do-while
(PHP 4, PHP 5)
do-while loops are very similar to
while loops, except the truth expression is
checked at the end of each iteration instead of in the beginning.
The main difference from regular while loops is
that the first iteration of a do-while loop is
guaranteed to run (the truth expression is only checked at the end
of the iteration), whereas it may not necessarily run with a
regular while loop (the truth expression is
checked at the beginning of each iteration, if it evaluates to
FALSE right from the beginning, the loop
execution would end immediately).
There is just one syntax for do-while loops:
<?php
$i = 0;
do {
echo $i;
} while ($i > 0);
?>
The above loop would run one time exactly, since after the first
iteration, when truth expression is checked, it evaluates to
FALSE ($i is not bigger than 0) and the loop
execution ends.
Advanced C users may be familiar with a different usage of the do-while loop, to allow stopping execution in the middle of code blocks, by encapsulating them with do-while (0), and using the break statement. The following code fragment demonstrates this:
<?php
do {
if ($i < 5) {
echo "i is not big enough";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
echo "i is ok";
/* process i */
} while (0);
?>
Don't worry if you don't understand this right away or at all. You can code scripts and even powerful scripts without using this 'feature'. Since PHP 5.3.0, it is possible to use goto operator instead of this hack.
Коментарии
There is one major difference you should be aware of when using the do--while loop vs. using a simple while loop: And that is when the check condition is made.
In a do--while loop, the test condition evaluation is at the end of the loop. This means that the code inside of the loop will iterate once through before the condition is ever evaluated. This is ideal for tasks that need to execute once before a test is made to continue, such as test that is dependant upon the results of the loop.
Conversely, a plain while loop evaluates the test condition at the begining of the loop before any execution in the loop block is ever made. If for some reason your test condition evaluates to false at the very start of the loop, none of the code inside your loop will be executed.