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

Question

Derived from this question : (Java) How does java do modulus calculations with negative numbers?

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) =?

updated ... looking for solution without using hard-guessing method — ajreal, Dec 10 '10 at 13:38
There is no configuration setting to change how math works in PHP. If there was then it would probably break PHP, which I would imagine uses lots of calculations internally — Gareth, Dec 10 '10 at 13:40
That's not an intended algorithm. Surprisingly Python and Perl accept it, but PHP doesn't. Use a workaround. Btw, `fmod()` and `bcmod()` and `gmp_mod()` don't either. — mario, Dec 10 '10 at 13:46

Spiny Norman · Answer 1 · 2010-12-10T18:20:51.393

16

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.

edited Dec 10 '10 at 18:20

answered Dec 10 '10 at 13:34

Spiny Norman

8,277
1
30
55

is there a configuration for modulus instead guess ourself? – ajreal Dec 10 '10 at 13:36
1

There's no guessing involved, just calculating which (wow) you're already doing some of. – Gareth Dec 10 '10 at 13:42
1

The `if` should preferrably be a `while`. – mario Dec 10 '10 at 13:47
1

@mario It doesn't hurt to use `while`, but due to the nature of modulus, $r will never be < -63 (that is, -($n-1)). – Spiny Norman Dec 10 '10 at 13:58
2

The while is not necessary, but adding the absolute value of n to r rather than just n would also solve the case where n is negative. – abc Dec 10 '10 at 17:06

Gordon · Accepted Answer · 2010-12-10T15:48:15.417

7

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.}

edited Dec 10 '10 at 15:48

answered Dec 10 '10 at 13:43

Gordon

312,688
75
539
559

I really wouldn't use the GMP library for this. It's far too heavyweight. The answer below by Spiny Norman is much better. – Richard Smith Oct 16 '20 at 14:00

score 4 · Answer 3 · answered Jan 14 '11 at 11:10

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.

score 2 · Answer 4 · answered Feb 08 '22 at 09:39

2

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

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

when $n is positive.

answered Feb 08 '22 at 09:39

Anurat Chapanond

2,837
3
18
31

score 0 · Answer 5 · answered Dec 10 '10 at 13:42

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

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

5 Answers5

Linked

Related