std::fmod, std::fmodf, std::fmodl

The floating-point remainder of the division operation x / y calculated by this function is exactly the value x - iquot * y, where iquot is x / y

The returned value has the same sign as x and is less than y in magnitude.

Parameters

Return value

If successful, returns the floating-point remainder of the division x / y as defined above.

If a domain error occurs, an implementation-defined value is returned (NaN where supported).

If a range error occurs due to underflow, the correct result (after rounding) is returned.

Error handling

Errors are reported as specified in math_errhandling.

Domain error may occur if y is zero.

If the implementation supports IEEE floating-point arithmetic (IEC 60559),

• If x is ±0 and y is not zero, ±0 is returned.
• If x is ±∞ and y is not NaN, NaN is returned and FE_INVALID
• If y is ±0 and x is not NaN, NaN is returned and FE_INVALID
• If y is ±∞ and x is finite, x
• If either argument is NaN, NaN is returned.

Notes

POSIX requires that a domain error occurs if x is infinite or y

std::fmod, but not std::remainder is useful for doing silent wrapping of floating-point types to unsigned integer types: ( 0.0 <= (y = std:: fmod ( std::rint (x), 65536.0 ) ) ? y : 65536.0 + y) is in the range [ -0.0 ,  65535.0 ] , which corresponds to unsigned short, but std::remainder ( std::rint (x), 65536.0 is in the range [ -32767.0 ,  +32768.0 ] , which is outside of the range of signed short

The double version of std::fmod behaves as if implemented as follows:

double fmod(double x, double y)
{
#pragma STDC FENV_ACCESS ON
    double result = std::remainder(std::fabs(x), y = std::fabs(y));
    if (std::signbit(result))
        result += y;
    return std::copysign(result, x);
}

The expression x - std::trunc (x / y) * may not equal std::fmod(x, y), when the rounding of x / y to initialize the argument of std::trunc loses too much precision (example: x = 30.508474576271183309, y = 6.1016949152542370172

The additional overloads are not required to be provided exactly as (A). They only need to be sufficient to ensure that for their first argument num1 and second argument num2

Example

#include <cfenv>
#include <cmath>
#include <iostream>
// #pragma STDC FENV_ACCESS ON
 
int main()
{
    std::cout << "fmod(+5.1, +3.0) = " << std::fmod(5.1, 3) << '\n'
              << "fmod(-5.1, +3.0) = " << std::fmod(-5.1, 3) << '\n'
              << "fmod(+5.1, -3.0) = " << std::fmod(5.1, -3) << '\n'
              << "fmod(-5.1, -3.0) = " << std::fmod(-5.1, -3) << '\n';
 
    // special values
    std::cout << "fmod(+0.0, 1.0) = " << std::fmod(0, 1) << '\n'
              << "fmod(-0.0, 1.0) = " << std::fmod(-0.0, 1) << '\n'
              << "fmod(5.1, Inf) = " << std::fmod(5.1, INFINITY) << '\n';
 
    // error handling
    std::feclearexcept(FE_ALL_EXCEPT);
    std::cout << "fmod(+5.1, 0) = " << std::fmod(5.1, 0) << '\n';
    if (std::fetestexcept(FE_INVALID))
        std::cout << "    FE_INVALID raised\n";
}

fmod(+5.1, +3.0) = 2.1
fmod(-5.1, +3.0) = -2.1
fmod(+5.1, -3.0) = 2.1
fmod(-5.1, -3.0) = -2.1
fmod(+0.0, 1.0) = 0
fmod(-0.0, 1.0) = -0
fmod(5.1, Inf) = 5.1
fmod(+5.1, 0) = -nan
    FE_INVALID raised

See also