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copysign, copysignf, copysignl

Defined in header `<math.h>`
float copysignf( float x, float y );	(1)	(since C99)
double copysign( double x, double y );	(2)	(since C99)
long double copysignl( long double x, long double y );	(3)	(since C99)
Defined in header `<tgmath.h>`
#define copysign(from, to)	(7)	(since C99)

1-3) Composes a floating point value with the magnitude of x and the sign of y.

4) Type-generic macro: If any argument has type long double, copysignl is called. Otherwise, if any argument has integer type or has type double, copysign is called. Otherwise, copysignf is called.

Parameters

x, y

floating point values

Return value

If no errors occur, the floating point value with the magnitude of x and the sign of y is returned.

If x is NaN, then NaN with the sign of y is returned.

If y is -0, the result is only negative if the implementation supports the signed zero consistently in arithmetic operations.

Error handling

This function is not subject to any errors specified in math_errhandling.

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

The returned value is exact (FE_INEXACT is never raised) and independent of the current rounding mode.

Notes

copysign is the only portable way to manipulate the sign of a NaN value (to examine the sign of a NaN, signbit may also be used).

Example

#include <stdio.h>
#include <math.h>
 
int main(void)
{
    printf("copysign(1.0,+2.0)      = %+.1f\n", copysign(1.0,+2.0));
    printf("copysign(1.0,-2.0)      = %+.1f\n", copysign(1.0,-2.0));
    printf("copysign(INFINITY,-2.0) = %f\n",    copysign(INFINITY,-2.0));
    printf("copysign(NAN,-2.0)      = %f\n",    copysign(NAN,-2.0));
}

Possible output:

copysign(1.0,+2.0)      = +1.0
copysign(1.0,-2.0)      = -1.0
copysign(INFINITY,-2.0) = -inf
copysign(NAN,-2.0)      = -nan

References

C11 standard (ISO/IEC 9899:2011):

7.12.11.1 The copysign functions (p: 255)
7.25 Type-generic math <tgmath.h> (p: 373-375)
F.10.8.1 The copysign functions (p: 529)

C99 standard (ISO/IEC 9899:1999):

7.12.11.1 The copysign functions (p: 236)
7.22 Type-generic math <tgmath.h> (p: 335-337)
F.9.8.1 The copysign functions (p: 465)

fabsfabsffabsl (C99)(C99)	computes absolute value of a floating-point value (\|x\|) (function)
signbit (C99)	checks if the given number is negative (function)
C++ documentation for `copysign`