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

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 (var candidate in candidates) {
  candidate.interview();
}

In the previous example loop, candidate is defined within the loop body and set to reference one value from candidates at a time. candidate is a local variable. Reassigning candidate inside the loop body only changes the local variable for that iteration and doesn't modify the original candidates iterable.

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

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());

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());

A label is an identifier followed by a colon (labelName:) that you can place before a statement to create a labeled statement. Loops and switch cases are often used as labeled statements. A labeled statement can be referenced later in a break or continue statement as follows:

• break labelName; Terminates the execution of the labeled statement. This is useful for breaking out of a specific outer loop when you're within a nested loop.

• continue labelName; Skips the rest of the current iteration of the labeled statement loop and continues with the next iteration.

Labels are used to manage control flow. They are often used with loops and switch cases and allow you to specify which statement to break out of or continue, rather than affecting the innermost loop by default.

The following code demonstrates the usage of a label called outerLoop in a for loop with a break statement:

outerLoop:
for (var i = 1; i <= 3; i++) {
  for (var j = 1; j <= 3; j++) {
    print('i = $i, j = $j');
    if (i == 2 && j == 2) {
      break outerLoop;
    }
  }
}
print('outerLoop exited');

In the previous example, when i == 2 and j == 2, the break outerLoop; statement stops both inner and outer loops. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
outerLoop exited

The following code demonstrates the use of a label called outerLoop in a for loop with a continue statement:

outerLoop:
for (var i = 1; i <= 3; i++) {
  for (var j = 1; j <= 3; j++) {
    if (i == 2 && j == 2) {
      continue outerLoop;
    }
    print('i = $i, j = $j');
  }
}

In the previous example, when i == 2 and j == 2, continue outerLoop; skips the rest of the iterations for i = 2 and moves to i = 3. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

The following code demonstrates the use of a label called outerLoop in a while loop with a break statement:

var i = 1;
​
outerLoop:
while (i <= 3) {
  var j = 1;
  while (j <= 3) {
    print('i = $i, j = $j');
    if (i == 2 && j == 2) {
      break outerLoop;
    }
    j++;
  }
  i++;
}
print('outerLoop exited');

In the previous example, the program breaks out of both inner and outer while loops when i == 2 and j == 2. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
outerLoop exited

The following code demonstrates the use of a label called outerLoop in a while loop with a continue statement:

var i = 1;
​
outerLoop:
while (i <= 3) {
  var j = 1;
  while (j <= 3) {
    if (i == 2 && j == 2) {
      i++;
      continue outerLoop;
    }
    print('i = $i, j = $j');
    j++;
  }
  i++;
}

In the previous example, the iteration for i = 2 and j = 2 is skipped and the loop moves directly to i = 3. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

The following code demonstrates the use of a label called outerLoop in a do while loop with a break statement:

var i = 1;
​
outerLoop:
do {
  var j = 1;
  do {
    print('i = $i, j = $j');
    if (i == 2 && j == 2) {
      break outerLoop;
    }
    j++;
  } while (j <= 3);
  i++;
} while (i <= 3);
​
print('outerLoop exited');

In the previous example, the program breaks out of both inner and outer loops when i == 2 and j == 2. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
outerLoop exited

The following code demonstrates the use of a label called outerLoop in a do while loop with a continue statement:

var i = 1;
​
outerLoop:
do {
  var j = 1;
  do {
    if (i == 2 && j == 2) {
      i++;
      continue outerLoop;
    }
    print('i = $i, j = $j');
    j++;
  } while (j <= 3);
  i++;
} while (i <= 3);

In the previous example, the loop skips i = 2 and j = 2 and moves directly to i = 3. As a result, the output is:

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3