PHP 5.6.0 released

加算子/減算子

PHP は C 言語形式の加算子/減算子(前置・後置ともに)をサポートします。

注意: 加算子/減算子は、数値や文字列にしか影響を及ぼしません。 配列やオブジェクトそしてリソースには、何も変更を加えません。 同じく NULL に減算子を適用しても何も起こりませんが、NULL に加算子を 適用すると 1 となります。

加算子/減算子
名前 効果
++$a 前置加算子 $a に 1 を加え、$a を返します。
$a++ 後置加算子 $a を返し、$a に1を加えます。
--$a 前置減算子 $a から 1 を引き、$a を返します。
$a-- 後置減算子 $a を返し、$a から 1 を引きます。

以下に簡単なスクリプトの例を示します。

<?php
echo "<h3>後置加算</h3>";
$a 5;
echo 
"5 となります: " $a++ . "<br>\n";
echo 
"6 となります: " $a "<br>\n";
 
echo 
"<h3>前置加算</h3>";
$a 5;
echo 
"6 となります: " . ++$a "<br>\n";
echo 
"6 となります: " $a "<br>\n";

echo 
"<h3>後置減算</h3>";
$a 5;
echo 
"5 となります: " $a-- . "<br>\n";
echo 
"4 となります: " $a "<br>\n";

echo 
"<h3>前置減算</h3>";
$a 5;
echo 
"4 となります: " . --$a "<br>\n";
echo 
"4 となります: " $a "<br>\n";
?>

PHP は、算術演算子で文字変数を扱った場合に C ではなく Perl の慣習に 従います。例えば、PHP や Perl では $a = 'Z'; $a++; の結果として $a'AA' になりますが C では a = 'Z'; a++; の結果として a'[' になります ('Z' の ASCII 値は 90、そして '[' の ASCII 値は 91 です)。 文字変数はインクリメントされることは可能ですがデクリメントは不可能であるということ、 またプレーンな ASCII 文字と数字 (a-z、A-Z、そして 0-9) のみがサポートされるということに注意しましょう。 その他の文字変数のインクリメント/デクリメントは何の効果もなく、元の文字列は変更されません。

例1 文字変数に対する算術演算子の使用

<?php
echo '== Alphabets ==' PHP_EOL;
$s 'W';
for (
$n=0$n<6$n++) {
    echo ++
$s PHP_EOL;
}
// 数字の挙動は異なります
echo '== Digits ==' PHP_EOL;
$d 'A8';
for (
$n=0$n<6$n++) {
    echo ++
$d PHP_EOL;
}
$d 'A08';
for (
$n=0$n<6$n++) {
    echo ++
$d PHP_EOL;
}
?>

上の例の出力は以下となります。

== Characters ==
X
Y
Z
AA
AB
AC
== Digits ==
A9
B0
B1
B2
B3
B4
A09
A10
A11
A12
A13
A14

論理型に対する加算/減算は何の影響も及ぼしません。

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User Contributed Notes 8 notes

up
10
dsbeam at gmail dot com
5 years ago
When using the ++ operator by itself on a variable, ++$var is faster than $var++ and uses slightly less memory (in my experiments).  It would seem like this could be optimized in the language during runtime (if $var++ is the only thing in the whole statement, it could be treated as ++$var).

I conducted many tests (I believe to be fair), and here's one of the results:

$i++ took 8.47515535355 seconds and 2360 bytes
++$i took 7.80081486702 seconds and 2160 bytes

Here's my code.  If anyone sees a bias in it, tell me.  I conducted it many times, each time going through a loop one million iterations and doing each test 10 - 15 times (10 - 15 million uses of the ++ operator).

<?php

ini_set
( 'MAX_EXEC_TIME', 120 );
ob_start( );

$num_tests = 10;
$startFirst = $startSecond = $endFirst = $endSecond = $startFirstMemory = $endFirstMemory = $startSecondMemory = $endSecondMemory = $someVal = 0;
$times = array( '$i++' => array( 'time' => 0, 'memory' => 0 ), '++$i' => array( 'total' => 0, 'memory' => 0 ) );

for(
$j = 0; $j < $num_tests; ++$j )
{
        for(
$i = 0, $startFirstMemory = memory_get_usage( ), $startFirst = microtime( true ); $i < 10000000; $i++ ){ $someval = 2; }
       
$endFirstMemory = memory_get_usage( );
       
