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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 >
< 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 >
< h2 id = "满分初印象" > < a class = "header" href = "#满分初印象" > 满分初印象< / a > < / h2 >
< p > 先来一段代码，看看格式化输出的初印象：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > println!("Hello"); // => "Hello"
println!("Hello, {}!", "world"); // => "Hello, world!"
println!("The number is {}", 1); // => "The number is 1"
println!("{:?}", (3, 4)); // => "(3, 4)"
println!("{value}", value=4); // => "4"
println!("{} {}", 1, 2); // => "1 2"
println!("{:04}", 42); // => "0042" with leading zeros
< span class = "boring" > }< / span > < / code > < / pre >
< 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 >
< h2 id = "printprintlnformat" > < a class = "header" href = "#printprintlnformat" > < code > print!< / code > ， < code > println!< / code > ， < code > format!< / code > < / a > < / h2 >
< p > 它们是 Rust 中用来格式化输出的三大金刚，用途如下：< / p >
< ul >
< li > < code > print!< / code > 将格式化文本输出到标准输出，不带换行符< / li >
< li > < code > println!< / code > 同上，但是在行的末尾添加换行符< / li >
< li > < code > format!< / code > 将格式化文本输出到 < code > String< / code > 字符串< / li >
< / ul >
< 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 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 >
< 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 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 >
< 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-" > < code > {}< / code > 与 < code > {:?}< / code > < / a > < / h2 >
< p > 与其它语言常用的 < code > %d< / code > ， < code > %s< / code > 不同， < code > {}< / code > 作为格式化占位符（ ， ， < 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 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 >
< 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 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 >
< 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 = "#" > < code > {:#?}< / code > < / 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 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 >
< 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， ， ， ， ，
< / 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 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 >
< 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 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 >
< 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 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 >
< 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 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 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 >
< 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 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 >
< 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 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，
println!("Hello {:5}!", "x");
// 使用参数5来指定宽度
println!("Hello {:1$}!", "x", 5);
// 使用x作为占位符输出内容，
println!("Hello {1:0$}!", 5, "x");
// 使用有名称的参数作为宽度
println!("Hello {:width$}!", "x", width = 5);
//-----------------------------------
// 使用参数5为参数x指定宽度， > Hello x !5
println!("Hello {:1$}!{}", "x", 5);
}< / code > < / 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 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， > Hello 00005!
println!("Hello {:05}!", 5);
// 负号也要占用一位宽度 => Hello -0005!
println!("Hello {:05}!", -5);
}< / code > < / pre >
< h3 id = "对齐" > < a class = "header" href = "#对齐" > 对齐< / a > < / h3 >
< 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 !
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 >
< 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 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 >
< 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 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， > 0b00011011!
println!("{:#010b}!", 27);
}< / code > < / pre >
< h3 id = "指数" > < a class = "header" href = "#指数" > 指数< / a > < / h3 >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
println!("{:2e}", 1000000000); // => 1e9
println!("{:2E}", 1000000000); // => 1E9
}< / code > < / pre >
< h3 id = "指针地址" > < a class = "header" href = "#指针地址" > 指针地址< / a > < / h3 >
< 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 >
< 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 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 >
< 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（ < / 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 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 >
< 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 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 >
< 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 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 >
< 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 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 >
< 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， < / p >
< / main >
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