for

(PHP 4, PHP 5)

O loop for é o loop mais complexo no PHP. Ele tem comportamento semelhante ao C. A sintaxe do loop for é:

for (expr1; expr2; expr3)
    statement

A primeira expressão (expr1) é executada incondicionalmente somente no começo do loop.

No começo de cada iteração a expr2 é avaliada. Se a avaliação resultar em TRUE, o loop continua e as instruções aninhada são executadas. Se a avaliação resultar em FALSE, a execução do loop termina.

No final de cada iteração, a expr3 é executada.

Cada uma das expressões podem sem vazias ou conter múltiplas expressões separadas por vírgulas. Na expr2, todas as expressões separadas por vírgula são avaliadas mas o resultado é obtido da última parte. Se a expr2 estiver vazia significa que o loop deve ser executado indefinidamente (O PHP considera implicitamente como TRUE, igual ao C). Isto pode não ser tão inútil quanto você pensa, pois muitas vezes você desejar que o loop termine usando a instrução break ao invés de usar a expressão verdade do for.

Analise os seguintes exemplos. Todos exibem números de 1 até 10:

<?php
/* exemplo 1 */

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

/* exemplo 2 2 */

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

/* exemplo 3 */

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

/* exemplo 4 */

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

É claro que o primeiro exemplo aparenta ser o melhor (ou talvez o quarto), mas você pode achar que usar expressões vazias no loop for seja vantajoso em algumas ocasiões.

O PHP também suporta a sintaxe alternativa "dois pontos" para o loop for.

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

É comum para muitos usuários iterar em arrays como no example abaixo.

<?php
/*
 * Esta é uma array com alguns dados que devem ser modificadoswant to modify
 * durante a execuçao do loop 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);
}
?>

O código acima pode se tornar lento, pois o tamanho do array será calculado a cada iteração. Desde que o tamanho nunca mude, o loop pode ser facilmente otimizado usando uma variável intermediária para guardar o tamanho ao invés de executar repetidamente o 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);
}
?>

add a note add a note

User Contributed Notes 16 notes

up
45
matthiaz
3 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
14
nzamani at cyberworldz dot de
14 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 $i gets changed after the copy for the function (post-increment).
up
4
eduardofleury at uol dot com dot br
8 years ago
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
   
   
$p = $i + $x;
   
    print
"\$i = $i , \$x = $x , \$z = $z <br />";
   
}

?>
up
3
user at host dot com
11 years ago
Also acceptable:

<?php
 
for($letter = ord('a'); $letter <= ord('z'); $letter++)
   print
chr($letter);
?>
up
1
Andrew
1 year 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
0
vincentamorij at hotmail dot com
1 year 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
Vincenzo Raco
1 year 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
0
lishevita at yahoo dot co (notcom) .uk
8 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
-2
Warbo
1 year 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
-1
AoKMiKeY
1 year ago
As a note for people just starting out and wanting to know if you can do some thing like this...

<?php For( $a = 0; $a < 10; $a++ ) { ?>

//Random html elements you would like to duplicate.

<?php } ?>

Then yes you can. It works like a charm.
up
-8
Philipp Trommler
2 years 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
-3
Anonymous
11 months ago
You can also work with arrays. For example, say you want to generate an array of 12 unique 2-letter strings:

<?php

for ($names = array(); count($names) < 12; $names = array_unique($names)) {
   
/**
     * we assume here we have some $faker object
     * which generates n-letter strings
     */
   
$names[] = $faker->word(2);
}

print_r($names);
?>

will print something like:

Array
(
    [0] => cc
    [1] => cb
    [2] => dd
    [3] => db
    [4] => bb
    [6] => cd
    [8] => aa
    [9] => ad
    [10] => ca
    [11] => ac
    [12] => dc
    [15] => ab
)
up
-6
JustinB at harvest dot org
9 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
-8
bishop
12 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
-15
kanirockz at gmail dot com
5 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
-31
kanirockz at gmail dot com
5 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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