Why is modulo not functioning correctly in this C code?

Question

I have the following code snippet:

#include <stdio.h>

int main(){
    printf("%d\r\n", -1 % 7);
    return 0;
}

When run, it prints -1. According to my own math and calculators like this one (http://www.miniwebtool.com/modulo-calculator/?number1=-1&number2=7), my answer should be 6. Im assuming there is some 'gotcha' in the C implementation of modulo that I am not grasping. Can someone explain why I am not getting the answer I expect?

Read http://stackoverflow.com/questions/11720656/modulo-operation-with-negative-numbers — Sinstein, Dec 21 '15 at 16:33
FWW: `-1 % 7` also returns `-1` in JavaScript, but it does return `6` in Python. — Yanick Rochon, Dec 21 '15 at 16:36

score 0 · Answer 1 · answered Dec 21 '15 at 16:32

0

The rule of C (after C99) is that the sign of the result of i%j is same as the sign of i. The answer you are getting is correct.

In C89 the result of i%j (if either i or j is negative) depends on the implementation. -1%7 could either be -1 or 6.

answered Dec 21 '15 at 16:32

haccks

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Why is modulo not functioning correctly in this C code?

1 Answers1