format! macro

Build a new String with the format! macro.

The format! macro

format! uses the same format string as println! but returns a String instead of printing:

let urun = "Kalem";
let adet = 3;
let fiyat = 12.5;
let line = format!("{urun} x{adet} = {:.2} TL", fiyat * adet as f64);
println!("{line}");   // Kalem x3 = 37.50 TL

The format string is checked at compile time and the arguments are borrowed, so the values can still be used afterwards.

Positional and named arguments

Arguments can be used in order, by number or by name:

format!("{0} {1} {0}", "a", "b");    // "a b a"
format!("{ad} {ad}", ad = "Ada");     // "Ada Ada"
format!("{:^7}", "ok");                // "  ok   "
format!("{:08.3}", 3.14159);           // "0003.142"
format!("{:x} {:b}", 255, 5);         // "ff 101"

Exercise Receipt line

Write fn line(urun: &str, adet: u32, fiyat: f64) -> String returning format!("{urun:<8}{adet:>3} x {fiyat:6.2}"). Print the result of line("Kalem", 3, 12.5) inside square brackets: println!("[{}]", ...).

Show example solution

fn line(urun: &str, adet: u32, fiyat: f64) -> String {
    format!("{urun:<8}{adet:>3} x {fiyat:6.2}")
}

fn main() {
    println!("[{}]", line("Kalem", 3, 12.5));
}

Test yourself

What does <code>format!("{:>5}", "ab")</code> produce?
- "ab "
- " ab "
- "ab"
- " ab"
Show answer

" ab" — > right-aligns and pads the 5-character field with spaces on the left; < left-aligns and ^ centers.

Produce text

let dosya = ____!("rapor-{}.csv", 2026);

Show answer

let dosya = format!("rapor-{}.csv", 2026);

Format and output

{:>5} "ab"

" ab"
{:05} 42

"00042"
{:x} 255

"ff"
{0}{0} "a"

"aa"

Next lesson: String length

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