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
Il ciclo do-while è simile al ciclo while, con l'unica differenza che il valore dell'espressione viene controllato alla fine di ogni iterazione anzichè all'inizio. La differenza più importante rispetto a while è che la prima iterazione di un blocco do-while verrà sempre eseguita (il valore dell'espressione viene controllato alla fine del ciclo), mentre non sarà necessariamente eseguito in un ciclo while (il valore dell'espressione viene controllato all'inizio del ciclo, e se tale valore è FALSE dall'inizio, l'esecuzione del ciclo termina immediatamente).
È ammessa una sola sintassi per il ciclo do-while:
<?php
$i = 0;
do {
echo $i;
} while ($i > 0);
?>
Il ciclo precedente verrà eseguito un'unica volta, dal momento che alla prima iterazione, quando si controlla l'espressione, il suo valore sarà FALSE ($i non è maggiore di 0) e il ciclo di esecuzioni, termina.
Chi ha utilizzato il linguaggio C conosce probabilmente un'altro modo di utilizzare il ciclo do-while, che permette di terminare l'esecuzione delle istruzioni durante l'esecuzione stessa, utilizzando do-while(0), e usando l'istruzione break. Il codice che segue esemplifica questa possibilità:
<?php
do {
if ($i < 5) {
echo "i non è abbastanza grande";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
echo "i è ok";
/* processa i */
} while (0);
?>
Non vi preoccupate se l'esempio non è sufficientemente chiaro. Si possono scrivere ottimi programmi PHP anche senza far ricorso a questa 'possibilità'.
add a note
User Contributed Notesdo-while
andrew at NOSPAM dot devohive dot com
15-Sep-2008 10:44
15-Sep-2008 10:44
Ryan
17-Apr-2008 10:59
17-Apr-2008 10:59
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
28-Nov-2007 05:56
28-Nov-2007 05:56
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
10-Apr-2007 03:36
10-Apr-2007 03:36
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.