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Rust Basics

Reading User Input in Rust

Read a line with read_line, trim it and parse it into a number without crashing on bad input.

  • Beginner
  • 2 min read
  • 1 runnable example
What you'll learn
  • io::stdin().read_line
  • trim and parse
  • Handling bad input with match
Run code →
From the keyboard to a number
  1. 1read_line"36\n"
  2. 2trim()"36"
  3. 3parse::<u32>()Ok(36)
  4. 4age + 137

Text in, number out

read_line waits for the user to type a line and press Enter, then adds the text to a String. The text always arrives as a string, so numbers must be parsed before you calculate with them.

trim removes the newline that read_line keeps at the end. parse::<u32>() returns Ok with the number or Err when the text is not a number, and match handles both cases. The sample output uses the answers Ada and 36: in the online compiler, type your answers when the prompts appear.

Key points

  1. read_line keeps the newline: call trim.
  2. parse returns Ok(number) or Err.
  3. match handles both cases.

Runnable Rust example

Rust
Run code →
main.rs
use std::io;

fn main() {
    println!("What is your name?");
    let mut name = String::new();
    io::stdin().read_line(&mut name).expect("failed to read");
    let name = name.trim();

    println!("How old are you?");
    let mut age = String::new();
    io::stdin().read_line(&mut age).expect("failed to read");

    match age.trim().parse::<u32>() {
        Ok(age) => println!("Hello, {name}! Next year you will be {}.", age + 1),
        Err(_) => println!("Hello, {name}! That is not a number."),
    }
}
Get a Run button for this example Result of running the code
What is your name?
How old are you?
Hello, Ada! Next year you will be 37.

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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