How does Javascript handle a negative modulo? (`-5 % 20`)

[bit5009167409]

-5 % 20 should be 15. When I enter this it returns -5. Is this a javascript thing or an artifact of this simulator?

thanks.

[midlindner]

Hello, @bit5009167409. Welcome to the forums.

20 goes into -5 how many times? 0 right? What is the remainder? -5 correct?
-5 + 20 = 15, but -5 % 20 = -5
The output is correct.

Node.js:

> -5 + 20
15
> -5 % 20
-5

[bit5009167409]

Case 1:
101/ 11 = 99 remainder 2
or
101 = 11 * 99 + 2

Case 2:
-11 / 3 =

-11 = -4*3 + 1
So the remainder is 1.

Remainders must always be positive.

Reference: Discrete Math & Its Applications, K. Rosen, 7th Edition page 217.

So -5 / 20 =

-5 = -1 * 20 + 15

So the remainder is 15.

[midlindner]

I’m not a mathematician, so I won’t argue, but this is curious. It would seem per your reference that Python knows how to do math where JavaScript and Haskell do not. It may also be that Python does the actual math where JavaScript and Haskell behave in a predictable manner for programmers. Idk

Examples:

//JavaScript
> -5 % 20
-5

//#Python
>>> -5 % 20
15

//-- Haskell
Prelude> -5 `mod` 20
-5

//C++
#include <iostream>

int main() {
  std::cout << -5 % 20; // -5
}

//C#
using System;

class MainClass {
  public static void Main (string[] args) {
    Console.WriteLine (-5 % 20); // -5
  }
}

//Java
public class YourClassNameHere {
    public static void main(String[] args) {
    System.out.println(-5 % 20); // -5
    }
}

//#Ruby
puts -5 % 20 // 15

//Swift
print(-5 % 20) // -5

Interesting. Python & Ruby behave as you’ve described the correct answer should be, but the others do not.

Perhaps someone with much more knowledge than me could weigh in. @ionatan, @mtf, @appylpye

Edit:
Haskell produces different results depending on how the mod method is used:

Prelude> mod (-5) 20
15
Prelude> (-5) `mod` 20
15
Prelude> -5 `mod` 20
-5

I suppose you just have to review the docs for the language that you’re using to know what the expected behavior is. Here’s a link to the JavaScript docs on the remainder operator.

[bit5009167409]

Thanks for checking all of those out! In the big picture, I don’t think it matters. I just found it curious! In the very very rare event that the topic comes up, I guess its good to know that one can get different interpretations.

Thank you.

[midlindner]

@bit5009167409

I don’t think it matters.

Other than not getting the expected outcome. I, not having much of a background in math, (at least not a lot that I remember anymore) saw the expected outcome. With your background, you saw a math error. Thanks for asking your original question. I learned something.
Happy coding!

[ionatan]

In modular arithmetic things go around, like a clock. You’ll always end up with a result in the valid range. For a clock, 23 + 4 is 3
There are two similar operations, rem and mod, and two ways to divide to go with them.

Haskell’s prelude exports both of those functions, and both ways to divide (rem mod div quot). (watch out with operator precedence) The documentation there also mentions their differences and relations:
https://hackage.haskell.org/package/base-4.12.0.0/docs/Prelude.html#v:quot
I suppose languages that give special meaning to % pick one or the other and then match division with it (div mod or quot rem) … and if you care about the difference then you’d probably notice.