cospi, cospif, cospil, cospid32, cospid64, cospid128

< c‎ | numeric‎ | math

Defined in header `<math.h>`
float cospif( float arg );	(1)	(since C23)
double cospi( double arg );	(2)	(since C23)
long double cospil( long double arg );	(3)	(since C23)
_Decimal32 cospid32( _Decimal32 arg );	(4)	(since C23)
_Decimal64 cospid64( _Decimal64 arg );	(5)	(since C23)
_Decimal128 cospid128( _Decimal128 arg );	(6)	(since C23)
Defined in header `<tgmath.h>`
#define cospi( arg )	(7)	(since C23)

1-6) Computes the cosine of π·arg measured in radians, thus regarding arg as a measurement in half-revolutions.

7) Type-generic macro: calls the correct function based on the type of arg. If the argument has integer type, (2) (cospi) is called.

The functions (4-6) are declared if and only if the implementation predefines __STDC_IEC_60559_DFP__ (i.e. the implementation supports decimal floating-point numbers).

(since C23)

Parameters

arg - floating-point value whose product with π represents an angle in radians

Return value

If no errors occur, the cosine of π·arg ( $cos(π\timesarg)$ ) in the range $[-1, +1]$ , is returned.

Error handling

Errors are reported as specified in math_errhandling.

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

if the argument is ±0, the result is 1.0;
if the argument is ±∞, NaN is returned and FE_INVALID is raised;
if the argument is NaN, NaN is returned.

Example

#include <errno.h>
#include <fenv.h>
#include <math.h>
#include <stdio.h>
 
#ifndef __GNUC__
#pragma STDC FENV_ACCESS ON
#endif
 
#if __STDC_VERSION__ < 202311L
// A naive implementation of a subset of cospi family
double cospi(double arg)
{
    return cos(arg * (double)3.1415926535897932384626433);
}
#endif
 
int main(void)
{
    const double pi = acos(-1);
 
    // typical usage
    printf("cospi(1) = %f, cos(pi) = %f\n", cospi(1), cos(pi));
    printf("cospi(0.5) = %f, cos(pi/2) = %f\n", cospi(0.5), cos(pi / 2));
    printf("cospi(-0.75) = %f, cos(-3*pi/4) = %f\n", cospi(-0.75), cos(-3 * pi / 4));
 
    // special values
    printf("cospi(+0) = %f\n", cospi(0.0));
    printf("cospi(-0) = %f\n", cospi(-0.0));
 
    // error handling
    feclearexcept(FE_ALL_EXCEPT);
    printf("cospi(INFINITY) = %f\n", cospi(INFINITY));
    if (fetestexcept(FE_INVALID))
        puts("    FE_INVALID raised");
}

Possible output:

cospi(1) = -1.000000, cos(pi) = -1.000000
cospi(0.5) = 0.000000, cos(pi/2) = 0.000000
cospi(-0.75) = -0.707107, cos(-3*pi/4) = -0.707107
cospi(+0) = 1.000000
cospi(-0) = 1.000000
cospi(INFINITY) = -nan
    FE_INVALID raised

References

C23 standard (ISO/IEC 9899:2023):

7.12.4.12 The cospi functions (p: 247)