Read and validate console input

A typed mark from a user arrives as text, not as a validated number. Reading, parsing and validation are separate steps. Standard input can fail, contain invalid text or reach its end, so a useful program handles those situations deliberately.

read_line appends into a mutable String and returns a Result containing the number of bytes read. Trim removes surrounding whitespace, including the newline. Parsing into u32 rejects nonnumeric text and negative integers; a separate comparison rejects marks above 100.

Connect this idea with result and recoverable errors.

Save the complete source as main.rs in the Rust environment. Supply sample standard input 80 followed by a newline.

Rust 2024 · complete program

use std::io;
fn main() {
  let mut input = String::new();
  match io::stdin().read_line(&mut input) {
    Ok(0) => println!("No input supplied"),
    Ok(_) => match input.trim().parse::<u32>() {
      Ok(score) if score <= 100 => println!("Recorded: {score}"),
      _ => println!("Enter a whole mark from 0 to 100"),
    },
    Err(_) => println!("Could not read input"),
  }
}

Expected output

Recorded: 80

Follow the execution

With sample input 80 followed by a newline, read_line fills the buffer. Trim leaves 80, parsing returns Ok(80), and the guard accepts its range. Invalid text takes the fallback arm; end of input takes the separate Ok(0) arm.

Try the complete program in the Rust compiler. For tests, Cargo commands and local files, use the specified local environment.

Your task: What should happen when the user enters 120?

Rust 2024 · complete program

use std::io;
fn main() {
  let mut input = String::new();
  match io::stdin().read_line(&mut input) {
    Ok(0) => println!("No input supplied"),
    Ok(_) => match input.trim().parse::<u32>() {
      Ok(score) if score <= 100 => println!("Recorded: {score}"),
      _ => println!("Enter a whole mark from 0 to 100"),
    },
    Err(_) => println!("Could not read input"),
  }
}

Worked answer

The program should print Enter a whole mark from 0 to 100. Parsing succeeds, but the range guard rejects the value.

Reusing a buffer without clearing it can accumulate multiple lines. End of input is not identical to a blank line. A console example does not validate an HTTP request or provide a graphical input form; those environments have their own input boundaries.

The program should print Enter a whole mark from 0 to 100. Parsing succeeds, but the range guard rejects the value. Reusing a buffer without clearing it can accumulate multiple lines.

Further reading: the official Rust Book on read and validate console input.