How to format strings

Strings
2026-04-17T16:32:01+00:00
Use the `format!` macro for creating new strings in memory and the `println!` or `print!` macros for outputting to the console.

Use the format! macro for creating new strings in memory and the println! or print! macros for outputting to the console. Both rely on the same curly brace {} placeholder syntax, supporting positional arguments, named arguments, and specific formatting options like width, alignment, and type conversion.

Here is a practical example showing the difference between creating a string and printing one, along with common formatting flags:

fn main() {
    let name = "Alice";
    let age = 30;

    // 1. Create a new String in memory using format!
    let greeting = format!("Hello, {}! You are {} years old.", name, age);
    println!("{}", greeting); 
    // Output: Hello, Alice! You are 30 years old.

    // 2. Print directly to stdout using println!
    println!("User: {name}, Age: {age}"); // Named arguments (Rust 2021+)
    
    // 3. Advanced formatting: width, alignment, and padding
    let number = 42;
    println!("Right aligned (width 5): {:5}", number);  // "   42"
    println!("Left aligned (width 5): {:<5}", number);  // "42   "
    println!("Zero-padded (width 5):  {:05}", number);  // "00042"
    println!("Hexadecimal:            {:x}", number);   // "2a"
    println!("Binary:                 {:b}", number);   // "101010"
    println!("Float (2 decimals):     {:.2}", 3.14159); // "3.14"
}

Key syntax rules to remember:

  • Positional: Use {} for arguments in order, or {0}, {1} to reference specific indices.
  • Named: Use {variable_name} if you pass named arguments (e.g., println!("Hi {name}", name = "Bob")).
  • Modifiers: Place a colon : after the placeholder to add formatting flags (e.g., {:width}, {:0width}, {:type}).
  • Debug vs Display: Use {:?} to print the Debug representation (useful for structs/vectors) and {} for the Display trait (default string representation).

If you need to format a string inside a loop or a function, format! is your go-to tool because it returns a String type, whereas println! returns (). Always prefer format! when you need to store, manipulate, or return the formatted text.