do-while
(PHP 4, PHP 5, PHP 7, PHP 8)
Los bucles do-while
son muy similares a los
bucles while
, excepto que la expresión verdadera es
verificada al final de cada iteración en lugar que al principio.
La diferencia principal con los bucles while
es
que está garantizado que corra la primera iteración de un bucle
do-while
(la expresión verdadera sólo es verificada al
final de la iteración), mientras que no necesariamente va a correr con un
bucle while
regular (la expresión verdadera es
verificada al principio de cada iteración, si se evalúa como
false
justo desde el comienzo, la ejecución del
bucle terminaría inmediatamente).
Hay una sola sintaxis para bucles do-while
:
El bucle de arriba se ejecutaría exactamente una sola vez, ya que después de la primera
iteración, cuando la expresión verdadera es verificada, se evalúa como
false
($i no es mayor que 0) y termina la ejecución
del bucle.
Los usuarios avanzados de C pueden estar familiarizados con un uso distinto del
bucle do-while
, para permitir parar la ejecución en
medio de los bloques de código, mediante el encapsulado con
do-while
(0), y utilizando la sentencia break
.
El siguiente fragmento de código demuestra esto:
No se preocupe si no se entiende esto completamente o en absoluto.
Se pueden codificar scripts e incluso scripts potentes sin usar esta
'característica'.
Desde PHP 5.3.0, es posible utilizar el operador
goto
en lugar de este hack.
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.
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>';
?>
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);
?>
andrew at NOSPAM dot devohive dot com ¶15 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() ); ?>
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;
}
?>
M. H. S. ¶4 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.