do-while

(PHP 4, PHP 5, PHP 7)

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);
?>

Don't worry if you don't understand this right away or at all. You can code scripts and even powerful scripts without using this 'feature'. Since PHP 5.3.0, it is possible to use goto operator instead of this hack.

User Contributed Notes

Martin

1 year 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>';
?>

gried at NOSPAM dot nsys dot by

3 months ago


It might be obvious but still, for those who need to use do-while with alternative syntax you can use while-break combination like in a code below:

<?php
  do{

    // ... some statements

  } while ($expr);
?>

is equal to

<?php
  while(1): ?>

    some html code

<?php 
  if (! $expr) break;
  endwhile;
?>

jayreardon at gmail dot com

9 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.

shaida dot mca at gmail dot com

5 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);
?>

Ryan

8 years ago


I've found that the most useful thing to use do-while loops for is multiple checks of file existence. The guaranteed iteration means that it will check through at least once, which I had trouble with using a simple "while" loop because it never incremented at the end.

My code was:

<?php
$filename = explode(".", $_FILES['file']['name']); // File being uploaded
$i=0; // Number of times processed (number to add at the end of the filename)
do {
  /* Since most files being uploaded don't end with a number,
      we have to make sure that there is a number at the end
      of the filename before we start simply incrementing. I
      admit there is probably an easier way to do this, but this
      was a quick slap-together job for a friend, and I find it
      works just fine. So, the first part "if($i > 0) ..." says that
      if the loop has already been run at least once, then there
      is now a number at the end of the filename and we can 
      simply increment that. Otherwise, we have to place a
      number at the end of the filename, which is where $i
      comes in even handier */

  if($i > 0) $filename[0]++;
  else $filename[0] = $filename[0].$i;
  $i++;
} while(file_exists("uploaded/".$filename[0].".".$filename[1]));

/* Now that everything is uploaded, we should move it
    somewhere it can be accessed. Hence, the "uploaded"
    folder. */
move_uploaded_file($_FILES['file']['tmp_name'], "uploaded/".$filename[0].".".$filename[1]);
?>

I'm sure there are plenty of ways of doing this without using the do-while loop, but I managed to toss this one together in no-time flat, and I'm not a great PHP programmer. =) It's simple and effective, and I personally think it works better than any "for" or "while" loop that I've seen that does the same thing.

andrew at NOSPAM dot devohive dot com

7 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
?>

jantsch at gmail dot com

8 years ago


Useful when you want to continue to read a recordset that was already being read like in:

<?
$sql = "select * from customers";
$res = mysql_query( $sql );

// read the first record
if( $rs = mysql_fetch_row( $res ) ){
   // do something with this record

}

// do another stuff here

// keep reading till the end
if( mysql_num_rows( $res )>1 ){
   do{
      // processing the records till the end

   }while( $rs = mysql_fetch_row( $res ));

}

?>

david dot schueler at tel-billig dot de

5 years ago


If you are trying to use a construct like this:
<?php
do {
    // some code to run only one more time if expression is true
    if ($your_expression == true)
        continue;
} while (false);
?>
It will NOT loop as expected, because the continue tries to run the "next" loop, but the expression says just "false" so there is no next loop. The continue exits the while loop.
To get around this you may use an other expression, like this
<?php
do {
    // some code to run only one more time if expression is true
    if ($your_expression == true)
        continue;
    break;
} while (true);
?>
or use the goto statement since PHP 5.3.

xiaomao5 at live dot com

5 years ago


Another hack

If you want $type to only have a value of 0 or 1, you can do this:



<?php

do {

    echo 'Choose a type, 0 or 1: ';

    $type = trim(fgets(STDIN));

    if ($type == 0) {

        /* do stuff */

    } elseif ($type == 1) {

        /* do stuff */    

    }

} while ($type != 0 && $type != 1);

?>

info at pkrules dot in

2 years ago


Notice the way of using the loops:-
 <?php
$arr=array("orange","banana","apple","raspberry");

$i=0;
while($i < count($arr)){
  $a=$arr[$i];
  echo $a."<br />";
  $i++;
}
/*
orange
banana
apple
raspberry

*/
echo "<br /> &nbsp;";
$i=0;
$c=count($arr);
while($i < $c){
 $a=$arr[$i];
 echo $a."<br />";
 $i++;
}
/*
orange
banana
apple
raspberry

*/
echo "&nbsp;<br />";
while($a =$arr[1*--$i]){
echo $a."<br />";
if($i<=0)break;

}
echo "<br />";
/*
orange
banana
apple
raspberry

*/
foreach($arr as $i){
print $i;
echo "<br />";
}
/*
orange
banana
apple
raspberry

*/
?>