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.
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.
add a note
User Contributed Notes 9 notes
jayreardon at gmail dot com ¶
18 years ago
Martin ¶
9 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>';
?>
mparsa1372 at gmail dot com ¶
4 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);
?>
anonymous at example dot com ¶
8 years ago
The last example on this page is simply abuse of the `break` keyword. Also, the suggestion to use `goto` if you don't understand the abuse of `break` is unsettling. (See the manual page for `goto` for more than enough reasons not to use it.)
The final example is generally better expressed using a typical if-else statement.
<?php
if ($i < 5) {
echo "i is not big enough";
} else {
$i *= $factor;
if ($i >= $minimum_limit) {
echo "i is ok";
/* process i */
}
}
?>
This version is easier to read and understand. And arguments for code golf are invalid as well as this version is 3 lines shorter.
In conclusion, although you can certainly write code that abuses the `break` keyword, you shouldn't in practice. Keep the code easy to read and understand for whoever inherits your code. And remember, code is for humans not computers.
andrew at NOSPAM dot devohive dot com ¶
16 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 = $child->next_sibling() ); // we have to use next_sibling for everything after first_child
?>
fuhse at data-quest dot de ¶
8 years ago
What actually surprised me: There is no alternative-syntax or template syntax for a do-while-loop.
So you can write
<?php
while ($a < 10) :
$a++;
endwhile;
?>
But this won't work:
<?php
do :
$a++
while ($a <= 10);
?>
M. H. S. ¶
5 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 ¶
7 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"
?>
shaida dot mca at gmail dot com ¶
14 years ago
Example of Do while :-
<?php
$i = 0;
echo 'This code will run at least once because i default value is 0.<br/>';
do {
echo 'i value is ' . $i . ', so code block will run. <br/>';
++$i;
} while ($i < 10);
?>
