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while-Schleifen sind der einfachste Typ von Schleifen in PHP. Sie verhalten sich wie ihre Pendants in C. Die Grundform einer while-Schleife ist:
while (bedingung)
anweisung
Die Bedeutung einer while-Schleife ist simpel. Die Schleife weist PHP an, die untergeordnete(n) Anweisung(en) wiederholt auszuführen, solange die while-Bedingung zutrifft. Die Bedingung wird jedes Mal am Anfang der Schleife überprüft, Das bedeutet, dass die Ausführung der untergeordneten Anweisungen nicht stoppt, auch wenn sich der Wahrheitsgehalt der Bedingung zwischendurch ändert (jedes Mal, wenn PHP die Anweisungen ausführt, handelt es sich um eine Runde). Wenn die while-Bedingung von Anfang an nicht zutrifft, werden die untergeordneten Anweisung nicht ein einziges Mal ausgeführt.
Wie bei if-Abfragen können mehrere Statements innerhalb der selben Schleife mit geschleiften Klammern oder mithilfe der alternativen Syntax gruppiert werden:
while (bedingung):
anweisung
...
endwhile;
Die folgenden Beispiele sind identisch und geben die Zahlen von 1 bis 10 aus:
<?php
/* Beispiel 1 */
$i = 1;
while ($i <= 10) {
echo $i++; /* der ausgegebene Wert ist $i bevor
er erhöht wird (post-increment) */
}
/* Beispiel 2 */
$i = 1;
while ($i <= 10):
echo $i;
$i++;
endwhile;
?>
I find it often clearer to set a simple flag ($finished) to false at the start of the loop, and have the program set it to true when it's finished doing whatever it's trying to do. Then the code is more self-documenting: WHILE NOT FINISHED keep going through the loop. FINISHED EQUALS TRUE when you're done. Here's an example. This is the code I use to generate a random filename and ensure that there is not already an existing file with the same name. I've added very verbose comments to it to make it clear how it works:
<?php
$finaldir = 'download';
$finished = false; // we're not finished yet (we just started)
while ( ! $finished ): // while not finished
$rn = rand(); // random number
$outfile = $finaldir.'/'.$rn.'.gif'; // output file name
if ( ! file_exists($outfile) ): // if file DOES NOT exist...
$finished = true; // ...we are finished
endif;
endwhile; // (if not finished, re-start WHILE loop)
?>
Just a note about using the continue statement to forego the remainder of a loop - be SURE you're not issuing the continue statement from within a SWITCH case - doing so will not continue the while loop, but rather the switch statement itself.
While that may seem obvious to some, it took a little bit of testing for me, so hopefully this helps someone else.
Due to the fact that php only interprets the necessary elements to get a result, I found it convenient to concatenate different sql queries into one statement:
<?php
$q1 = 'some query on a set of tables';
$q2 = 'similar query on a another set of tables';
if ( ($r1=mysql_query($q1)) && ($r2=mysql_query($q2)) ) {
while (($row=mysql_fetch_assoc($r1))||($row=mysql_fetch_assoc($r2))) {
/* do something with $row coming from $r1 and $r2 */
}
}
?>
[EDIT BY danbrown AT php DOT net: Contains a bugfix supplied by "Ira" on 14-AUG-09 to address an extra '(' in the leading `if` statement.]
@stuart:
There's nothing strange or unexpected about your loop's behaviour.
> So in effect the main while loop is only doing one iteration... and not 4 as expected....
That's the wrong conclusion. The outer "while" does all four iterations. However the "inner" loop does nothing for the second, third and fourth run.
> I think it would be good to have an explaination of this strange behaviour.
Here it is:
<?PHP
$i=0;
while($i < count($one)) {
while($a = each($two)) {
echo $a[1]." - ".$one[$i].", ";
}
$i++;
}
?>
The "problem" is your use of "each", which reached the last item after the first iteration of the outer loop. After that, when you come back to the second iteration with the outer loop, "each" still is at the end of the array $two.
If you add a reset($two) in front of the inner "while", you'll get the result you expect.
Talking about while, dropdown menus, and ternary operator which was mentionned before, you can combine them to have drop menu built with a value selected according to your wishses.
