Strange Modulus Operator Result - PHP

Question

I am working on a PHP application and mathematical operation was resulting wrong answer which was displaying wrong results. So, I started digging down and after few hours effort I was able to detect the issue.

Here is the problematic Expression:

echo -1 % 26;

The answer should be 25 but it gives -1. I don't know, is there anything wrong with my expression?

PHP Output:

Calculator:

Can anyone please identify, where is the problem?

clinton3141 · Accepted Answer · 2017-09-30T04:57:08.720

1

This is the expected behaviour. From the PHP manual

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

If the sign of the dividend (the part to the left of the %) changes, the result will also change. You can find the positive equivalent of a negative remainder by adding the divisor. -1 is equivalent to 25 modulo 26 since -1 + 26 = 25.

Hence you can do the following to get the positive result:

function modulo($dividend, $divisor) {
  $result = $dividend % $divisor;
  return $result < 0 ? $result + $divisor : $result;
}

$calculation = modulo(-1, 26); // 25
$calculation2 = modulo(51, 26); // 25

edited Sep 30 '17 at 04:57

answered Sep 22 '17 at 02:38

clinton3141

4,751
3
33
46

it is not about sign, result are also different – Alena Sep 22 '17 at 02:39
@Alena Answer updated to show why this is the case :) – clinton3141 Sep 22 '17 at 02:46
thanks but I don't think if this is the proper way to handle it. Because I would need to add extra check, if number is negative then add it instead of calculating modulus. – Alena Sep 22 '17 at 02:55
@Alena updated again to add function which you can use and handles the negatives for you. – clinton3141 Sep 22 '17 at 02:57

score 0 · Answer 2 · answered Sep 22 '17 at 02:54

0

How about ?

$ cat test.php
<?php

function truemod($num, $mod) {
  return ($mod + ($num % $mod)) % $mod;
}

echo truemod(-1, 26).PHP_EOL;
?>

Output

$ php test.php 
25

answered Sep 22 '17 at 02:54

Akshay Hegde

16,536
2
22
36

it works for this but causes problem with other calculation. But anyway, thanks! – Alena Sep 22 '17 at 05:17

Strange Modulus Operator Result - PHP

2 Answers2