = "test"); // => "test"
|
|
|
|
|
|
println!("{name} {}", 1, name = 2); // => "2 1"
|
|
|
|
|
|
println!("{a} {c} {b}", a = "a", b = 'b', c = 3); // => "a 3 b"
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>需要注意的是：<strong>带名称的参数必须放在不带名称参数的后面</strong>，例如下面代码将报错：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>println!("{abc} {1}", abc = "def", 2);
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>error: positional arguments cannot follow named arguments
|
|
|
|
|
|
--> src/main.rs:4:36
|
|
|
|
|
|
|
|
|
|
|
|
|
4 | println!("{abc} {1}", abc = "def", 2);
|
|
|
|
|
|
| ----- ^ positional arguments must be before named arguments
|
|
|
|
|
|
| |
|
|
|
|
|
|
| named argument
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<h2 id="格式化参数"><a class="header" href="#格式化参数">格式化参数</a></h2>
|
|
|
|
|
|
<p>格式化输出，意味着对输出格式会有更多的要求，例如只输出浮点数的小数点后两位：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
let v = 3.1415926;
|
|
|
|
|
|
// Display => 3.14
|
|
|
|
|
|
println!("{:.2}", v);
|
|
|
|
|
|
// Debug => 3.14
|
|
|
|
|
|
println!("{:.2?}", v);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>上面代码只输出小数点后两位。同时我们还展示了 <code>{}</code> 和 <code>{:?}</code> 的用法，后面如无特殊区别，就只针对 <code>{}</code> 提供格式化参数说明。</p>
|
|
|
|
|
|
<p>接下来，让我们一起来看看 Rust 中有哪些格式化参数。</p>
|
|
|
|
|
|
<h3 id="宽度"><a class="header" href="#宽度">宽度</a></h3>
|
|
|
|
|
|
<p>宽度用来指示输出目标的长度，如果长度不够，则进行填充和对齐：</p>
|
|
|
|
|
|
<h4 id="字符串填充"><a class="header" href="#字符串填充">字符串填充</a></h4>
|
|
|
|
|
|
<p>字符串格式化默认使用空格进行填充，并且进行左对齐。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
//-----------------------------------
|
|
|
|
|
|
// 以下全部输出 "Hello x !"
|
|
|
|
|
|
// 为"x"后面填充空格，补齐宽度5
|
|
|
|
|
|
println!("Hello {:5}!", "x");
|
|
|
|
|
|
// 使用参数5来指定宽度
|
|
|
|
|
|
println!("Hello {:1$}!", "x", 5);
|
|
|
|
|
|
// 使用x作为占位符输出内容，同时使用5作为宽度
|
|
|
|
|
|
println!("Hello {1:0$}!", 5, "x");
|
|
|
|
|
|
// 使用有名称的参数作为宽度
|
|
|
|
|
|
println!("Hello {:width$}!", "x", width = 5);
|
|
|
|
|
|
//-----------------------------------
|
|
|
|
|
|
|
|
|
|
|
|
// 使用参数5为参数x指定宽度，同时在结尾输出参数5 => Hello x !5
|
|
|
|
|
|
println!("Hello {:1$}!{}", "x", 5);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h4 id="数字填充符号和-0"><a class="header" href="#数字填充符号和-0">数字填充:符号和 0</a></h4>
|
|
|
|
|
|
<p>数字格式化默认也是使用空格进行填充，但与字符串左对齐不同的是，数字是右对齐。</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
// 宽度是5 => Hello 5!
|
|
|
|
|
|
println!("Hello {:5}!", 5);
|
|
|
|
|
|
// 显式的输出正号 => Hello +5!
|
|
|
|
|
|
println!("Hello {:+}!", 5);
|
|
|
|
|
|
// 宽度5，使用0进行填充 => Hello 00005!
|
|
|
|
|
|
println!("Hello {:05}!", 5);
|
|
|
|
|
|
// 负号也要占用一位宽度 => Hello -0005!
|
|
|
|
|
|
println!("Hello {:05}!", -5);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h3 id="对齐"><a class="header" href="#对齐">对齐</a></h3>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
// 以下全部都会补齐5个字符的长度
|
|
|
|
|
|
// 左对齐 => Hello x !
