/* --------------------------------------------------------------------- 
**  Figure 20.11: Selecting a function from a menu.
** --------------------------------------------------------------------- 
*/
#include "tools.h"                  

typedef double (*funcptr)( double ); 

double                     /* Base 2 logarithm */
log_2( double x ) { return log( x ) / log( 2 ); }

void main( void )
{   
    int choice;     /* user's function selection index */
    double x, fx;   /* data input, calculated function value */
    funcptr fp;     /* function chosen by user */

    #define CHOICES  7
    string greeting = " Select a function:";
    string names[CHOICES] = { "quit", "sine", "cosine", "tangent", 
                              "natural log", "base 2 log", "e to the x" };
    funcptr function[CHOICES] = { NULL, sin, cos, tan, log, log_2, exp };

    puts( " A Menu of Functions" );
    for (;;) {
        choice = menu_i( greeting, CHOICES, names );
        if (choice == 0) break;
        printf( " Enter x: " );
        scanf( "%lg", &x );
        fp = function[choice];
        fx = fp(x);
        printf( "\n %s(%g) = %g \n", names[choice], x, fx );
    }
    bye();
}