<select name="whatever">
<?php
while ($data = mysql_fetch_assoc($requeteID))
{
$menu .= '<option value="'.$data['id'].'"';
$menu .= ($data['id'] == $_GET['id'] ? ' selected>' :'>');
$menu .= $data['name'].'</option>';
}
echo $menu;
?>
</select>
Therefore if you are creating a form to select data from database, and want the form displayed when search is done to show what parameters have been chosen that will do the trick !!
Let's say I make a search between different sports, I choose football in my form, send my query... then displays are show, the menu will have football selected because of the ternary operator that displays "selected>" on the <option> ;) Enjoy ^^
I was reading the excellent post by wbryson at gmail dot com and I wanted to just add that the ? : syntax is known as the 'ternary operator' for those who want to learn more about it.
Just a note to stuart - the reason for this behaviour is because using the while(value = each(array)) construct increments the internal counter of the array as its looped through. Therefore if you intend to repeat the loop, you need to reset the counter. eg:
$one = array("10", "20", "30", "40");
$two = array("a", "b", "c", "d");
$i=0;
while($i < count($one)) {
reset($two);
while($a = each($two)) {
echo $a[1]." - ".$one[$i].", ";
}
$i++;
}
This produces:
a - 10, b - 10, c - 10, d - 10, a - 20, b - 20, c - 20, d - 20, a - 30, b - 30, c - 30, d - 30, a - 40, b - 40, c - 40, d - 40,
A note to anyone nesting a while loop inside a while loop....
Consider the example below:
$one = array("10", "20", "30", "40");
$two = array("a", "b", "c", "d");
$i=0;
while($i < count($one)) {
while($a = each($two)) {
echo $a[1]." - ".$one[$i].", ";
}
$i++;
}
This will return the following:
a - 10, b - 10, c - 10, d - 10
So in effect the main while loop is only doing one iteration... and not 4 as expected....
Now the example below works as expected..
$i=0;
while($i < count($one)) {
foreach($two as $a) {
echo $a." - ".$one[$i]."\n";
}
$i++;
}
by returning:
a - 10, b - 10, c - 10, d - 10, a - 20, b - 20, c - 20, d - 20, a - 30, b - 30, c - 30, d - 30, a - 40, b - 40, c - 40, d - 40
So there is clearly a difference on how while statements work in comparison to other looping structures.
I think it would be good to have an explaination of this strange behaviour.
virtualjosh at yahoo dot com (Hosh) wrote on: 16-Aug-2003 12:52
The speedtest is interesting. But the seemingly fastest way contains a pitfall for beginners who just use it because it is fast and fast is cool ;)
Walking through an array with next() will cut of the first entry, as this is the way next() works ;)
If you really need to do it this way, make sure your array contains an empty entry at the beginning. Another way would be to use
<?php
while ($this = current($array) ){
do_something($this);
next($array);
}
?>
There is an impact on speed for sure but I did not test it. I would advise to stick with conventional methods because current(),next() in while loops is too error prone for me.
For Perl programmers, break is similar to last
while (1) {
while(cond) {
if (error) {
break 2; // in perl this could have been last;
}
}
}
While can do wonders if you need something to queue writing to a file while something else has access to it.
Here is my simple example:
<?php
function write ($data, $file, $write_mode="w") {
$lock = $file . ".lock";
// run the write fix, to stop any clashes that may occur
write_fix($lock);
// create a new lock file after write_fix() for this writing session
touch( $lock );
// write to your file
$open = fopen($file, $write_mode);
fwrite($open, $data);
fclose($open);
// kill your current lock
unlink($lock);
}
function write_fix ($lock_file) {
while( file_exists($lock_file){
// do something in here?
