Strings and Text Processing

String Formatting

Rust's formatting machinery (std::fmt) uses compile-time-checked format strings ("{arg:spec}") to control alignment, fill characters, width, numeric precision, sign display, and radix across format!, println!, and write!.

Anatomy of a Format Specifier

{ variable : [fill][align][sign][#][0][width][.precision][type] }
Specifier Component Syntax Effect Example (x = 42)
Positional / Named {0}, {name} Selects argument by index or variable name "{x}" -> "42"
Alignment & Width :<w, :^w, :>w Left, center, or right-aligns to minimum width w "{x:>5}" -> " 42"
Fill Character :*<w, :_>w, :0w Pads unused width with custom fill character or 0 "{x:05}" -> "00042"
Sign :+ Always prefixes positive numbers with + "{x:+}" -> "+42"
Precision (floats / strings) :.p Decimal places for floats, or max byte/char truncation for &str "{3.14159:.2}" -> "3.14"
Radix / Base :x, :X, :b, :o Lower hex, upper hex, binary, or octal (add # for 0x/0b/0o prefix) "{x:#06x}" -> "0x002a"
Trait Selection (empty), :?, :#?, :e, :p Display, Debug, pretty Debug, scientific notation, pointer address "{x:?}" -> "42"

Alignment, Padding, Numeric Radix, and Precision Examples

fn main() {
    let name = "compute-node";
    let status = 255_u16;
    let latency_ms = 12.3456_f64;

    // Alignment and fill characters:
    println!("|{name:<16}|{name:^16}|{name:>16}|");
    println!("|{name:.<16}|");

    // Numeric base and zero-padding:
    println!(
        "dec={status}, hex={status:#06x}, bin={status:#010b}"
    );

    // Floating-point precision and string truncation:
    println!("Latency: {latency_ms:+08.2} ms");
    println!("Truncated string (5 chars): '{name:.5}'");
}

|compute-node    |  compute-node  |    compute-node|
|compute-node....|
dec=255, hex=0x00ff, bin=0b11111111
Latency: +0012.35 ms
Truncated string (5 chars): 'compu'

Dynamic Width and Precision (width$ and prec$)

You can pass the minimum width or decimal precision dynamically at runtime from a usize variable using the $ suffix:

fn main() {
    let value = std::f64::consts::PI;

    for (width, prec) in [(8, 2), (10, 4), (12, 6)] {
        println!("{value:>width$.prec$}");
    }
}

    3.14
    3.1416
    3.141593

Escaping Literal Braces ({{ and }})

To output a literal { or } inside a format string, double the brace ("{{" produces "{", and "}}" produces "}").