do-while
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 $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
for
(PHP 4, PHP 5)
for-Schleifen sind die komplexesten Schleifen in PHP. Sie verhalten sich wie ihre Pendants in C. Die Syntax einer for-Schleife ist:
for (expr1; expr2; expr3)
statement
Der erste Ausdruck (expr1) wird vor Ausführung der Schleife ausgeführt.
Am Anfang jedes Schleifendurchlaufs wird die Anweisung expr2
ausgeführt. Wenn diese wahr ist (TRUE), wird die Schleife fortgesetzt
und die untergeordneten Anweisungen werden ausgeführt.
Andernfalls (FALSE) endet die Ausführung der Schleife.
Am Ende jedes Schleifendurchlaufs wird die Anweisung expr3 ausgeführt.
Jede der Anweisungen kann leer sein oder mehrere durch Kommata getrennte
Anweisungen enthalten. In letzterem Fall werden bei expr2
zwar alle Anweisungen ausgeführt aber das Ergebnis wird nur von der letzten
Anweisung verwendet.
Wenn expr2 leer ist, läuft die Schleife unendlich
lange (PHP nimmt dies als TRUE, ebenso wie C). Dies ist nicht so sinnlos,
wie es scheint, denn oft will man die Schleife manuell mit break
beenden.
Die folgenden Beispiele geben alle die Zahlen 1 bis 10 aus:
<?php
/* Beispiel 1 */
for ($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; $i < sizeof($personen); ++$i)
{
$personen[$i]['salt'] = rand(000000, 999999);
}
?>
Das Problem liegt in der zweiten Anweisung: Dieses Programmbeispiel kann sehr langsam sein, weil die Größe des Arrays bei jedem Schleifendurchlauf berechnet wird. Da sich die Größe nie ändert, kann das Programm optimiert werden, indem man die Größe in eine Variable speichert und diese anstelle von sizeof nutzt, wie im folgenden Beispiel:
<?php
$personen = Array(
Array('name' => 'Hans', 'salt' => 856412),
Array('name' => 'Martin', 'salt' => 215863)
);
for($i = 0, $groesse = sizeof($personen); $i < $groesse; ++$i)
{
$personen[$i]['salt'] = rand(000000, 999999);
}
?>
add a note
User Contributed Notes
for
matthiaz
08-Feb-2012 11:37
kanirockz at gmail dot com
21-Mar-2010 08:49
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)
kanirockz at gmail dot com
21-Mar-2010 08:48
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)
Steven
11-Jan-2009 07:50
Alternating form rows:
<?php
$rows = 4;
echo '<table><tr>';
for($i = 0; $i < 10; $i++){
echo '<td>' . $i . '</td>';
if(($i + 1) % $rows == 0){
echo '</tr><tr>';
}
}
echo '</tr></table>';
?>
Changing $rows will change how many columns are in a row.
dkimbel13 at gmail dot com
08-Jan-2009 12:41
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.
http://badluck.tv
24-Mar-2008 02:05
Nested For Loop with the same iterator as the parent.
(Well formatted so the resulting code is clean when executed).
Useful for outputting a data array into a table, ie. images.
<?php
//Dummy data
$data = array(73,74,75,76,78,79,80,81,82,83,84,85,86,87);
//Our 'stepping' variable
$g = 0;
//Our rowcount
$rowcount = 0;
echo "<table cellspacing='0'>\r";
for ($i=0; $i<count($data); ) {
$rowcount++;
echo " <tr>\r"; //New row
$g = $i + 3; //Set our nested limit
for( ; $i<$g; $i++) { //nested for loop
if (!isset($data[$i])) { //Allow us to break on incomplete rows
break;
}
echo " <td style='border: 1px #000 solid;'>\r"; //Out put a cell
echo " <p>Row $rowcount <br/> Cell: $i <br/> Data: $data[$i]</p>\r";
echo " </td>\r";
}
echo " </tr> \r"; //End New Row
}
echo "</table>\r";?>
eduardofleury at uol dot com dot br
14-Jun-2007 03:18
<?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 = $z <br />";
}
?>
lishevita at yahoo dot co (notcom) .uk
08-Sep-2006 09:33
On the combination problem again...
It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.
The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:
<?php
for($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 $d and then do nested foreach loops as above.
- Elizabeth
JustinB at harvest dot org
05-Aug-2005 01:23
For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can. Although many have stated this fact, most have not stated that there is still a way to do this:
<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
// Do Something with All Those Fun Numbers
}
?>
user at host dot com
19-Apr-2004 12:53
Also acceptable:
<?php
for($letter = ord('a'); $letter <= ord('z'); $letter++)
print chr($letter);
?>
bishop
17-Jul-2003 10:23
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++;
}
?>
$val++ can be whatever operation you need to perform ITERATIONS number of times.
On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version: 7.9857 s
Obvious version: 27.608 s
nzamani at cyberworldz dot de
18-Jun-2001 08:47
The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:
<?php
for ($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).