Different between % operator of C language and mod of google calculator

Question

I calc -15 mod 18 and here are results:
C: -15 % 18 = -15 (http://codepad.org/DhzkZYHk)
Google: -15 mod 18 = 3 (Type -15 mod 18 into google's search box)

and results of -9 mod 5:
C: -9 % 5 = -4
Google: -9 mod 5 =1

Why these are differents? And how google calculate their mod?

Because Google is not a C compiler and is not required to follow the same rules as C compilers? — DCoder, May 05 '13 at 16:15
possible duplicate of [C,Python - different behaviour of the modulo (%) operation](http://stackoverflow.com/questions/1907565/c-python-different-behaviour-of-the-modulo-operation) — Hasturkun, May 05 '13 at 16:18
I'll also note [Why does Java's % operator give different results than my calculator for a negative dividend?](http://stackoverflow.com/questions/7479526/why-does-javas-operator-give-different-results-than-my-calculator-for-a-negat?rq=1) which is also relevant. — Hasturkun, May 05 '13 at 16:18

Varad · Accepted Answer · 2013-05-05T17:59:09.080

3

Google's calculator does -15 mod 18 as

-15 = 18*(-1) + 3

giving a remainder of 3, whereas C evaluates it as

-15 = 18*(0) - 15

and hence the expression becomes -15.

In general,

a = (a/b)*b + a%b

holds.

edited May 05 '13 at 17:59

answered May 05 '13 at 16:35

Varad

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`a = (a/b)*b + a%b` always holds no matter which definition you're using. – R.. GitHub STOP HELPING ICE May 05 '13 at 17:07
Put `a = -15` and `b = 18`. Then, `a%b = a - (a/b)*b = -15 - (-15/18)*18 = -15 + 0 = -15` since '/' is integer division. Google's calculator (and even Python) do not evaluate it this way. – Varad May 05 '13 at 17:38
If `(-15)%18` is defined as 3, then `(-15)/18` should be defined as -1, not 0. Making inconsistent definitions for the two is just stupid. – R.. GitHub STOP HELPING ICE May 05 '13 at 17:44
Okay, I get it. I was thinking along the lines of C, where integer division of -15 and 18 evaluates to 0 and not -1. My bad. – Varad May 05 '13 at 17:58

score 2 · Answer 2 · answered May 05 '13 at 16:17

Because the % operator in Google is a modulus operator and the % operator in C is a remainder operator.

Modulus and remainder operators differ with respect to negative values. With the modulus operator the sign of the result is the sign of the divisor and with the remainder operator the sign of the result is the sign of the dividend.

Different between % operator of C language and mod of google calculator

2 Answers2