do-while
(PHP 4, PHP 5, PHP 7, PHP 8)
Les boucles do-while
ressemblent beaucoup
aux boucles while
, mais l'expression est
testée à la fin de chaque itération plutôt
qu'au début. La principale différence par rapport
à la boucle while
est que la
première itération de la boucle
do-while
est toujours exécutée
(l'expression n'est testée qu'à la fin de
l'itération), ce qui n'est pas le cas lorsque vous
utilisez une boucle while
(la condition
est vérifiée dès le début de chaque itération, et si elle
s'avère false
dès le début, la boucle sera immédiatement arrêtée).
Il n'y a qu'une syntaxe possible pour les boucles do-while
:
La boucle ci-dessus ne va être exécutée
qu'une seule fois, car lorsque l'expression est
évaluée, elle vaut false
(car
la variable $i n'est pas plus grande que 0) et l'exécution
de la boucle s'arrête.
Les utilisateurs familiers du C sont habitués à
une utilisation différente des boucles
do-while
, qui permet de stopper
l'exécution de la boucle au milieu des instructions, en
encapsulant dans un do-while
(0) la
fonction break
.
Le code suivant montre une utilisation possible :
Il est possible d'utiliser l'opérateur
goto
au lieu de ce bidouillage.
jayreardon at gmail dot com ¶16 years ago
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.
mparsa1372 at gmail dot com ¶2 years ago
The example below first sets a variable $x to 1 ($x = 1). Then, the do while loop will write some output, and then increment the variable $x with 1. Then the condition is checked (is $x less than, or equal to 5?), and the loop will continue to run as long as $x is less than, or equal to 5:
<?php
$x = 1;
do {
echo "The number is: $x <br>";
$x++;
} while ($x <= 5);
?>
Martin ¶8 years ago
Do-while loops can also be used inside other loops, for example:
<?php
$numbers = array();
$array_size = 10;
for ($i=0;$i<$array_size;$i++) {
do {
$random = rand(1,1000);
} while (($random % 2) == 1);
$numbers[] = $random;
}
asort($numbers);
echo '<pre>';
print_r($numbers);
echo '</pre>';
?>
andrew at NOSPAM dot devohive dot com ¶14 years ago
I'm guilty of writing constructs without curly braces sometimes... writing the do--while seemed a bit odd without the curly braces ({ and }), but just so everyone is aware of how this is written with a do--while...
a normal while:
<?php
while ( $isValid ) $isValid = doSomething($input);
?>
a do--while:
<?php
do $isValid = doSomething($input);
while ( $isValid );
?>
Also, a practical example of when to use a do--while when a simple while just won't do (lol)... copying multiple 2nd level nodes from one document to another using the DOM XML extension
<?php
$fileDoc = domxml_open_file('example.xml'); $fileRoot = $fileDoc->document_element();
$newDoc = domxml_new_doc('1.0'); $newRoot = $newDoc->create_element('rootnode');
$newRoot = $newDoc->append_child($newRoot); $child = $fileRoot->first_child(); do $newRoot->append_child($child->clone_node(true)); while ( $child = $child->next_sibling() ); ?>
M. H. S. ¶3 years ago
<!-- if you write with WHILE: -->
<?php
$i = 100
while ($i < 10) :
echo "\$i is $i.";
endwhile;
?>
<!-- returning: -->
<!-- if you write with DO/WHILE: -->
<?php
$i = 100;
do {
echo "\$i is $i.";
} while ($i < 10);
?>
<!-- returning: -->
$i is 100.
iamjeffjack at gmail dot com ¶5 years ago
If you put multiple conditions in the while check, a do-while loop checks these conditions in order and runs again once it encounters a condition that returns true. This can be helpful to know when troubleshooting why a do-while loop isn't finishing. An (illustrative-only) example:
<?php
$numberOne = 0;
do {
echo $numberOne;
$numberOne++;
} while( $numberOne < 5 || incrementNumberTwo() );
function incrementNumberTwo() {
echo "function incrementNumberTwo called";
return false;
}
?>