# C preprocessor evaluates sin () constants

Is there a way to convince the **C** preprocessor to evaluate the constant transcendental function at compile time?

For example, replace `(int)256*sin(PI/4)`

with `181`

. This will help me keep the magic numbers out of my code.

If it matters, I'm using MSPGCC 4.5.3 and I don't have `sin()`

or `cos()`

are available at runtime.

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As long as your arguments are within the range [-ฯ / 4, + ฯ / 4], you can use the same standard libm implementations to compute sin. It is corrected to the last place (maximum allowable error is 1), as in the IEEE standard:

```
static const double
half = 5.00000000000000000000e-01, /* 0x3FE00000, 0x00000000 */
S1 = -1.66666666666666324348e-01, /* 0xBFC55555, 0x55555549 */
S2 = 8.33333333332248946124e-03, /* 0x3F811111, 0x1110F8A6 */
S3 = -1.98412698298579493134e-04, /* 0xBF2A01A0, 0x19C161D5 */
S4 = 2.75573137070700676789e-06, /* 0x3EC71DE3, 0x57B1FE7D */
S5 = -2.50507602534068634195e-08, /* 0xBE5AE5E6, 0x8A2B9CEB */
S6 = 1.58969099521155010221e-10; /* 0x3DE5D93A, 0x5ACFD57C */
double __kernel_sin(double x, double y, int iy)
{
double z,r,v;
int ix;
ix = __HI(x)&0x7fffffff; /* high word of x */
if(ix<0x3e400000) /* |x| < 2**-27 */
{if((int)x==0) return x;} /* generate inexact */
z = x*x;
v = z*x;
r = S2+z*(S3+z*(S4+z*(S5+z*S6)));
if(iy==0) return x+v*(S1+z*r);
else return x-((z*(half*y-v*r)-y)-v*S1);
}
```

Source: http://www.netlib.org/fdlibm/k_sin.c

While not what I would call nice, you can definitely convert this whole function into a macro that will evaluate a floating point constant expression (compile time). (Ignore the hacker bit at the beginning, which has nothing to do with the value, and as far as I know, you should assume it `iy`

is 0.)

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The preprocessor can only allow macros, which is no different from executing a function. The closest solution I can think of to reduce your magic numbers is to create a header with the most commonly used sin or coincidence meanings:

```
#define SIN_PI (-1)
#define SIN_PI2 0
#define SIN_PI4 0.707106781186548
...
```

Then you can write:

`256*SIN_PI2`

And let the compiler optimization reduce it to one constant.

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