PHP 5.4.31 Released

for

(PHP 4, PHP 5)

for ループは、PHPで最も複雑なループです。 for は、Cのforループと同様に動作します。 forループの構文は、次のようになります。

for (式1; 式2; 式3)
    文

最初の式(式1)は、ループ開始時に無条件に 評価(実行)されます。

各繰り返しの開始時に、式2が評価されます。 その式の値がTRUEが場合、ループは継続され、括弧 内の文が実行されます。値がFALSEの場合、ループの 実行は終了します。

各繰り返しの後、式3が評価(実行)されます。

各式は空にすることもできますし、複数の式をカンマで区切って指定することもできます。 式2 でカンマ区切りの式を使用すると、 すべての式を評価します。しかし、結果として取得するのは最後の式の結果となります。 式2 を空にすると、無限実行ループになります (PHP は、この状態を C 言語のように暗黙の内に TRUE とみなします)。 この機能は、初心者が想像するよりもずっと便利です。 for の式の結果ではなく条件付き break 文を使ってループを終了させたくなることもしばしばあります。

次の例について考えてみましょう。以下の例はすべて 1 から 10 までの数を表示します。

<?php
/* 例 1 */

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

/* 例 2 */

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

/* 例 3 */

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

/* 例 4 */

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

もちろん、最初の例(もしくは 4番目の例)が最善であると考えられます。 しかし、forループにおいて空の式を使用できると、 多くの場合、便利だということに気づかれるかと思います。

PHPは、forループ用に"コロン構文"もサポートします。 for loops.

for (式1; 式2; 式3):
    文;
    ...
endfor;

多くのユーザーにとって、次の例のように配列をループ処理することはよくあるでしょう。

<?php
/*
 * データが入った配列で、ループ処理中に
 * その中身を書き換えたいと考えています
 */
$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);
}
?>

このコードは実行速度が遅くなることでしょう。 というのも、配列のサイズを毎回取得しているからです。 サイズが変わることはありえないのだから、これは簡単に最適化することができます。 配列のサイズを変数に格納して使うようにすれば、 何度も count() を呼ばずに済むのです。

<?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 15 notes

up
5
Vincenzo Raco
4 months 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
8
nzamani at cyberworldz dot de
13 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
5
Andrew
6 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
5
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
5
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 $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
up
4
eduardofleury at uol dot com dot br
7 years ago
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
   
   
$p = $i + $x;
   
    print
"\$i = $i , \$x = $x , \$z = $z <br />";
   
}

?>
up
1
user at host dot com
10 years ago
Also acceptable:

<?php
 
for($letter = ord('a'); $letter <= ord('z'); $letter++)
   print
chr($letter);
?>
up
0
AoKMiKeY
17 days 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
0
vincentamorij at hotmail dot com
3 months 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
-1
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
-2
Warbo
5 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
-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
-8
bishop
11 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
-10
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
-24
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)
To Top