while
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.
do-while
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'.
add a note
User Contributed Notesdo-while
Ryan
18-Apr-2008 07:59
18-Apr-2008 07:59
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.