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 with its fractional part truncated.

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

Notes

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

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 ) * 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