for

(PHP 4, PHP 5)

Los bucles for son los más complejos en PHP. Se comportan como sus homólogos en C. La sintaxis de un bucle for es:

for (expr1; expr2; expr3)
    sentencia

La primera expresión (expr1) es evaluada (ejecutada) una vez incondicionalmente al comienzo del bucle.

En el comienzo de cada iteración, se evalúa expr2. Si se evalúa como TRUE, el bucle continúa y se ejecutan la/sy sentencia/s anidada/s. Si se evalúa como FALSE, finaliza la ejecución del bucle.

Al final de cada iteración, se evalúa (ejecuta) expr3.

Cada una de las expresiones puede estar vacía o contener múltiples expresiones separadas por comas. En expr2, todas las expresiones separadas por una coma son evaluadas, pero el resultado se toma de la última parte. Que expr2 esté vacía significa que el bucle debería ser corrido indefinidamente (PHP implícitamente lo considera como TRUE, como en C). Esto puede no ser tan inútil como se pudiera pensar, ya que muchas veces se debe terminar el bucle usando una sentencia condicional break en lugar de utilizar la expresión verdadera del for.

Considere los siguientes ejemplos. Todos ellos muestran los números del 1 al 10:

<?php
/* ejemplo 1 */

for ($i 1$i <= 10$i++) {
    echo 
$i;
}

/* ejemplo 2 */

for ($i 1; ; $i++) {
    if (
$i 10) {
        break;
    }
    echo 
$i;
}

/* ejemplo 3 */

$i 1;
for (; ; ) {
    if (
$i 10) {
        break;
    }
    echo 
$i;
    
$i++;
}

/* ejemplo 4 */

for ($i 1$j 0$i <= 10$j += $i, print $i$i++);
?>

Por supuesto, el primer ejemplo parece ser el mejor (o quizás el cuarto), pero se puede observar que la posibilidad de usar expresiones vacías en los bucles for resulta útil en muchas ocasiones.

PHP también admite la sintaxis alternativa de los dos puntos para bucles for.

for (expr1; expr2; expr3):
    sentencia
    ...
endfor;

Es una cosa común a muchos usuarios iterar por medio de arrays como en el siguiente ejemplo.

<?php
/*
* Este es un array con algunos datos que se quieren modificar
* cuando se recorra el bucle for.
*/
$people = array(
    array(
'name' => 'Kalle''salt' => 856412),
    array(
'name' => 'Pierre''salt' => 215863)
);

for(
$i 0$i count($people); ++$i) {
    
$people[$i]['salt'] = mt_rand(000000999999);
}
?>

El código anterior puede ser lento, debido a que el tamaño del array se capta en cada iteración. Dado que el tamaño nunca cambia, el bucle ser fácilmente optimizado mediante el uso de una variable intermedia para almacenar el tamaño en lugar de llamar repetidamente a count():

<?php
$people 
= array(
    array(
'name' => 'Kalle''salt' => 856412),
    array(
'name' => 'Pierre''salt' => 215863)
);

for(
$i 0$size count($people); $i $size; ++$i) {
    
$people[$i]['salt'] = mt_rand(000000999999);
}
?>

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User Contributed Notes 14 notes

up
3
Vincenzo Raco
23 days ago
In this code:

<?php

    $array
= array(
       
'pop0',
       
'pop1',
       
'pop2',
       
'pop3',
       
'pop4',
       
'pop5',
       
'pop6',
       
'pop7',
       
'pop8'
   
);
    echo
"Tot Before: ".count($array)."<br><br>";
    for (
$i=0; $i<count($array); $i++) {
        if (
$i === 3) {
            unset(
$array[$i]);
        }
        echo
"Count: ".count($array). " - Position: ".$i."<br>";
    }
    echo
"<br> Tot After: ".count($array)."<br>";

?>

The result is:

---

Tot Before: 9

Count: 9 - Position: 0
Count: 9 - Position: 1
Count: 9 - Position: 2
Count: 8 - Position: 3
Count: 8 - Position: 4
Count: 8 - Position: 5
Count: 8 - Position: 6
Count: 8 - Position: 7

Tot After: 8

---

The position 8 is skipped, because the "expr2" {{ $i<count($array) }} is evaluated again, for each cycle.

