Level 29. Asking for Input

So far, your programs have been one-way conversations: the computer talks, but you can’t talk back. Let’s change that! The input function prints a question and waits for the user to type an answer.

fun main() {
  let name = input("What is your name? ")
  println("Hello, " + name + "!")
}

When you run this, the computer prints the prompt, then waits. You type your answer, press Enter, and the program continues with whatever you typed.

Reading numbers

input always gives you a string. If you want a number, use parse_int or parse_float to convert it, and match to handle the case where the user types something that isn’t a number:

fun main() {
  let age_str = input("How old are you? ")
  match parse_int(age_str) {
    Some(age) => println("In 10 years you'll be {age + 10}"),
    None      => println("That's not a number!")
  }
}

A guessing game!

Combine input, parse_int, and match guards for a mini game:

fun main() {
  let secret = 7
  println("I'm thinking of a number between 1 and 10...")
  let guess_str = input("Your guess: ")
  match parse_int(guess_str) {
    Some(n) if n == secret => println("Correct!"),
    Some(_)               => println("Wrong! It was {secret}"),
    None                  => println("Please enter a number!")
  }
}

Notice how we use a match guard (if n == secret). That’s the pattern matching trick from level 11!

🎯 Try it: Write a program that asks for your name and your favourite colour, then prints "Hi ___, your favourite colour is ___!".

🎯 Challenge: Make a simple calculator: ask for two numbers and an operator (+, -, *, /), then print the result. Use match on the operator string!