$endFirst = microtime( true );

        for(
$i = 0, $startSecondMemory = memory_get_usage( ), $startSecond = microtime( true ); $i < 10000000; ++$i ){ $someval = 2; }
       
$endSecondMemory = memory_get_usage( );
       
$endSecond = microtime( true );

       
$times[ '$i++' ][ $j ] = array( 'startTime' => $startFirst, 'endTime' => $endFirst, 'startMemory' => $startFirstMemory, 'endMemory' => $endFirstMemory );
       
$times[ '++$i' ][ $j ] = array( 'startTime' => $startSecond, 'endTime' => $endSecond, 'startMemory' => $startSecondMemory, 'endMemory' => $endSecondMemory );
}

for(
$i = 0; $i < $num_tests; ++$i )
{
       
$times[ '$i++' ][ 'time' ] += ( $times[ '$i++' ][ $i ][ 'endTime' ] - $times[ '$i++' ][ $i ][ 'startTime' ] );
       
$times[ '++$i' ][ 'time' ] += ( $times[ '++$i' ][ $i ][ 'endTime' ] - $times[ '++$i' ][ $i ][ 'startTime' ] );
       
$times[ '$i++' ][ 'memory' ] += ( $times[ '$i++' ][ $i ][ 'endMemory' ] - $times[ '$i++' ][ $i ][ 'startMemory' ] );
       
$times[ '++$i' ][ 'memory' ] += ( $times[ '++$i' ][ $i ][ 'endMemory' ] - $times[ '++$i' ][ $i ][ 'startMemory' ] );
}

echo
'There were ' . $num_tests . ' tests conducted, here\'s the totals<br /><br />
$i++ took '
. $times[ '$i++' ][ 'time' ] . ' seconds and ' . $times[ '$i++' ][ 'memory' ] . ' bytes<br />
++$i took '
. $times[ '++$i' ][ 'time' ] . ' seconds and ' . $times[ '++$i' ][ 'memory' ] . ' bytes';

ob_end_flush( );

?>

Try it yourself, ;)
up
8
hartmut at php dot net
2 years ago
Note that

$a="9D9"; var_dump(++$a);   => string(3) "9E0"

but counting onwards from there

$a="9E0"; var_dump(++$a);   => float(10)

this is due to "9E0" being interpreted as a string representation of the float constant 9E0 (or 9e0), and thus evalutes to 9 * 10^0 = 9 (in a float context)
up
3
cleong at letstalk dot com
12 years ago
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.
up
2
michiel ed thalent circle nl
2 years ago
BEWARE:

If incrementing an uninitialized variable you will not get an E_NOTICE error. This may caused you to not find issue's like the visibility of a property.

<?php
class a {
    private
$foo = 1;
}

class
b extends a {
    public function
inc() {
        echo ++
$this->foo;
    }
}

$b = new b;
$b->inc();
?>

Will output 1 and not 2 (if $foo was accessible).
Also no notices are given.
up
3
Brad Proctor
3 years ago
I ran some tests (on PHP 5.3.3) of my own and was surprised to find $i += 1 to be the fastest method of incrementing.  Here are the methods fastest to slowest:

$i += 1;
++$i;
$i++;
$i = $i + 1;
up
3
fred at surleau dot com
13 years ago
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"
up
-1
johnnythan at nospam dot gmx dot com
2 years ago
If you have a trailing zero and use the increment, the trailing zero will not remain. Was at least unexpected for me at first, although it's logical if you think about it.

<?php
$start
= '01';
$start++;
print
$start; //Outputs '2' not '02'
?>
up
-3
sneskid at hotmail dot com
5 years ago
(related to what "Are Pedersen" wrote)
With arrays it can lead to much confusion if your index variable is altered on the right side of the = sign, either with ++|-- or even when passed to a function by reference..
Consider these (PHP 5):
<?php
$A
[$a] = ++$a; // [1]=1
$B[++$b] = ++$b; // [1]=2
$C[$c+=0] = ++$c; // [0]=1
?>
In 'A' you have to be aware that PHP evaluates $A[$a] last.
Yet in 'B' and 'C' PHP evaluates the index and saves it in a temporary variable.

You can always force PHP to evaluate a variable without explicitly storing it as a named variable first, with a simple "+=0" like in example 'C'.

Compared to 'A', 'C' gives the more logically expected result, when we expect evaluation occurs left to right.
PHP does evaluate left to right BUT it will attempt to cut down on temporary variables, which can lead to confusing results.

So just be aware and use either behavior to your advantage for the desired functionality.
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