Dart Lesson 17 of 102 3 min read

Loops in Dart: for, for-in, while and do-while

Repeat code in Dart with for, for-in, while and do-while loops and forEach. Learn which loop to pick and how to avoid infinite loops.

A loop runs the same block of code many times. Dart has four loop statements; which you choose depends on what you know before the loop starts.

for: when you know how many times #

void main() {
  for (var i = 1; i <= 5; i++) {
    print('Count: $i');
  }
}

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

The three parts in the brackets are: start (var i = 1), keep going while (i <= 5) and after each round (i++).

for-in: once for each item #

This is the loop you will use most. It hands you each element of a collection in turn.

void main() {
  var fruits = ['apple', 'mango', 'kiwi'];

  for (final fruit in fruits) {
    print('I like $fruit');
  }
}

I like apple
I like mango
I like kiwi

Need the position as well? Use indexed.

void main() {
  var fruits = ['apple', 'mango', 'kiwi'];

  for (final (index, fruit) in fruits.indexed) {
    print('${index + 1}. $fruit');
  }
}

1. apple
2. mango
3. kiwi

while: until a condition changes #

A while loop checks the condition before each round. If it is false at the start, the body never runs.

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

  while (savings < 500) {
    savings += 150;
    months++;
  }
  print('Reached $savings after $months months');
}

Reached 550 after 3 months

do-while: at least once #

A do-while loop checks the condition after each round, so the body always runs at least one time.

void main() {
  var attempts = 0;

  do {
    attempts++;
    print('Attempt $attempts');
  } while (attempts < 3);
}

Attempt 1
Attempt 2
Attempt 3

forEach #

Collections have a forEach method that takes a function. It is fine for one-liners; for anything longer, for-in is easier to read and works with break and await.

void main() {
  [1, 2, 3].forEach(print);

  var prices = {'tea': 20, 'coffee': 50};
  prices.forEach((item, price) => print('$item costs $price'));
}

1
2
3
tea costs 20
coffee costs 50

Nested loops #

void main() {
  for (var row = 1; row <= 3; row++) {
    var line = '';
    for (var col = 1; col <= 3; col++) {
      line += '${row * col}\t';
    }
    print(line);
  }
}

Which loop should I use? #

Avoiding infinite loops #

A loop never ends if its condition never becomes false. The usual cause is forgetting to update the variable the condition depends on. Before running a while loop, check that something inside the body moves it towards the exit.

Try it yourself #

Print the multiplication table for 7 from 7 x 1 = 7 to 7 x 10 = 70. Then use a while loop to find the first power of 2 that is greater than 1000.