Rust Output

You’ve already used println! a bunch of times. This page covers the other printing macros you’ll run into and when to reach for each one.

println! and print!

println! prints text and adds a newline at the end. print! does the same thing but doesn’t add a newline:

fn main() {
    print!("Hello, ");
    print!("World");
    println!("!");
}

Output:

Hello, World!

▶ Try it Yourself

Notice all three calls printed on the same line — only println! breaks to a new one. In practice you’ll use println! almost all the time; print! is mainly useful when you’re building up a line piece by piece.

Placeholders: {} and named variables

You’ve seen {} used as a placeholder. You can fill it either by passing a value after the string, or by putting the variable name directly inside the braces:

fn main() {
    let name = "Ferris";
    let age = 10;

    println!("{} is {} years old", name, age); // positional
    println!("{name} is {age} years old");     // inline (same result)
}

Both lines print the same thing. The inline style ({name}) is a bit newer and usually easier to read, so prefer it when you can.

format! — build a string instead of printing it

Sometimes you want the formatted text as a String instead of printing it immediately — say, to store it or pass it to another function. That’s what format! is for. It works exactly like println! but returns a String instead of printing:

fn main() {
    let name = "Ferris";
    let message = format!("Hello, {name}!");
    println!("{message}");
}

▶ Try it Yourself

Debug printing with {:?}

Plain {} only works for values that know how to display themselves as text — numbers, strings, booleans. For more complex values like tuples or structs, {} won’t compile. Use {:?} (the “debug” format) instead:

fn main() {
    let point = (3, 7);
    println!("{:?}", point);
}

Output:

(3, 7)

▶ Try it Yourself

For a nicer, multi-line layout on bigger values, use {:#?} (pretty-print):

fn main() {
    let point = (3, 7);
    println!("{:#?}", point);
}

Note

If Rust ever tells you a type “doesn’t implement Display,” that’s your cue to try {:?} instead of {}. It’s one of the most common errors beginners hit while printing things.

Number formatting

A few formatting tricks that come up often:

fn main() {
    let pi = 3.14159;
    println!("{:.2}", pi);   // 2 decimal places -> 3.14
    println!("{:5}", 42);    // pad to width 5  -> "   42"
    println!("{:08.2}", pi); // pad with zeros  -> 00003.14
}

You don’t need to memorize these — just know they exist for when you need to line up numbers or round decimals.

Recap

• println! prints with a newline; print! doesn’t.
• {} and {name} are placeholders for values that can Display themselves.
• format! works like println! but returns a String instead of printing.
• {:?} (and {:#?}) prints values for debugging, including ones {} can’t handle.

Read more

• Hello, World! — The Rust Book
• std::fmt documentation — the full list of formatting options