### _fyl2xp1 -- Calculate y * log2(x + 1)

Format

>     #include <builtin.h>
>     double _fyl2xp1(double x, double y);

Language Level: Extension
_fyl2xp1 computes the base-2 logarithm of x+1 and multiplies it by y (y * log2(x+1)). The variable x must be in the range -(1-(sqrt(2)/2)) to (sqrt(2) - 1). _fyl2xp1 provides improved accuracy over _fyl2x when logarithms of numbers very close to 1 are computed. When x is small, you can retain more significant digits by providing x to the _fyl2xp1 function than by providing x+1 as an argument to the _fyl2x function.

Because it is a built-in function and has no backing code in the library:

- You cannot take the address of _fyl2xp1.
- You cannot parenthesize a _fyl2xp1 function call, because parentheses specify a call to the backing code for the function in the library.

Return Value
_fyl2xp1 returns the result of the formula y * log2(x+1).

Example
This example calculates (y * log2(x + 1)), after prompting the user for values of x and y.

    #include <builtin.h>
    #include <stdio.h>

    int main(void)
    {
       double x, y;

       printf("Enter a value for x:\n");
       scanf("%lf", &x);
       printf("Enter a value for y:\n");
       scanf("%lf", &y);
       printf("%lf * log2(%lf + 1) is %lf.\n", y, x, _fyl2xp1(x, y));
       return 0;

       /*************************************************************
          Assuming you enter 0.001 for x and 2.0 for y.

          The output should be:

          2.000000 * log2(0.001000 + 1) is 0.002884.
       *************************************************************/
    }

_fyl2x -- Calculate y * log2(x)
_f2xm1 -- Calculate (2**x) - 1
<builtin.h>