The solution is:

<?php
   
    $array
= array(
       
'pop0',
       
'pop1',
       
'pop2',
       
'pop3',
       
'pop4',
       
'pop5',
       
'pop6',
       
'pop7',
       
'pop8'
   
);
    echo
"Tot Before: ".count($array)."<br><br>";
   
$count = count($array);
    for (
$i=0; $i<$count; $i++) {
        if (
$i === 3) {
            unset(
$array[$i]);
        }
        echo
"Count: ".count($array). " - Position: ".$i."<br>";
    }
    echo
"<br> Tot After: ".count($array)."<br>";
   
?>
up
7
nzamani at cyberworldz dot de
12 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).
up
6
matthiaz
2 years 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 $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
up
5
eduardofleury at uol dot com dot br
6 years ago
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
   
   
$p = $i + $x;
   
    print
"\$i = $i , \$x = $x , \$z = $z <br />";
   
}

?>
up
4
Andrew
3 months ago
You can use strtotime with for loops to loop through dates

<?php
for ($date = strtotime("2014-01-01"); $date < strtotime("2014-02-01"); $date = strtotime("+1 day", $date)) {
    echo
date("Y-m-d", $date)."<br />";
}
?>
up
4
lishevita at yahoo dot co (notcom) .uk
7 years ago
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
up
1
vincentamorij at hotmail dot com
8 days ago
<html>
<head>
<title>
drikhoek

</title>
</head>
<body bgcolor="#99FF66">
<form action="driehoek.php" method="post" >
<input type="text" name="breedte"  />
<input type="submit" value="klikken"  />
</form>
<?

$hoogte=$_POST["breedte"];
print(" ");

for($teller=1; $teller<=$hoogte; $teller++)
{
    

    for($breedte=1; $breedte<=$teller; $breedte++)
       { print("*");
       }
print("   </br>");      


?>

it's an example!!!!!

</body>
</html>
up
0
Warbo
2 months ago
Remember that for-loops don't always need to go 'forwards'. For example, let's say I have the following code:

<?php
for ($i = 0; $i < calculateLoopLength(); $i++) {
 
doSomethingWith($i);
}
>?

As
other comments have pointed out, if "calculateLoopLength" will keep giving back the same value, it can be moved outside the loop:

<?
php
$loopLength
= calculateLoopLength();
for (
$i=0; $i < $loopLength; $i++) {
 
doSomethingWith($i);
}
?>

However, if the order the looping doesn't matter (ie. each iteration is independent) then we don't need to use an extra variable either, we can just count down (ie. loop 'backwards') instead:

<?php
for ($i=calculateLoopLength(); $i > 0; $i--) {
 
doSomething($i);
}
?>

In fact, we can simplify this even more, since "$i > 0" is equivalent to "$i" (due to type casting):

<?php
for ($i=calculateLoopLength(); $i; $i--) {
 
doSomething($i);
}
?>

Finally, we can switch to a 'pre-decrement' instead of a 'post-decrement' to be slightly more efficient (see, for example, http://dfox.me/2011/04/php-most-common-mistakes-part-2-using-post-increment-instead-of-pre-increment/ ):

<?php
for ($i = calculateLoopLength(); $i; --$i) {
 
doSomething($i);
}
?>

In this case we could also replace the entire loop with a map, which might make your algorithm clearer (although this won't work if calculateLoopLength() == 0):

<?php
array_map
('doSomething',
         
range(0, calculateLoopLength() - 1));
?>
up
0
JustinB at harvest dot org
8 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
}
?>
up
0
user at host dot com
10 years ago
Also acceptable:

<?php
 
for($letter = ord('a'); $letter <= ord('z'); $letter++)
   print
chr($letter);
?>
up
-8
Philipp Trommler
1 year 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.
up
-9
kanirockz at gmail dot com
4 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)
up
-9
bishop
10 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
up
-20
kanirockz at gmail dot com
4 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)
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