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do-while
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's 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'.