|
|
|
|
|
|
println!("Hello {:<5}!", "x");
|
|
|
|
|
|
// 右对齐 => Hello x!
|
|
|
|
|
|
println!("Hello {:>5}!", "x");
|
|
|
|
|
|
// 居中对齐 => Hello x !
|
|
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|
|
println!("Hello {:^5}!", "x");
|
|
|
|
|
|
|
|
|
|
|
|
// 对齐并使用指定符号填充 => Hello x&&&&!
|
|
|
|
|
|
// 指定符号填充的前提条件是必须有对齐字符
|
|
|
|
|
|
println!("Hello {:&<5}!", "x");
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h3 id="精度"><a class="header" href="#精度">精度</a></h3>
|
|
|
|
|
|
<p>精度可以用于控制浮点数的精度或者字符串的长度</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
let v = 3.1415926;
|
|
|
|
|
|
// 保留小数点后两位 => 3.14
|
|
|
|
|
|
println!("{:.2}", v);
|
|
|
|
|
|
// 带符号保留小数点后两位 => +3.14
|
|
|
|
|
|
println!("{:+.2}", v);
|
|
|
|
|
|
// 不带小数 => 3
|
|
|
|
|
|
println!("{:.0}", v);
|
|
|
|
|
|
// 通过参数来设定精度 => 3.1416，相当于{:.4}
|
|
|
|
|
|
println!("{:.1$}", v, 4);
|
|
|
|
|
|
|
|
|
|
|
|
let s = "hi我是Sunface孙飞";
|
|
|
|
|
|
// 保留字符串前三个字符 => hi我
|
|
|
|
|
|
println!("{:.3}", s);
|
|
|
|
|
|
// {:.*}接收两个参数，第一个是精度，第二个是被格式化的值 => Hello abc!
|
|
|
|
|
|
println!("Hello {:.*}!", 3, "abcdefg");
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h3 id="进制"><a class="header" href="#进制">进制</a></h3>
|
|
|
|
|
|
<p>可以使用 <code>#</code> 号来控制数字的进制输出：</p>
|
|
|
|
|
|
<ul>
|
|
|
|
|
|
<li><code>#b</code>, 二进制</li>
|
|
|
|
|
|
<li><code>#o</code>, 八进制</li>
|
|
|
|
|
|
<li><code>#x</code>, 小写十六进制</li>
|
|
|
|
|
|
<li><code>#X</code>, 大写十六进制</li>
|
|
|
|
|
|
<li><code>x</code>, 不带前缀的小写十六进制</li>
|
|
|
|
|
|
</ul>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
// 二进制 => 0b11011!
|
|
|
|
|
|
println!("{:#b}!", 27);
|
|
|
|
|
|
// 八进制 => 0o33!
|
|
|
|
|
|
println!("{:#o}!", 27);
|
|
|
|
|
|
// 十进制 => 27!
|
|
|
|
|
|
println!("{}!", 27);
|
|
|
|
|
|
// 小写十六进制 => 0x1b!
|
|
|
|
|
|
println!("{:#x}!", 27);
|
|
|
|
|
|
// 大写十六进制 => 0x1B!
|
|
|
|
|
|
println!("{:#X}!", 27);
|
|
|
|
|
|
|
|
|
|
|
|
// 不带前缀的十六进制 => 1b!
|
|
|
|
|
|
println!("{:x}!", 27);
|
|
|
|
|
|
|
|
|
|
|
|
// 使用0填充二进制，宽度为10 => 0b00011011!
