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Input from user

Reading User Input in Rust (std::io)

User input in Rust is handled through the standard library module std::io. Most input operations return a Result<T, E> because I/O can fail. You must decide:

• Crash on error -> expect() or unwrap()
• Handle gracefully -> match or ?

(Use VSCode for demo)

Example 1

cargo new input_demo
cd input_demo

// Input String

use std::io;

fn main() {
    println!("Please enter your name:");

    let mut name = String::new();

    io::stdin()
        .read_line(&mut name)
        .expect("Failed to read input");

    println!("Welcome {}", name.trim());
}

- String::new() creates an empty growable string on the heap.
- read_line(&mut name):
    - Takes a mutable reference.
    - Appends input (including newline).
    - Returns Result<usize>.

- .expect("..."):
    - If Err, program panics with the message.
    - If Ok, continues execution.

- .trim() removes trailing newline.
- read_line() keeps \n. Without trim(), output formatting looks messy.

Example 2

// Accept two numbers

use std::io;

fn main() {
    println!("Enter First Number:");

    let mut s1 = String::new();
    io::stdin().read_line(&mut s1).expect("Invalid input");
    let n1: u32 = s1.trim().parse().expect("Not a valid number");

    println!("Enter Second Number:");

    let mut s2 = String::new();
    io::stdin().read_line(&mut s2).expect("Invalid input");
    let n2: u32 = s2.trim().parse().expect("Not a valid number");

    let result = n1 + n2;

    println!("Result: {result}");
}

Example 3 - Better Version

use std::io;

fn read_number(prompt: &str) -> u32 {
    loop {
        println!("{prompt}");

        let mut input = String::new();
        io::stdin().read_line(&mut input).expect("Read failed");

        match input.trim().parse::<u32>() {
            Ok(num) => return num,
            Err(_) => println!("Please enter a valid number."),
        }
    }
}

fn main() {
    let n1 = read_number("Enter First Number:");
    let n2 = read_number("Enter Second Number:");

    println!("Result: {}", n1 + n2);
}

• No panic
• Keeps asking until valid input
• Real-world pattern

#input #stdio #panicVer 2.0.12