PHP: Opérateurs d'incrémentation et décrémentation

Last updated: Fri, 29 Aug 2008

Opérateurs d'incrémentation et décrémentation

PHP supporte les opérateurs de pre- et post-incrémentation et décrémentation, comme en langage C.

Note: Les opérateurs d'incrémentation/décrémentation n'affectent pas les valeurs booléennes. La décrémentation des valeurs NULL n'a également aucun effet, mais leur incrémentation donnera comme résultat 1.

**Opérateurs d'incrémentation et décrémentation**
Exemple	Nom	Résultat
++$a	Pre-incrémente	Incrémente `$a` de 1, puis retourne `$a`.
$a++	Post-incrémente	Retourne `$a`, puis l'incrémente de 1.
--$a	Pré-décrémente	Décrémente `$a` de 1, puis retourne `$a`.
$a--	Post-décrémente	Retourne `$a`, puis décrémente `$a` de 1.

Voici un exemple simple :


<?php
echo '<h3>Post-incrémentation</h3>';
$a = 5;
echo "Devrait valoir  5: " . $a++ . "<br />\n";
echo "Devrait valoir  6: " . $a . "<br />\n";
echo '<h3>Pre-incrémentation</h3>';
$a = 5;
echo "Devrait valoir  6: " . ++$a . "<br />\n";
echo "Devrait valoir  6: " . $a . "<br />\n";
echo '<h3>Post-décrémentation</h3>';
$a = 5;
echo "Devrait valoir  5: " . $a-- . "<br />\n";
echo "Devrait valoir  4: " . $a . "<br />\n";
echo '<h3>Pre-décrémentation</h3>';
$a = 5;
echo "Devrait valoir  4: " . --$a . "<br />\n";
echo "Devrait valoir  4: " . $a . "<br />\n";
?>

PHP suit les conventions de Perl pour la gestion des opérateurs arithmétiques, et non pas celle du C. Par exemple, en Perl 'Z'+1 retourne 'AA', alors qu'en C, 'Z'+1 retourne '[' (ord('Z') == 90, donc ord('[') == 91). Notez que les variables de caractères peuvent être incrémentées, mais pas décrémentées et même seuls les caractères ASCII (a-z et A-Z) sont supportés.

Exemple #1 Opérations arithmétiques sur un caractère


<?php
$i = 'W';
for($n=0; $n<6; $n++) {
  echo ++$i . "\n";
}
?>

L'exemple ci-dessus va afficher :

X
Y
Z
AA
AB
AC

L'incrémentation ou la décrémentation d'un booléen n'a aucun effet.

Les opérateurs logiques

Opérateur d'exécution

Last updated: Fri, 29 Aug 2008

add a note User Contributed Notes
Opérateurs d'incrémentation et décrémentation

pov at fingerprint dot fr
28-Mar-2008 08:15


In reply to Anonymous :



What is strange is that you didn't get an error : ++$var is an expression and can't therefore not be referenced.

Now, if you suppose an implicit assignment to an invisible variable, your code becomes :



<?php

 $var = 1;

 $plus_plus_var = ++$var;

 change($plus_plus_var);

 echo "var=$var";

?>



Written as such, change clearly acts on $plus_plus_var, not on $var. So PHP5 got right, and it's not a "strange behaviour", it's only a solved bug.



Anyway, it's always a bad idea to pass anything other than a variable as a by-reference parameter...

Anonymous
09-Jan-2008 11:17


Some strange behaviour between PHP 4 and 5.



Code :

<?php

 function change (&$var) {

   $var += 10;

 }



 $var = 1;

 ++$var;

 change($var);

 echo "var=$var";



 $var = 1;

 change(++$var);

 echo "var=$var";

 

?>



Output in PHP4

var=12

var=12



Output in PHP5

var=12

var=2

michal dot kocarek at NO_SPAM dot seznam dot cz
23-Sep-2007 11:04


Speed tip:



Do not use post-incrementation/post-decrementation ($i++, $i--) where you do not work with the result of this expression.

(For novices: Yes, every expression returns an result, also $a = '5' returns result, same as $a && $b. And this consumes more time and resources.)



When writing loops, replace the post-incrementation with pre-incrementation, it is around 3times faster than post-incrementation.



Why? In post-incrementation, PHP needs to copy variable value somewhere, then it increments the value, then returns the value which was stored before the incrementation was done. No matter if you don't expect the return value, PHP is scripting language, not compiled one, so it doesn't optimize use of return values.



<?php

// Good practice for loop:

$array_count = count($array); // Store temporarily instead of calling everytime in loop

for ($i = 0; $i < $max_count; ++$i) { // Use pre-incrementation here, it is faster

   // do something here

}

?>

rowan dot collins at gmail dot com
14-Jun-2007 03:34


As the manual says, decrementing NULL in this way yields NULL, although incrementing it yields 1, as you might expect. Can't quite see why this makes sense, but if you need to work around it, you can use '-= 1' instead:



<?php

$i = null;



--$i;

var_dump($i); // NULL

$i--;

var_dump($i); // NULL



$i-=1;

var_dump($i); // int(-1)

?>



Note that -= returns the value assigned, so treat it like '--$i', not '$i--' if you're testing the value.

