while, do-while, break, and continue

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

while

void main() {
  var balance = 100;
  var months = 0;

  while (balance < 200) {
    balance += balance ~/ 10;
    months++;
  }

  print('Doubled after $months months: $balance');
}

Doubled after 8 months: 212

do-while

do { } while (condition); checks after each pass, so the body always runs at least once:

void main() {
  var attempts = 0;
  do {
    attempts++;
    print('Attempt $attempts');
  } while (attempts < 3);
}

Attempt 1
Attempt 2
Attempt 3

break and continue

• break stops the loop completely.
• continue skips to the next pass.

void main() {
  for (var n = 1; n <= 10; n++) {
    if (n.isEven) continue;
    if (n > 7) break;
    print(n);
  }
}

1
3
5
7

Common mistakes

• Forgetting to change the variable in a while condition, so the loop never ends. Stop a runaway program with Ctrl + C.

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.

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

1024