开发者

How is -13 % 64 = -13 in PHP?

Derived from this question : (Java) How does java do modulus calculations with negative numbers开发者_运维技巧?

Anywhere to force PHP to return positive 51?

update

Looking for a configuration setting to fix, instead hard-guessing

Or other math function like bcmath?

updated

Not entire convinced by that java answer, as it does not take account of negative modulus -13+(-64) =?


Anyway, the post you referenced already gave the correct answer:

$r = $x % $n;
if ($r < 0)
{
    $r += abs($n);
}

Where $x = -13 and $n = 64.


If GMP is available, you can use gmp_mod

Calculates n modulo d. The result is always non-negative, the sign of d is ignored.

Example:

echo gmp_strval(gmp_mod('-13', '64')); // 51

Note that n and d have to be GMP number resources or numeric strings. Anything else won't work¹

echo gmp_strval(gmp_mod(-13, 64));
echo gmp_mod(-13, 64);

will both return -51 instead (which is a bug).

¹ running the above in this codepad, will produce 51 in all three cases. It won't do that on my development machine.


The modulo operation should find the remainder of division of a number by another. But strictly speaking in most mainstream programming languages the modulo operation malfunctions if dividend or/and divisor are negative. This includes PHP, Perl, Python, Java, C, C++, etc.

Why I say malfunction? Because according to mathematic definition, a remainder must be zero or positive.

The simple solution is to handle the case yourself:

if r < 0  then r = r + |divisor|;

|divisor| is the absolute value of divisor.

Another solution is to use a library (as @Gordon pointed). However I wouldn't use a library to handle a simple case like this.


I hate using if in this case when you can calculate it right away.

$r = ($x % $n + $n) % $n;

when $n is positive.


The PHP manual says that

The result of the modulus operator % has the same sign as the dividend — that is, the result of $a % $b will have the same sign as $a. For example

so this is not configurable. Use the options suggested in the question you linked to

0

上一篇:

下一篇:

精彩评论

暂无评论...
验证码 换一张
取 消

最新问答

问答排行榜