在 Rust 的格式化字符串中，美元符号语法是什么意思？

Question

在 Rust 的格式化字符串中，美元符号语法是什么意思？

7

我正在学习 Rust by Example，但在这个例子中使用的美元符号 $ 并不清楚：

// You can right-align text with a specified width. This will output
// "     1". 5 white spaces and a "1".
println!("{number:>width$}", number=1, width=6);

// You can pad numbers with extra zeroes. This will output "000001".
println!("{number:>0width$}", number=1, width=6);

我在 Rust语言标准库文档中的std::fmt发现了这篇文章，但是它并没有解决我的问题：

format_string := <text> [ maybe-format <text> ] *
maybe-format := '{' '{' | '}' '}' | <format>
format := '{' [ argument ] [ ':' format_spec ] '}'
argument := integer | identifier

format_spec := [[fill]align][sign]['#']['0'][width]['.' precision][type]
fill := character
align := '<' | '^' | '>'
sign := '+' | '-'
width := count
precision := count | '*'
type := identifier | ''
count := parameter | integer
parameter := argument '$'

我稍微查看了一下代码，发现没有美元符号就无法编译通过，但是"width"可以替换为任何标识符。也就是说，以下代码等价于第一个代码块的第三行：

println!("{number:>test$}", number=1, test=6);

- Fred

1个回答

网页内容由stack overflow 提供, 点击上面的

可以查看英文原文，
原文链接

- Shepmaster · Accepted Answer

它允许将另一个格式化项的宽度或精度作为参数提供，而不是硬编码为格式字符串的一部分。该参数可以使用数字索引或名称指定。

文档说明：

The value for the width can also be provided as a [usize] in the list of parameters by using the dollar syntax indicating that the second argument is a [usize] specifying the width, for example:
// All of these print "Hello x    !"
println!("Hello {:5}!", "x");
println!("Hello {:1$}!", "x", 5);
println!("Hello {1:0$}!", 5, "x");
println!("Hello {:width$}!", "x", width = 5);
Referring to an argument with the dollar syntax does not affect the "next argument" counter, so it's usually a good idea to refer to arguments by position, or use named arguments.

它还说：

There are three possible ways to specify the desired precision:

[...]

An integer or name followed by dollar sign .N$:

use format argument N (which must be a usize) as the precision.

[...]

For example, the following calls all print the same thing Hello x is 0.01000:

// Hello {arg 0 ("x")} is {arg 1 (0.01) with precision specified inline (5)}
println!("Hello {0} is {1:.5}", "x", 0.01);

// Hello {arg 1 ("x")} is {arg 2 (0.01) with precision specified in arg 0 (5)}
println!("Hello {1} is {2:.0$}", 5, "x", 0.01);

// Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
println!("Hello {0} is {2:.1$}", "x", 5, 0.01);

// Hello {next arg ("x")} is {second of next two args (0.01) with precision
//                          specified in first of next two args (5)}
println!("Hello {} is {:.*}",    "x", 5, 0.01);

// Hello {next arg ("x")} is {arg 2 (0.01) with precision
//                          specified in its predecessor (5)}
println!("Hello {} is {2:.*}",   "x", 5, 0.01);

// Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
//                          in arg "prec" (5)}
println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);