Dart For In Loop

A for in loop runs a block once for each value in a group. The loop name takes each value in turn.

Visiting three numbers

The name n takes 4, then 8, then 15. Each value reaches the screen on its own line.

void main() {
  for (var n in [4, 8, 15]) {
    print(n);
  }
}

Visiting pieces of text

The name color takes each word in turn. The words print in the order they sit in the group.

void main() {
  for (var color in ['red', 'blue', 'green']) {
    print(color);
  }
}

Working with each value

Each run doubles the current value before printing. The doubled values keep the order of the group.

void main() {
  for (var n in [1, 2, 3]) {
    print(n * 2);
  }
}

Example

void main() {
  var total = 0;
  for (var n in [4, 8, 15]) {
    total += n;
  }
  print(total);
}

Key points

Use a for in loop to visit each value in a group.
The word in sits between the loop name and the group.
The loop name holds one value on each run.
The runs follow the order of the values in the group.
The block runs once per value, with no counter to manage.

In this example, total starts at 0. The loop visits 4 first and total grows to 4. The loop visits 8 next and total grows to 12. The loop visits 15 last and total grows to 27. The print after the loop shows 27.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

void main() {
  var total = 0;
  for (var n in [4, 8, 15]) {
    total += n;
  }
  print(total);
}

Exercise

How many times does the block run in a for in loop?

Once in total Once for each value in the group Twice for each value in the group Until a condition turns false