Mini project: reading input from the user

Why read input?

Programs become useful when they respond to people. In this lesson you build a small interactive tool: a GST calculator that asks for a price. Along the way you meet three ideas you will use everywhere: the standard library, mutable references, and handling errors.

Step 1: bring in the io module

use std::io;

std is the standard library that comes with Rust. io is its input/output module. A module is a named group of code; you will create your own in module 7.

Step 2: read a line

let mut text = String::new();
io::stdin()
    .read_line(&mut text)
    .expect("Failed to read input");

• String::new() creates an empty String. String is text that can grow; module 4 explains it fully.
• The variable is mut because read_line changes it.
• &mut text passes a mutable reference: read_line may change our String without taking it away from us. Module 4 is all about references.
• read_line returns a Result, which is either Ok or Err. .expect("...") means: if this is an Err, stop the program with this message. That is fine for now; module 6 shows better ways.

Step 3: turn text into a number

If the user types 250 and presses Enter, text holds "250\n" (on Windows "250\r\n"). The newline would break the conversion, so call trim(), which removes spaces and newlines from both ends. Then parse() converts the text into a number. You must say which number type you want.

let price: u32 = match text.trim().parse() {
    Ok(n) => n,
    Err(_) => {
        println!("Please type a whole number.");
        return;
    }
};

match compares a value against patterns and runs the branch that fits. Here, Ok(n) means "it worked, and the number is n". Err(_) means "it failed"; the underscore ignores the error details. You will study match properly in module 5.

Putting it together

use std::io;

const GST_PERCENT: u32 = 18;

fn main() {
    println!("Enter the price in rupees:");
    let mut text = String::new();
    io::stdin().read_line(&mut text).expect("Failed to read input");

    let price: u32 = match text.trim().parse() {
        Ok(n) => n,
        Err(_) => {
            println!("Please type a whole number.");
            return;
        }
    };

    let gst = price * GST_PERCENT / 100;
    println!("Price: {price}, GST: {gst}, Total: {}", price + gst);
}

Asking again in a loop

To keep asking until the input is valid, wrap the reading in a loop and use continue in the Err branch and break in the Ok branch. Remember to create a new String (or call text.clear()) each time, because read_line adds to what is already there.

Common mistakes

• Forgetting trim(). parse sees "250\n" and fails every time.
• Not saying the target type. let n = text.trim().parse().unwrap(); cannot know which number you want; write let n: u32 = ... or parse::<u32>().
• Reusing the same String in a loop without clear(). The second line is appended to the first.
• Using unwrap() on user input. One typo crashes the program; use match.