PHP: 증가/감소 연산자

Last updated: Sun, 25 Nov 2007

증가/감소 연산자

PHP는 C-스타일의 전처리와 후처리 증가/감소 연산자를 지원한다.

**증가/감소 연산자**
Example	Name	Effect
++$a	Pre-increment	Increments $a by one, then returns $a.
$a++	Post-increment	Returns $a, then increments $a by one.
--$a	Pre-decrement	Decrements $a by one, then returns $a.
$a--	Post-decrement	Returns $a, then decrements $a by one.

단순한 예제 스크립트:


<?php
echo "<h3>Postincrement</h3>";
$a = 5;
echo "Should be 5: " . $a++ . "<br />\n";
echo "Should be 6: " . $a . "<br />\n";

echo "<h3>Preincrement</h3>";
$a = 5;
echo "Should be 6: " . ++$a . "<br />\n";
echo "Should be 6: " . $a . "<br />\n";

echo "<h3>Postdecrement</h3>";
$a = 5;
echo "Should be 5: " . $a-- . "<br />\n";
echo "Should be 4: " . $a . "<br />\n";

echo "<h3>Predecrement</h3>";
$a = 5;
echo "Should be 4: " . --$a . "<br />\n";
echo "Should be 4: " . $a . "<br />\n";
?>

PHP는 문자 변수에 관한 산술 연산을 할때 C 방식이 아니라 펄 방식을 따른다. 예를 들면, 펄에서 'Z'+1은 'AA'를 전환되는 반면, C에서 'Z'+1은 '[' ( ord('Z') == 90, ord('[') == 91 )으로 전환된다. 문자 변수는 증가될수는 있으나 감소될수는 없다는 것에 주의해야 한다.

Example#1 문자 변수에 대한 산술 연산


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

/*
  Produces the output similar to the following:

X
Y
Z
AA
AB
AC

*/
?>

논리 연산자

실행 연산자

Last updated: Sun, 25 Nov 2007

add a note User Contributed Notes
증가/감소 연산자

pov at fingerprint dot fr
28-Mar-2008 01:15


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 03:17


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 03:04


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 07:34


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
20-Apr-2007 06:52


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 03:08


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 07:55


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 08:45


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 01:19


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
06-Feb-2004 04:11


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
14-Oct-2002 09:11


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
17-Oct-2001 07:52


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 12:02


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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Last updated: Sun, 25 Nov 2007