Loops

This page shows how you can control the flow of your Dart code using loops and supporting statements:

• for loops
• while and do while loops
• break and continue

You can also manipulate control flow in Dart using:

• Branching, like if and switch
• Exceptions, like try, catch, and throw

For loops

You can iterate with the standard for loop. For example:

var message = StringBuffer('Dart is fun');
for (var i = 0; i < 5; i++) {
  message.write('!');
}

Closures inside of Dart’s for loops capture the value of the index. This avoids a common pitfall found in JavaScript. For example, consider:

var callbacks = [];
for (var i = 0; i < 2; i++) {
  callbacks.add(() => print(i));
}

for (final c in callbacks) {
  c();
}

The output is 0 and then 1, as expected. In contrast, the example would print 2 and then 2 in JavaScript.

Sometimes you might not need to know the current iteration counter when iterating over an Iterable type, like List or Set. In that case, use the for-in loop for cleaner code:

for (final candidate in candidates) {
  candidate.interview();
}

To process the values obtained from the iterable, you can also use a pattern in a for-in loop:

for (final Candidate(:name, :yearsExperience) in candidates) {
  print('$name has $yearsExperience of experience.');
}

Iterable classes also have a forEach() method as another option:

var collection = [1, 2, 3];
collection.forEach(print); // 1 2 3

While and do-while

A while loop evaluates the condition before the loop:

while (!isDone()) {
  doSomething();
}

A do-while loop evaluates the condition after the loop:

do {
  printLine();
} while (!atEndOfPage());

Break and continue

Use break to stop looping:

while (true) {
  if (shutDownRequested()) break;
  processIncomingRequests();
}

Use continue to skip to the next loop iteration:

for (int i = 0; i < candidates.length; i++) {
  var candidate = candidates[i];
  if (candidate.yearsExperience < 5) {
    continue;
  }
  candidate.interview();
}

If you’re using an Iterable such as a list or set, how you write the previous example might differ:

candidates
    .where((c) => c.yearsExperience >= 5)
    .forEach((c) => c.interview());