How to code a modulo operator in C C Obj-C that handles negative numbers

Question

As a mathematician, one of my pet peeves of C-derived languages is :

(-1) % 8 // comes out as -1, and not 7

fmodf(-1,8) // fails similarly

What is the most effective solution?

Templates and operator overloading are both possible in C++, but both are unfamiliar to me. 

Thank you for providing examples.

Answer 1

First and foremost, I'd like to point out that you cannot rely on the assumption that (-1) % 8 == -1.

The sole certainty is that (x / y) * y + (x % y) == x.

However, whether or not the remainder is negative is determined by the implementation.

The binary / operator returns the quotient, whereas the binary percent operator returns the remainder of dividing the first expression by the second.

The behaviour is unknown if the second operand of / or percent is 0; otherwise, (a/b)*b + a % b equals a.

If both operands are nonnegative, the remainder is also nonnegative; otherwise, the remainder's sign is implementation-defined.

Here is a variant that handles both negative operands so that the result of subtracting the remainder from the divisor may be  subtracted from the dividend to get the real division's floor.

mod(-1,8) yields 7, but mod(13, -8) yields -3.