The while loop

Learn loops that run while a condition holds with while.

The while loop

A while loop repeats the block as long as a condition is true. The form is while (condition) { ... }. The condition is checked before each iteration; when false the loop ends.

For example var i = 5; while (i > 0) { print(i); i--; } prints 5, 4, 3, 2, 1. Note: without i-- the loop is infinite because i stays 5 and the condition stays true.

Condition and update

In while the condition is written in parentheses and checked before each iteration. If true the body runs; if false the loop ends. The body must include a change (increase or decrease) that progresses the condition; otherwise the loop is infinite.

An infinite loop stops the program from continuing. The form while (true) always produces true; without a break inside it becomes an infinite loop. Practical tip: when writing while make sure the condition will become false at some point.

Exercise Count down from 5 to 1

Start with var i = 5;. Build a while (i > 0) loop; in the body print with print(i); and decrease with i--;. The output must be 5 lines: 5, 4, 3, 2, 1 (one number per line).

Show example solution

var i = 5;
while (i > 0) {
  print(i);
  i--;
}

Test yourself

Write the while condition

var i = 5;
while (____) {
  print(i);
  i--;
}

Show answer

var i = 5;
while (i > 0) {
  print(i);
  i--;
}

Which one causes an infinite loop?
```
// while examples
```
- while (i < 10) { i++; }
- while (true) { print(i); }
- while (i > 10) { i--; }
- while (i < 10) { break; }
Show answer

while (true) { print(i); } — The condition while (true) is always true, so the loop is infinite.
Match each part to its job

while (cond)

Condition check
{ ... }

Body
i--

Decrease
i > 0

Condition example

Next lesson: The do-while loop

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