while and repeat-while Loops

A while loop repeats as long as its condition stays true. Use it when you don't know in advance how many times to loop.

Real-world analogy: "While the kettle isn't boiling, keep waiting."

while

var balance = 100
var months = 0

while balance < 200 {
    balance += balance / 10
    months += 1
}

print("Doubled after \(months) months: \(balance)")

Doubled after 8 months: 212

The condition is checked before each pass. If it's false from the start, the loop body never runs.

repeat-while

repeat { } while condition checks after each pass, so the body always runs at least once — ideal for "ask until the answer is valid":

var attempts = 0

repeat {
    attempts += 1
    print("Attempt \(attempts)")
} while attempts < 3

Attempt 1
Attempt 2
Attempt 3

Common mistakes

• Forgetting to change the variable in the condition, so the loop never ends. Stop a runaway program with Ctrl + C.
• Using while to walk through an array with a counter. for-in is clearer and can't go out of range.

Practice

Starting from 1, keep doubling a number while it's below 1,000, then print the first value that reached 1,000 or more.

var n = 1
while n < 1000 {
    n *= 2
}
print(n)

1024