while
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
?>
do..while
do..while loops lijken erg veel op while loops, alleen wordt de vergelijking nu gecontroleerd aan het einde van de interatie, in plaats van aan het begin. Het grootste verschil met normale while loops is dat er altijd minimaal één iteratie van een do..while loop wordt uitgevoerd. De expressie wordt namelijk pas aan het einde van de iteratie geëvalueerd, dit in tegenstelling tot de normale while loop, waar de expressie aan het begin van een iteratie wordt geëvalueerd, Als de expressie meteen al aan het begin evauleerd tot FALSE, dan zal de loop niet één keer worden uitgevoerd.
Er is maar één systax voor do..while loops:
$i = 0;
do {
print $i;
} while ($i > 0);
De bovenstaande loop wordt exact één keer uitgevoerd. Als na de eerste iteratie, wanneer de expressie wordt geëvalueerd, de waarheids expressie evalueerd tot FALSE ($i is niet groter dan 0) dan eindigd de loop.
Gevorderde C programmeurs zullen bekend zijn met een ander gebruik van de do..while loop, namelijk om de executie in het midden van een blok code to stoppen. Dit kan door een groep statements te omvatten met do..while(0), en dan het break statement te gebruiken om naar het einde van de loop te springen. Het volgende voorbeeld demonstreert dit:
do {
if ($i < 5) {
print "i is niet groot genoeg";
break;
}
$i *= $factor;
if ($i < $minimum_limiet) {
break;
}
print "i is ok";
...doe wat met i...
} while(0);
Vrees niet als je dit niet meteen bergijpt, of helemaal niet. Je kunt zeer complexe PHP scripts schrijven zonder gebruik te maken van deze `feature'.
add a note
User Contributed Notesdo..while
andrew at NOSPAM dot devohive dot com
15-Sep-2008 07:44
15-Sep-2008 07:44
Ryan
18-Apr-2008 07:59
18-Apr-2008 07:59
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.
jantsch at gmail dot com
29-Nov-2007 02:56
29-Nov-2007 02:56
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 ));
}
?>
jayreardon at gmail dot com
11-Apr-2007 12:36
11-Apr-2007 12:36
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.