PHP: do-while - Manual

do-while

(PHP 4, PHP 5, PHP 7, PHP 8)

do-while-Schleifen sind sehr ähnlich zu while-Schleifen, außer dass der Wahrheitsausdruck erst am Ende eines jeden Durchlaufs statt zu dessen Beginn geprüft wird. Der Hauptunterschied zu einer normalen while-Schleife ist, dass die do-while-Schleife garantiert mindestens einmal durchlaufen wird (Der Wahrheitsausdruck wird ja nur am Ende jeden Durchlaufs geprüft), wohingegen es nicht zwingend ist, dass eine reguläre while-Schleife immer ausgeführt wird (hier wird der Wahrheitsausdruck bereits zu Beginn eines jeden Durchlaufs überprüft. Evaluiert er dabei zu false, wird die Verarbeitung der Schleife sofort abgebrochen).

Es gibt nur eine Syntax für do-while-Schleifen:


<?php
$i = 0;
do {
    echo $i;
} while ($i > 0);
?>

Die obige Schleife wird exakt einmal durchlaufen, da nach dem ersten Durchlauf, wenn der Wahrheitsausdruck geprüft wird, dieser false ergibt ($i ist nicht größer als 0), so dass die Schleifenausführung beendet wird.

Fortgeschrittenen C-Programmierern ist möglicherweise eine etwas andere Verwendung von do-while-Schleifen bekannt, die es erlaubt, die Ausführung in der Mitte des Codeblocks zu unterbrechen. Dies wird durch ein Kapseln in do-while(0) und die Verwendung des break-Statements erreicht. Das folgende Codefragment demonstriert dieses Verhalten:


<?php
do {
    if ($i < 5) {
        echo "i ist nicht groß genug";
        break;
    }
    $i *= $factor;
    if ($i < $minimum_limit) {
        break;
    }
   echo "i ist ok";

    /* i verarbeiten */

} while (0);
?>

Es ist möglich, statt dieses Hacks den goto-Operator zu verwenden.

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User Contributed Notes 6 notes

down

jayreardon at gmail dot com ¶

15 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.

down

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);
?>

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-2

Martin ¶

7 years ago


Do-while loops can also be used inside other loops, for example:

<?php
// generating an array with random even numbers between 1 and 1000

$numbers = array();
$array_size = 10;

// for loop runs as long as 2nd condition evaluates to true
for ($i=0;$i<$array_size;$i++) { 

      // always executes (as long as the for-loop runs)
      do { 
         $random = rand(1,1000);

     // if the random number is even (condition below is false), the do-while-loop execution ends
     // if it's uneven (condition below is true), the loop continues by generating a new random number
     } while (($random % 2) == 1);

     // even random number is written to array and for-loop continues iteration until original condition is met
     $numbers[] = $random; 
}

// sorting array by alphabet

asort($numbers);

// printing array

echo '<pre>';
print_r($numbers);
echo '</pre>';
?>

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-25

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
   # open up/create the documents and grab the root element
   $fileDoc  = domxml_open_file('example.xml'); // existing xml we want to copy
   $fileRoot = $fileDoc->document_element();
   $newDoc   = domxml_new_doc('1.0'); // new document we want to copy to
   $newRoot  = $newDoc->create_element('rootnode');
   $newRoot  = $newDoc->append_child($newRoot); // this is the node we want to copy to

   # loop through nodes and clone (using deep)
   $child = $fileRoot->first_child(); // first_child must be called once and can only be called once
   do $newRoot->append_child($child->clone_node(true)); // do first, so that the result from first_child is appended
   while ( $child = $child->next_sibling() ); // we have to use next_sibling for everything after first_child
?>

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-25

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.

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-29

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;
    }
    // outputs "01234function incrementNumberTwo called"
?>

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