while
while (0)
only executed once for me (PHP 5.2.1)
while (1)
looped until I issued a break statement.
do-while
Loops do-while são bem similares aos loops while, exceto pelo fato de que a condição é verificada no fim de cada iteração em vez de no começo. A diferença principal dos loops while regulares é que a primeira iteração de um loop do-while é garantidamente executada (a condição só é verificada no fim da iteração) enquanto que ele pode não rodar necessariamente em um loop while normal (a condição é verificada no começo de cada iteração, se ela é avaliada como FALSE logo no começo, a execução do loop terminaria imediatamente).
Há apenas uma sintaxe para loops do-while:
<?php
$i = 0;
do {
echo $i;
} while ($i > 0);
?>
O loop acima rodaria exatamente uma vez, desde que depois da primeira iteração, quando a condição é verificada, ela é avaliada como FALSE ($i não é maior que zero 0) e a execução do loop termina.
Usuários avançados de C podem estar familiarizados com o uso diferenciado do loop do-while, para permitir o fim da execução no meio dos blocos de código, englobando-os com do-while (0), e usando a instrução break . O fragmento de código a seguir demonstra isso:
<?php
do {
if ($i < 5) {
echo "i não é grande o suficiente";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
echo "i está Ok";
/* process i */
} while (0);
?>
Não se preocupe se você não entendeu isto da forma certa ou de jeito nenhum. Você pode codificar scripts simples ou mesmo poderosos sem usar esse 'recurso'.
add a note
User Contributed Notesdo-while
onejaguar
06-May-2008 02:59
06-May-2008 02:59
Ryan
18-Apr-2008 01:59
18-Apr-2008 01:59
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 09:56
29-Nov-2007 09:56
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 06:36
11-Apr-2007 06:36
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.