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Chapter 11. Control Structures |
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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:
$i = 0;
do {
print $i;
} while ($i>0);
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The above loop would run one time exactly, since after the first iteration, when truth
expression is checked, it evaluates to FALSE ($i is not bigger than 0) and the loop
execution ends.
Advanced C users may be familiar with a different usage of the do..while loop, to
allow stopping execution in the middle of code blocks, by encapsulating them with
do..while(0), and using the break
statement. The following code fragment demonstrates this:
do {
if ($i < 5) {
print "i is not big enough";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
print "i is ok";
...process i...
} while(0);
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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'.
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