代数演算子

学校で習った基礎代数を憶えていますか? この演算子はそれらと同様に動作します。

代数演算子
名前 結果
-$a 負にする $a の逆
$a + $b 加算 $a および $b の合計
$a - $b 減算 $a$b の差
$a * $b 乗算 $a および $b の積
$a / $b 除算 $a および $b の商
$a % $b 剰余 $a$b で割った余り
$a ** $b 累乗 $a$b 乗。PHP 5.6 で導入されました。

除算演算子 ("/") の返す値は浮動小数点数となります。 ただし、ふたつのオペランドがともに整数 (あるいは整数に変換できる文字列) であり、かつ結果が割り切れる場合には整数値を返します。

剰余演算子は、まず両方のオペランドを整数に直し (小数点以下を切り捨てます) てから処理を行います。

剰余演算子 % の結果の符号は、被除数の符号と同じになります。 つまり、$a % $b の結果の符号は $a と同じになるということです。

<?php

echo (3)."\n";           // 2
echo (% -3)."\n";          // 2
echo (-3)."\n";          // -2
echo (-% -3)."\n";         // -2

?>

マニュアルの 数学関数の項も参照してください。

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User Contributed Notes 13 notes

up
24
Jonathon Reinhart
8 years ago
A very simple yet maybe not obvious use of the modulus (%) operator is to check if an integer is odd or even.
<?php
 
if (($a % 2) == 1)
  { echo
"$a is odd." ;}
  if ((
$a % 2) == 0)
  { echo
"$a is even." ;}
?>

This is nice when you want to make alternating-color rows on a table, or divs.

<?php
 
for ($i = 1; $i <= 10; $i++) {
    if((
$i % 2) == 1//odd
     
{echo "<div class=\"dark\">$i</div>";}
    else  
//even
     
{echo "<div class=\"light\">$i</div>";}
   }
?>
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13
arjini at gmail dot com
10 years ago
When dealing purely with HTML, especially tables, or other things in "grids"  the modulous operator is really useful for splitting up the data with a seperator.

This snippet reads any gif files from the directory the script is in, prints them out and puts in a break every 5th image.

<?php
    $d
= dir('./');
   
$i = 0;
    while(
false !== ($e = $d->read())){
        if(
strpos($e,'.gif')){
            ++
$i;
            echo
'<img src="'.$e.'"/>'.chr(10);
            if(!(
$i%5))
                echo
'<br/>';
        }
    }
?>

For tables just put </tr><tr> in place of the break.
up
10
info at sima-pc dot com
10 years ago
Note that operator % (modulus) works just with integers (between -214748348 and 2147483647) while fmod() works with short and large numbers.

Modulus with non integer numbers will give unpredictable results.
up
7
TheWanderer
6 years ago
It is worth noticing that when working with large numbers, most noticably using the modulo operator, the results depend on your CPU architecture. Therefore, running a decent 64-bit machine will be to your advantage in case you have to perform complex mathematical operations. Here is some example code - you can compare its output on x86 and x86_64 machines:
<?php
/* tested under PHP 5.2.6-1 with Suhosin-Patch 0.9.6.2 (cli) on both i386 and amd64, Debian lenny/sid */
$a = 2863311530;
$b = 256;
$c = $a % $b;
echo
"$c <br />\n";
echo (
2863311530 % 256)." <br />\n"; /* directly with no variables, just to be sure */
?>

The code is expected to produce '170' if working correctly (try it in spreadsheet software).
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2
lmc at trendicy dot com
3 months ago
If you are running a php version older than 5.6, you can calculate $a ** $b by using exp($b*log($a))
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3
Andrew
1 year ago
The % operator doesn't behave as many people with a maths background would expect, when dealing with negative numbers. For example, -1 mod 8 = 7, but in PHP, -1 % 8 = -1.

The following function has the expected behaviour:

function mod($a, $n) {
    return ($a % $n) + ($a < 0 ? $n : 0);
}

mod(-1, 8) returns 7 as expected.
up
0
Dominik Buechler
23 days ago
In addition to Jonathan's comment, there is a way simpler way to determine if an integer is even or not:

<? $odd = $i % 2; ?>
or
<? $even = !($i % 2); ?>

This works because a modulo division by 2 will always return either 0 or the rest 1. Since those are valid boolean values you can just invert them by adding a prefixed ! if wanted.
up
-3
calmarius at atw dot hu
6 years ago
Be careful when using % with large numbers.

The code:

<?php
   
echo 3333333333 % 3
?>

puts out -1 instead of zero!

(Due to the overflow)
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-3
pww8 at cornell dot edu
9 years ago
It appears floating-point infinity (INF) is not returned from divide by zero (in PHP 5.0.0).  Instead a warning is given and Boolean FALSE is returned.

I searched the various manuals and did not find relevant explanation, so am adding this.
up
-4
glenn at benge dot co dot nz
10 years ago
a real simple method to reset an integer to a the next lowest multiple of a divisor

$startSeq = $startSeq - ($startSeq % $entriesPerPage);

if $startSeq was already a multiple, then " $startSeq % $entriesPerPage " will return 0 and $startSeq will not change.
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-9
php at richardneill dot org
3 years ago
For larger numbers (above PHP_INT_MAX), use fmod() rather than %.
The other operators (+-*/) work correctly with floats and integer overflow, but % uses integer wrap. Eg.

<?php
var_dump
(0xffffffff % 2);
//Prints  int(-1)   which is WRONG

var_dump(intval(fmod(0xffffffff,2)));
//Prints int(1)   which is the right answer
?>

(The reason this matters is that PHP's float is actually a double, and can accurately represent integers up to 52-bits, even on 32-bit systems)
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-12
antickon at gmail dot com
2 years ago
not listed here is the absolutely useless unary plus.

<?php
$a
= -3;
$a = +$a;
var_dump( $a ); // int(-3)
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-25
hassan dot beydoun at live dot com
1 year ago
<?php

//using range function()
//$numbers = array;range = value

$numbers= range (1,100);
// foreach ($array as $value)
//{Do something with $value}
foreach ($numbers as $value)
{echo
"<p>This is number: $value</P>";}

?>
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