PHP: do-while - Manual

do-while

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

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 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);
?>

It is possible to use the goto operator instead of this hack.

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User Contributed Notes 6 notes

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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.

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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 ¶

8 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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