Why Does C Modulo Return Negative Numbers?

Why C Output Negative Numbers When Using Modulo?

In contrast to Python, C outputs negative values when using modulo with a positive divisor. This occurs because the operation is implementation-defined in C according to ISO/IEC 14882:2003(E):

"If both operands are nonnegative then the remainder is nonnegative; if not, the sign of the remainder is implementation-defined."

This definition allows for optimizations that are common in processor instruction sets, where division and modulo are performed together.

The negative remainder ensures consistency with the following rules:

• Quotients are rounded towards zero for integer division.
• The dividend equation (dividend = quotient * divisor remainder) is satisfied by the results.

This means that when dividing a negative number by a positive number, the quotient will be negative (or zero).

The rationale for this specification includes:

• Efficiency and simplicity in processor instruction sets.
• Consistency in rounding and respecting the division equation.
• C's preference for efficiency and simplicity.
• C 's compatibility with C.

While the negative remainder may seem counterintuitive, it is a consequence of the implementation-defined nature of modulo in C . It ensures consistency with integer division and optimizes performance.

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