Dart Lesson 18 of 102 3 min read

break and continue in Dart

Use break to leave a loop early and continue to skip one round. Includes labels for controlling nested loops in Dart.

Sometimes you need to change the flow of a loop from the inside. break stops the loop completely. continue skips the rest of the current round and moves on to the next.

break #

void main() {
  var numbers = [4, 9, 15, 22, 8];

  for (final n in numbers) {
    if (n > 10) {
      print('First number over 10 is $n');
      break;
    }
    print('Checked $n');
  }
}

Checked 4
Checked 9
First number over 10 is 15

Once the answer is found there is no reason to look at 22 and 8, so break saves the work.

continue #

void main() {
  for (var i = 1; i <= 6; i++) {
    if (i.isEven) continue; // skip even numbers
    print(i);
  }
}

1
3
5

break in a while (true) loop #

A loop with the condition true runs until a break. This is a common way to write “keep going until done”.

void main() {
  var total = 0;
  var n = 1;

  while (true) {
    total += n;
    if (total > 20) break;
    n++;
  }
  print('Stopped at n = $n with total $total');
}

Stopped at n = 6 with total 21

Nested loops and labels #

break and continue only affect the loop they are written in. To control an outer loop, give it a label.

void main() {
  outer:
  for (var row = 1; row <= 3; row++) {
    for (var col = 1; col <= 3; col++) {
      if (col == 2 && row == 2) break outer; // leaves both loops
      print('row $row, col $col');
    }
  }
  print('Done');
}

row 1, col 1
row 1, col 2
row 1, col 3
row 2, col 1
Done

continue outer; works the same way: it jumps to the next round of the labelled loop.

Labels are rarely needed. If you reach for one, consider moving the loops into a function and using return instead.

A practical example #

void main() {
  var entries = ['12', 'abc', '7', '', 'stop', '99'];
  var sum = 0;

  for (final entry in entries) {
    if (entry == 'stop') break;   // stop reading
    final value = int.tryParse(entry);
    if (value == null) continue;  // ignore bad input
    sum += value;
  }
  print('Sum: $sum');
}

Sum: 19

Things to remember #

• break and continue do not work inside forEach. Use a for-in loop when you need them.
• In a while loop, make sure the counter is updated before continue, or the loop will never end.
• Since Dart 3 you do not need break at the end of a switch case.

Try it yourself #

Loop from 1 to 50. Skip every number divisible by 3, and stop completely at the first number divisible by both 5 and 7. Print the numbers you visit.