do-while
On the combination problem again...
It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.
The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:
<?php
for($a=0;$a<10;$a++){
for($b=0;$b<10;$b++){
for($c=0;$c<10;$c++){
for($d=0;$d<10;$d++){
echo $a.$b.$c.$d.", ";
}
}
}
}
?>
Of course, if you know that the numbers you had used were in a smaller subset, you could just plunk your possible numbers into arrays $a, $b, $c, and $d and then do nested foreach loops as above.
- Elizabeth
for
(PHP 4, PHP 5)
for-Schleifen sind die komplexesten Schleifen in PHP. Sie verhalten sich wie ihre Pendants in C. Die Syntax einer for-Schleife ist:
for (expr1; expr2; expr3)
statement
Der erste Ausdruck (expr1) wird vor Ausführung der Schleife ausgeführt.
Am Anfang jedes Schleifendurchlaufs wird die Anweisung expr2
ausgeführt. Wenn diese wahr ist (TRUE), wird die Schleife fortgesetzt
und die untergeordneten Anweisungen werden ausgeführt.
Andernfalls (FALSE) endet die Ausführung der Schleife.
Am Ende jedes Schleifendurchlaufs wird die Anweisung expr3 ausgeführt.
Jede der Anweisungen kann leer sein oder mehrere durch Kommata getrennte
Anweisungen enthalten. In letzterem Fall werden bei expr2
zwar alle Anweisungen ausgeführt aber das Ergebnis wird nur von der letzten
Anweisung verwendet.
Wenn expr2 leer ist, läuft die Schleife unendlich
lange (PHP nimmt dies als TRUE, ebenso wie C). Dies ist nicht so sinnlos,
wie es scheint, denn oft will man die Schleife manuell mit break
beenden.
Die folgenden Beispiele geben alle die Zahlen 1 bis 10 aus:
<?php
/* Beispiel 1 */
for ($i = 1; $i <= 10; $i++) {
echo $i;
}
/* Beispiel 2 */
for ($i = 1; ; $i++) {
if ($i > 10) {
break;
}
echo $i;
}
/* Beispiel 3 */
$i = 1;
for (; ; ) {
if ($i > 10) {
break;
}
echo $i;
$i++;
}
/* Beispiel 4 */
for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i, $i++);
?>
Sicher ist das erste das hübscheste (vielleicht auch das vierte), aber in manchen Fällen sind leere Anweisungen in for-Schleifen durchaus praktisch.
PHP untersützt bei for-Schleifen ebenfalls die alternative "Doppelpunkt-Syntax":
for (expr1; expr2; expr3):
statement
...
endfor;
Es ist üblich, dass Arrays wie in dem folgenden Beispiel durchlaufen werden.
<?php
/*
* Dies ist ein Array mit Daten, die wir in der
* Schleife verändern wollen.
*/
$personen = array(
array('name' => 'Hans', 'salt' => 856412),
array('name' => 'Martin', 'salt' => 215863)
);
for($i = 0; $i < count($personen); ++$i) {
$personen[$i]['salt'] = mt_rand(000000, 999999);
}
?>
Der obige Code kann langsam sein, weil die Größe des Arrays bei jedem Schleifendurchlauf abgerufen wird. Da sich die Größe nie ändern, kann die Schleife optimiert werden, indem man die Größe in einer Variablen zwischenspeichert, anstatt sie immer wieder mit count() abzurufen:
<?php
$personen = array(
array('name' => 'Hans', 'salt' => 856412),
array('name' => 'Martin', 'salt' => 215863)
);
for($i = 0, $groesse = count($personen); $i < $groesse; ++$i) {
$personen[$i]['salt'] = mt_rand(000000, 999999);
}
?>
add a note
User Contributed Notes
for - [11 notes]
lishevita at yahoo dot co (notcom) .uk ¶
6 years ago
JustinB at harvest dot org ¶
7 years ago
For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can. Although many have stated this fact, most have not stated that there is still a way to do this:
<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
// Do Something with All Those Fun Numbers
}
?>
user at host dot com ¶
9 years ago
Also acceptable:
<?php
for($letter = ord('a'); $letter <= ord('z'); $letter++)
print chr($letter);
?>
kanirockz at gmail dot com ¶
3 years ago
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
eduardofleury at uol dot com dot br ¶
5 years ago
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
$p = $i + $x;
print "\$i = $i , \$x = $x , \$z = $z <br />";
}
?>
nzamani at cyberworldz dot de ¶
11 years ago
The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:
<?php
for ($i = 0; $i <= somewhat_calcMax(); $i++) {
somewhat_doSomethingWith($i);
}
?>
Faster would be:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; $i++) {
somewhat_doSomethingWith($i);
}
?>
And here a little trick:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i++)) ;
?>
The $i gets changed after the copy for the function (post-increment).
Philipp Trommler ¶
6 months ago
Note, that, because the first line is executed everytime, it is not only slow to put a function there, it can also lead to problems like:
<?php
$array = array(0 => "a", 1 => "b", 2 => "c", 3 => "d");
for($i = 0; $i < count($array); $i++){
echo $array[$i];
unset($array[$i]);
}
?>
This will only output the half of the elements, because the array is becoming shorter everytime the for-expression counts it.
bishop ¶
9 years ago
If you're already using the fastest algorithms you can find (on the order of O(1), O(n), or O(n log n)), and you're still worried about loop speed, unroll your loops using e.g., Duff's Device:
<?php
$n = $ITERATIONS % 8;
while ($n--) $val++;
$n = (int)($ITERATIONS / 8);
while ($n--) {
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
}
?>
(This is a modified form of Duff's original device, because PHP doesn't understand the original's egregious syntax.)
That's algorithmically equivalent to the common form:
<?php
for ($i = 0; $i < $ITERATIONS; $i++) {
$val++;
}
?>
$val++ can be whatever operation you need to perform ITERATIONS number of times.
On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version: 7.9857 s
Obvious version: 27.608 s
matthiaz ¶
1 year ago
Looping through letters is possible. I'm amazed at how few people know that.
for($col = 'R'; $col != 'AD'; $col++) {
echo $col.' ';
}
returns: R S T U V W X Y Z AA AB AC
Take note that you can't use $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
dkimbel13 at gmail dot com ¶
4 years ago
Just a note on looping through an array using the for() loop.
with the array...
<?php $array = array("value1","value2","value3"); ?>
then...
<?php
for(reset($array),current($array),next($array){
echo("Element ".key($array)." contains ".current($array)."<br/>";
}
?>
is the equivalent of...
<?php
for($i=0;$i<count($array);$i++){
echo("Element $i contains $array[$i]<br/>");
}
?>
I don't know if there is any advantage, just thought I would mention it.
kanirockz at gmail dot com ¶
3 years ago
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)