Q1712 at online dot ms
21-Apr-2007 02:52


A more detailed explanation of the string incremant is:



First of all it is checked wether the string is a standart representaion of a number wich is true if it equals the regex /^ *[+-]?[0-9]*(\.[0-9]|[0-9]\.)[0-9]*([eE]?[+-]?[0-9]+)?$/

but not the regex /\+\./ (no idea why).

if it does, the type is changed to integer (if it equals /^ *[+-]?[0-9]+$/) or to float and then incremented by one.



An empty string becomes the string "1".



Otherwise if the last character is one of [0-8], [a-y] or [A-Y] it is incremented. If it is Z it puts it back to A, is z to a, if 9 to 0 and trys to do the same with the previouse character.

If a character is reatched that is not in [0-9a-zA-Z], nothing is done anymore (that's why " Z" will increment to " A").

If the begining is reached a new caracter is prepended. "1" "a" or "A" depending on wether the first character was "9", "z" or "Z".



If the last character was not [0-9a-zA-Z] the string isn't chaged.



hope this helps someone

Are Pedersen
28-Feb-2007 11:08


Something to think about:



$a=1; 

$a  +=  $a++   +   ++$a; 

echo $a;



will give you 7.



Why is this?



1. ++$a is first incremented. Now $a is 2.

$a += $a++  +  2

$a is 2



2. $a++ is added to 2 then $a is incremented

$a += 2 + 2

$a is 3



3. now the value of 2 + 2 is added to $a ($a is 3)

$a = $a + 2 + 2



Answer: 3 + 2 + 2 = 7

julien-bernie-laurent at polenord.com
01-Mar-2006 03:55


to thus trying to increment a string and are blocked by the exponential typecast explained in the message below, here is a small function :



function increment($var) {

    $var2 = '_'.$var;

    return substr(++$var2,1);

}

timo at frenay dot net
25-Aug-2004 04:45


JMcCarthy AT CitiStreet DOT com:



As for your March 31 post, at least in PHP version 4.3 this no longer holds for 'D'. Your point is still valid for 'e' or 'E' and worth noting.



Your comment from May 12 is simply not true, although it might be a bug in your specific version of PHP but that would seem very strange.



<?php

    $Align = array('a', 'b', 'c');

    $i = 0;

    echo $Align[$i++]; // Prints 'a', as expected

?>



It might be interesting to know that pre-/postincrement assumes a value of 0 for undefined variables, but pre-/postdecrement does not:



<?php

    echo var_dump(++$foo); // int(1)

    echo var_dump(--$bar); // NULL!

?>

31-Mar-2004 10:19


Note that incrementing strings can give unpredictable results due to type changes.  For example:



<?php



$i = '9C6';

for($n=0; $n<10; $n++)

  echo ++$i . "\n";



?>



Gives you:

 9C7

 9C8

 9C9

 9D0

 10

 11

 12

..etc.



The 'D' (and also 'E') characters are interpreted here as exponents of 10 (i.e., scientific notation) formatted numbers.  Using '9D6' will give 9000001, 9000002, etc.



You might want to use all alphabetical or all numerical, but not mix the two otherwise you may not get what you expect..

chris at free-source dot com
07-Feb-2004 12:11


Interesting performance note:





$i++ seems to be slightly slower than ++$i, when used on a line by itself the 2 have the same purpose.  It's not much, but over 100,000 incements the pre-increment is about .004 seconds faster on average.

mu at despammed dot net
15-Oct-2002 05:11


The exact moment when post-increment and post-decrement happen is _just immediately after the variable is evaluated_ (not "after the line is processed" or something like that)



Example 1:

$i = 2;

echo $i++ + $i;

Result: 5. The first i is evaluated as 2, gets incremented to 3. i is then evaluated as 3 for the second occurance.



Example 2:

$i = 2;

echo $i + $i++;

Result: 4. The first i is 2. Second i is 2 too, gets incremented afterwards.

cleong at letstalk dot com
18-Oct-2001 03:52


Note that the ++ and -- don't convert a boolean to an int. The following code will loop forever.





function a($start_index) {


for($i = $start_index; $i < 10; $i++) echo "\$i = $i\n";


}





a(false);





This behavior is, of course, very different from that in C. Had me pulling out my hair for a while.

fred at surleau dot com
18-Jul-2001 08:02


Other samples :


$l="A";      $l++; -> $l="B"


$l="A0";     $l++; -> $l="A1"


$l="A9";     $l++; -> $l="B0"


$l="Z99";    $l++; -> $l="AA00"


$l="5Z9";    $l++; -> $l="6A0"


$l="9Z9";    $l++; -> $l="10A0"


$l="9z9";    $l++; -> $l="10a0"


$l="J85410"; $l++; -> $l="J85411"


$l="J99999"; $l++; -> $l="K00000"


$l="K00000"; $l++; -> $l="K00001"

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Les opérateurs logiques

Opérateur d'exécution

Last updated: Fri, 29 Aug 2008