The do-while loop

Learn the do-while loop that runs at least once and how it differs from while.

The do-while loop

A do-while loop runs the body first, then checks the condition. Because of this, the body always runs at least once. Its shape is do { ... } while (condition);.

For example, $n = 1; do { echo $n; $n++; } while ($n <= 3); prints 1, 2 and 3 in turn. Since the condition is checked at the end, the body is guaranteed to run once.

do-while versus while

The main difference between do-while and while is where the condition is checked. In while it is checked at the top, in do-while at the bottom.

Because of this, a while body may never run, but a do-while body always runs at least once. If you want to show a menu at least once, do-while fits well: do { echo $menu; } while ($continue);.

Exercise Print 1 to 3 with do-while

Use a do-while loop to print the numbers from 1 to 3, one per line. Start $n at 1, increase it by one each step, and use the condition $n <= 3.

Show example solution

Start with do { }, write echo $n . "\n" and $n++ in the body, then end with while ($n <= 3);.
Example solution:
<?php
$n = 1;
// Numbers 1 to 3
do {
    echo $n . "\n";
    $n++;
} while ($n <= 3);

Test yourself

At least how many times does a do-while body run?
- 1
- 0
- 2
- Only if the condition is true
Show answer

1 — Correct. do-while runs the body first, then checks the condition.

Complete the do-while

$n = 3;
do {
    echo $n;
    $n--;
} ____ ($n >= 1);

Show answer

$n = 3;
do {
    echo $n;
    $n--;
} while ($n >= 1);

Match each loop type to its trait

for

Fixed repeats with a counter
while

Condition checked first
do-while

Body runs at least once
What is the output of the code below?
```
$n = 5;
do {
    echo $n;
    $n--;
} while ($n > 5);
```
- 43210
- 5432
- 5
- Empty
Show answer

5 — Correct. The body runs once and prints 5; then the condition is false.

Next lesson: The foreach loop

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