Loops: for and for-in

A loop repeats code.

The counting for loop

void main() {
  for (var i = 1; i <= 3; i++) {
    print('Pass $i');
  }
}

Pass 1
Pass 2
Pass 3

The three parts in the parentheses: start (var i = 1), keep going while (i <= 3), and after each pass (i++).

for-in: loop over a collection

void main() {
  final fruits = ['apple', 'banana', 'cherry'];

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

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

I like apple
I like banana
I like cherry
1. apple
2. banana
3. cherry

indexed pairs each element with its position; (index, fruit) unpacks both (a record pattern — Level 2 covers records).

Counting down and stepping

void main() {
  for (var i = 10; i >= 0; i -= 5) {
    print(i);
  }
}

10
5
0

forEach

Collections also have a forEach method that takes a function:

void main() {
  ['a', 'b'].forEach(print);
}

a
b

The style guide prefers for-in loops for most code; forEach is neat when you already have a function to pass, like print here.

Common mistakes

• Off-by-one errors: i <= list.length goes one past the end — use i < list.length, or better, for-in.

Practice

Print the 5 times table from 5 x 1 = 5 to 5 x 10 = 50.

void main() {
  for (var n = 1; n <= 10; n++) {
    print('5 x $n = ${5 * n}');
  }
}