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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);
?>
Seien Sie nicht traurig, wenn Sie dies nicht oder nicht ganz verstehen. Sie können trotzdem Skripte - und wirklich leistungsfähige Skripte! - programmieren, ohne dieses "Feature" zu verwenden. Seit PHP 5.3.0 ist es möglich, statt dieses Hacks den goto-Operator zu verwenden.
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);
?>
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
?>
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.
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 ));
}
?>
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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