Level 16. While Loops: Keep Going Until…

A for loop runs a set number of times. But sometimes you don’t know how many times. You just want to keep going until something happens. That’s what while is for!

fun main() {
  var x = 5
  while x > 0 {
    println(x)
    x = x - 1
  }
  println("liftoff!")
}

This prints 5, 4, 3, 2, 1, liftoff!. It keeps running while x > 0, and each time around, x gets smaller by 1.

Notice the var. We need a changeable box because the loop updates x each time around. A let box can’t change, so it wouldn’t work here.

How while works

1. Check the condition (x > 0).
2. If true, run the block { ... }.
3. Go back to step 1.
4. If false, skip the block and continue after the loop.

It’s like a guard at a gate: “Are you still above zero? Yes? Go again. No? You’re done.”

Summing numbers with while

fun sum_to(n) {
  var total = 0
  var i = 1
  while i <= n {
    total = total + i
    i = i + 1
  }
  total
}

fun main() {
  println(sum_to(100))    // 5050
}

Be careful!

If the condition never becomes false, the loop runs forever! Make sure something in the body moves you toward the exit:

// ❌ BAD — x never changes, so x > 0 is always true!
// var x = 5
// while x > 0 { println(x) }

// ✅ GOOD — x decreases each time
var x = 5
while x > 0 {
  println(x)
  x = x - 1
}

🎯 Try it: Write a while loop that finds the first power of 2 bigger than 1000. Start with var n = 1 and keep doubling: n = n * 2.