|
|
|
|
|
|
println!("{:#010b}!", 27);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h3 id="指数"><a class="header" href="#指数">指数</a></h3>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
println!("{:2e}", 1000000000); // => 1e9
|
|
|
|
|
|
println!("{:2E}", 1000000000); // => 1E9
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h3 id="指针地址"><a class="header" href="#指针地址">指针地址</a></h3>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>let v= vec![1, 2, 3];
|
|
|
|
|
|
println!("{:p}", v.as_ptr()) // => 0x600002324050
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<h3 id="转义"><a class="header" href="#转义">转义</a></h3>
|
|
|
|
|
|
<p>有时需要输出 <code>{</code>和<code>}</code>，但这两个字符是特殊字符，需要进行转义：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
|
|
|
|
|
|
// "{{" 转义为 '{' "}}" 转义为 '}' "\"" 转义为 '"'
|
|
|
|
|
|
// => Hello "{World}"
|
|
|
|
|
|
println!(" Hello \"{{World}}\" ");
|
|
|
|
|
|
|
|
|
|
|
|
// 下面代码会报错，因为占位符{}只有一个右括号}，左括号被转义成字符串的内容
|
|
|
|
|
|
// println!(" {{ Hello } ");
|
|
|
|
|
|
// 也不可使用 '\' 来转义 "{}"
|
|
|
|
|
|
// println!(" \{ Hello \} ")
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<h2 id="在格式化字符串时捕获环境中的值rust-158-新增"><a class="header" href="#在格式化字符串时捕获环境中的值rust-158-新增">在格式化字符串时捕获环境中的值（Rust 1.58 新增）</a></h2>
|
|
|
|
|
|
<p>在以前，想要输出一个函数的返回值，你需要这么做：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
|
|
|
|
|
String::from("sunface")
|
|
|
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let p = get_person();
|
|
|
|
|
|
println!("Hello, {}!", p); // implicit position
|
|
|
|
|
|
println!("Hello, {0}!", p); // explicit index
|
|
|
|
|
|
println!("Hello, {person}!", person = p);
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>问题倒也不大，但是一旦格式化字符串长了后，就会非常冗余，而在 1.58 后，我们可以这么写：</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
|
|
|
|
|
String::from("sunface")
|
|
|
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let person = get_person();
|
|
|
|
|
|
println!("Hello, {person}!");
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>是不是清晰、简洁了很多？甚至还可以将环境中的值用于格式化参数:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
|
|
|
</span><span class="boring">fn main() {
|
|
|
|
|
|
</span>let (width, precision) = get_format();
|
|
|
|
|
|
for (name, score) in get_scores() {
|
|
|
|
|
|
println!("{name}: {score:width$.precision$}");
|
|
|
|
|
|
}
|
|
|
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
|
|
|
<p>但也有局限，它只能捕获普通的变量，对于更复杂的类型（例如表达式），可以先将它赋值给一个变量或使用以前的 <code>name = expression</code> 形式的格式化参数。
|
|
|
|
|
|
目前除了 <code>panic!</code> 外，其它接收格式化参数的宏，都可以使用新的特性。对于 <code>panic!</code> 而言，如果还在使用 <code>2015版本</code> 或 <code>2018版本</code>，那 <code>panic!("{ident}")</code> 依然会被当成 正常的字符串来处理，同时编译器会给予 <code>warn</code> 提示。而对于 <code>2021版本</code> ，则可以正常使用:</p>
|
|
|
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">fn get_person() -> String {
|
|
|
|
|
|
String::from("sunface")
|
|
|
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let person = get_person();
|
|
|
|
|
|
panic!("Hello, {person}!");
|
|
|
|
|
|
}</code></pre></pre>
|
|
|
|
|
|
<p>输出:</p>
|
|
|
|
|
|
<pre><code class="language-console">thread 'main' panicked at 'Hello, sunface!', src/main.rs:6:5
|
|
|
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
|
|
|
</code></pre>
|
|
|
|
|
|
<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
|
|
|
|
|
|
<blockquote>
|
|
|
|
|
|
<p><a href="https://practice-zh.course.rs/formatted-output.html">Rust By Practice</a>，支持代码在线编辑和运行，并提供详细的习题解答。（本节暂无习题解答）</p>
|
|
|
|
|
|
</blockquote>
|
|
|
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
|
|
|
<p>把这些格式化都牢记在脑中是不太现实的，也没必要，我们要做的就是知道 Rust 支持相应的格式化输出，在需要之时，读者再来查阅本文即可。</p>
|
|
|
|
|
|
<p>还是那句话，<a href="https://github.com/sunface/rust-course"><<Rust 语言圣经>></a>不仅仅是 Rust 学习书籍，还是一本厚重的工具书！</p>
|
|
|
|
|
|
<p>至此，Rust 的基础内容学习已经全部完成，下面我们将学习 Rust 的高级进阶内容，正式开启你的高手之路。</p>
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