Level 30. Random Numbers: Roll the Dice!

What if your program could surprise you? With random, it can! The random function picks a number for you: a different one each time you run the program.

Rolling a die

random(min, max) gives you a random integer. Both numbers are included — so random(1, 6) can give you 1, 2, 3, 4, 5, or 6. To roll a six-sided die:

fun main() {
  let die = random(1, 6)
  println("You rolled a {die}!")
}

This works just like for i in 1..6, both ends are included. Simple!

🎯 Try it: Run the die program several times. You’ll get a different number each time!

Coin flip

A coin has two sides. Use random(0, 1) to pick between them:

fun main() {
  let flip = random(0, 1)
  if flip == 0 { println("Heads!") }
  else { println("Tails!") }
}

Rolling many dice

Combine random with a loop to roll several dice:

fun main() {
  var total = 0
  for i in 1..3 {
    let roll = random(1, 6)
    println("Die {i}: {roll}")
    total = total + roll
  }
  println("Total: {total}")
}

This rolls 3 dice, prints each one, and adds them up. Just like a board game!

A real guessing game!

Remember the guessing game in level 29? The secret number was hard-coded. Now we can make it truly random:

fun main() {
  let secret = random(1, 10)
  println("I picked a number between 1 and 10...")
  let guess_str = input("Your guess: ")
  match parse_int(guess_str) {
    Some(n) if n == secret => println("You got it!"),
    Some(n) if n < secret  => println("Too low! It was {secret}"),
    Some(n)                => println("Too high! It was {secret}"),
    None                   => println("That's not a number!")
  }
}

Every time you play, the answer is different. Now it’s a real game!

Random choices

You can use random to pick a random item from a list by generating a random index:

fun main() {
  let snacks = ["apple", "banana", "cookie", "donut"]
  let pick = random(0, length(snacks) - 1)
  println("Today's snack: {snacks[pick]}")
}

🎯 Try it: Make a list of 5 activities (“read”, “draw”, “code”, etc.) and have the computer pick one at random. Run it a few times!

🎯 Challenge: Write a rock-paper-scissors game. The computer picks randomly (0 = rock, 1 = paper, 2 = scissors), and the player types their choice. Use match to decide who wins!