// maybe sleep for a few microseconds
// to maintain stability, if this is going to
// take a while ?? [just a suggestion]
}
}
?>
This method is not recommended for use with programs that will be needing a good few seconds to write to a file, as the while function will eat up alot of process cycles. However, this method does work, and is easy to implement. It also groups the writing functions into one easy to use function, making life easier. :-)
I made a test traversing an array (simple, but long, numeric array with numeric keys). My test had a cycle per method, and multiplied each array element by 100.. These were my results:
******************************************************
30870 Element Array Traversing
[test_time] [BEGINS/RESETS @ time_start = 1060977996.689]
0.2373 seg later -> while (list ($key, $val) = each ($array)) ENDS
[test_time] [BEGINS/RESETS @ time_start = 1060977996.9414]
0.1916 seg later -> while (list ($key,) = each ($array)) ENDS
[test_time] [BEGINS/RESETS @ time_start = 1060977997.1513]
0.1714 seg later -> foreach ($array AS $key=>$value) ENDS
[test_time] [BEGINS/RESETS @ time_start = 1060977997.3378]
0.0255 seg later -> while ($next = next($array)) ENDS
[test_time] [BEGINS/RESETS @ time_start = 1060977997.3771]
0.1735 seg later -> foreach ($array AS $value) ENDS
**************************************************************
foreach is fatser than a while (list - each), true.
However, a while(next) was faster than foreach.
These were the winning codes:
$array = $save;
test_time("",1);
foreach ($array AS $key=>$value)
$array[$key] = $array[$key] * 100;
test_time("foreach (\$array AS \$key=>\$value)");
$array = $save;
test_time("",1);
reset($array);
while ($next = next($array))
{ $key = key($array);
$array[$key] = $array[$key] * 100;
}
test_time("while (\$next = next(\$array))");
*********************************************************
The improvement seems huge, but it isnt that dramatic in real practice. Results varied... I have a very long bidimensional array, and saw no more than a 2 sec diference, but on 140+ second scripts. Notice though that you lose control of the $key value (unless you have numeric keys, which I tend to avoid), but it is not always necessary.
I generally stick to foreach. However, this time, I was getting Allowed Memory Size Exceeded errors with Apache. Remember foreach copies the original array, so this now makes two huge 2D arrays in memory and alot of work for Apache. If you are getting this error, check your loops. Dont use the whole array on a foreach. Instead get the keys and acces the cells directlly. Also, try and use unset and Referencing on the huge arrays.
Working on your array and loops is a much better workaround than saving to temporary tables and unsetting (much slower).
This is an easy way for all you calculator creators to make it do factorials. The code is this:
<?php
$c = ($a-1);
$d = $a;
while ($c>=1)
{
$a = ($a*$c);
$c--;
}
print (" $d! = $a");
?>
$a changes, and so does c, so we have to make a new variable, $d, for the end statement.
I recently did a performance analysis, comparing while() and foreach() when traversing an array.
Foreach() is nearly 2x faster - an effect most notable when traversing large, multi-dimensional arrays.
Here's my code:
<?
for ($i=0; $i<10000; $i++)
$a[$i]=$i*2;
echo "list time: \n".$start=mktime()."\n";
for ($i=0; $i<1000; $i++)
{
reset($a);
while(list($k, $v)=each($a))
{
echo "";
}
}
echo ($finish=mktime())."\n";
$result=$finish-$start;
echo "Result: $result seconds\n\n";
echo "for time: \n".$start=mktime()."\n";
for ($i=0; $i<1000; $i++)
{
foreach($a as $k => $v)
{
echo "";
}
}
echo ($finish=mktime())."\n";
$result=$finish-$start;
echo "Result: $result seconds\n\n";
?>
And here's the results on an 1800+ Athlon:
list time:
1056579474
1056579512
Result: 38 seconds
fore time:
1056579512
1056579533
Result: 21 seconds
At the end of the while (list / each) loop the array pointer will be at the end.
This means the second while loop on that array will be skipped!
You can put the array pointer back with the reset($myArray) function.
example:
<?php
$myArray=array('aa','bb','cc','dd');
while (list ($key, $val) = each ($myArray) ) echo $val;
reset($myArray);
while (list ($key, $val) = each ($myArray) ) echo $val;
?>
If you want to skip an iteration of a while loop, you can use continue.
This will result in the rest of the present iteration being skipped, and it will go back to the start of the loop for the next iteration.
Moriarty
If you want to traverse array, foreach() is faster than while() a little.
[Benched with PHP4.0.4pl1/Apache DSO/Linux]
i.e.
foreach ($array as $k => $v)
is a little faster than
while (list($k,$v) = each($array))
You might want to use foreach for large arrays.
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