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.
Cykly do..while jsou velmi podobné cyklům while kromě toho, že pravdivost výrazu se testuje na konci každé iterace namísto jejího začátku. Hlavní rozdíl oproti běžným cyklům while je ten, že první iterace cyklu do..while se provede vždy (pravdivostní výraz je testován až na konci iterace), což u cyklu while není zaručeno (pravdivostní výraz je testován na začátku iterace; pokud je ohodnocen jako FALSE, provádění cyklu hned skončí).
Toto je jediná syntaxe pro cykly do..while:
$i = 0;
do {
print $i;
} while ($i>0);
Výše uvedený cyklus by se provedl právě jednou, protože po první iteraci, když se testuje pravdivostní výraz, je tento ohodnocen jako FALSE ($i není větší než 0) a provádění cyklu končí.
Pokročilí programátoři v C mohou znát i odlišné použití cyklu do..while. Kód se uzavře do do..while(0) a použije se příkaz break. To umožňuje přerušit provádění cyklu uprostřed kódu, jak je znázorněno v tomto příkladu:
do {
if ($i < 5) {
print "i není dost velké";
break;
}
$i *= $factor;
if ($i < $minimum_limit) {
break;
}
print "i je ok";
...zpracuj i...
} while(0);
Nedělejte si nic z toho, že tomu hned a beze zbytku nerozumíte. Můžete psát skripty, a to i velmi účinné skripty, i bez použití této 'finty'.
add a note
User Contributed Notesdo..while
Ryan
17-Apr-2008 10:59
17-Apr-2008 10:59
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.