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.