Modulus with a negative number

Question

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Modulus operation with negatives values - weird thing ??
Mod of negative number is melting my brain!

Answer 1

In C89/90, either result was allowed. The results you got from division and remainder were required to "fit" together so that (a/b)*b + a%b == a.

Starting with C99, integer division with a negative input is required to truncate toward zero. The relationship between the results from division and remainder is still required though. This means that (in effect) the result from the remainder operation always has the same sign as the left operand, so 25 % -9 must yield 7, not -2.

For what it's worth, C++ followed roughly the same path, just a few years behind C. C++98/03 has the same rules as C89/90, so for your inputs the remainder could be either negative or positive (but still needs to fit together with the result from division). Starting with C++11, it requires the same behavior as C99, so 25 % - 9 == 7.

Some other languages (e.g., Python) require that the result from remainder have the same sign as the right operand instead.

Answer 2

If you come to think about it in a mathematical base of 9 they are the same thing as 9-2 = 7