NAME

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

Synopsis

Defined in header <cmath>
float fmod ( float x, float y ); (1) (constexpr since C++23)
float fmodf( float x, float y ); (2) (since C++11)
(constexpr since C++23)
double fmod ( double x, double y ); (3) (constexpr since C++23)
long double fmod ( long double x, long double y ); (4) (constexpr since C++23)
long double fmodl( long double x, long double y ); (5) (since C++11)
(constexpr since C++23)
Promoted fmod ( Arithmetic1 x, Arithmetic2 y ); (6) (since C++11)
(constexpr since C++23)

1-5) Computes the floating-point remainder of the division operation x/y.
6) A set of overloads or a function template for all combinations of arguments of
arithmetic type not covered by (1-5). If any argument has integral type, it is cast
to double. If any other argument is long double, then the return type is long
double, otherwise it is double.

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

x, y - floating point values

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

Example

// Run this code

#include <iostream>
#include <cmath>
#include <cfenv>

// #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";
}

Possible output:

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