Why Does the C Modulo Operator (%) Sometimes Return Negative Results?

Modulus Evaluation: Understanding Negative Results

In C , the modulo operator (%) is known to yield negative values under certain conditions. This can be puzzling, especially when these operations return different results:

std::cout << (-7 % 3) << std::endl;  // -1
std::cout << (7 % -3) << std::endl;  // 1

To clarify this behavior, we refer to the ISO C standard (ISO14882:2011). The modulo operator returns the remainder from the division of the first operand (numerator) by the second operand (denominator).

For the first operation, (-7) is divided by 3, resulting in a quotient of -2 with a remainder of -1. Therefore, (-7 % 3) equals -1.

For the second operation, 7 is divided by -3, also resulting in a quotient of -2. However, since the denominator is negative, the sign of the remainder is implementation-defined. In this case, the implementation chooses to return a positive remainder, which is 1.

It's important to note that the behavior of the modulo operator may differ depending on the specific platform and implementation. In general, it's recommended to handle negative values carefully and consider the potential for different results when working with the modulo operator.

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