The for loop

Learn to write counted loops with for.

The for loop

To repeat a code block a fixed number of times you use a for loop. The form is for (init; condition; update) { ... }. The init runs once; the condition is checked before each iteration; the update runs after each iteration.

For example for (var i = 1; i <= 3; i++) { print(i); } prints 1, 2, 3. i++ increases i by one. When the condition becomes false the loop ends.

Init, condition, update

The for loop has three parts: init (var i = 1), condition (i <= 5) and update (i++). The init runs once before the loop; the condition is checked before each iteration; the update runs after each iteration.

The update is not always i++. With i += 2 you count by twos; with i-- you count backwards. When counting backwards adjust the condition: for (var i = 5; i >= 1; i--) { print(i); } counts from 5 down to 1.

Exercise Count from 1 to 5

Build a for (var i = 1; i <= 5; i++) loop. In the body print each number with print(i);. The output must be 5 lines: 1, 2, 3, 4, 5 (one number per line).

Show example solution

for (var i = 1; i <= 5; i++) {
  print(i);
}

Test yourself

How many times does this loop run?
```
for (var i = 1; i <= 3; i++) {
  print(i);
}
```
- 3 times
- 1 time
- Infinitely
- 0 times
Show answer

3 times — The condition is true for i=1, 2, 3; 3 iterations run.

Complete the for header

for (____) {
  print(i);
}

Show answer

for (var i = 1; i <= 5; i++) {
  print(i);
}

Match each part to its job

var i = 1

Init
i <= 5

Condition
i++

Update
{ ... }

Body

Next lesson: The while loop

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