goalseek.h

#include <stdarg.h>
#include <math.h>

	/*
	* goalseek
	* --------
	* double goalseek(double* pf, int argn, int warg, double goal, ...)
	* - pf: is pointer to function (type double)
	* - argn: is arguments number function pf pointed to. 
	* 	eg double add(double a, double b) accepts 2 arguments
	* - warg: is index if which arguments you want to seek. 
	* 	eg, if function add(500,300) and you want to seek second 
	* 	value, the set warg value to 2
	* - goal: is the end value that you wants
	* - ...: is variadic variable you set to initialize the function
	* 
	* usage example
	* ------------
	* #include <stdio.h>
	* #include <goalseek.h>
	* 
	* double avg(double a, double b, double c){
	*	return (a + b + c) / 3;
	* }
	* 
	* int main() {
    *	double ans = 0;
    * 	printf("average of is 234, 456 and 876 is %lf\n", avg(234,456,876));
	* 	// avg(234,456,876) is 522. Replace argument b with any value, i want this to become 900
    * 	ans = goalseek(&avg,3,2,900.0,234.0,456.0,876.0);
    * 	printf("If you want average to 900, C should be %lf\n", ans);
    * 	gets(stdin);
    * 	return (0);
	* }	
	*/
	
	
double goalseek(double* pf, int argn, int warg, double goal, ...)
{

    double (*gn)(double, ...) = pf;
    double a[5], t1=0, t2=0, op=0;
    int i;
    va_list ap;
    va_start(ap, goal);
    for(i = 0; i < argn; i++) a[i] = va_arg(ap, double);
  
#define FARG (*gn)(a[0], a[1], a[2], a[3], a[4], a[5])

    t1 = FARG;
    a[warg] = a[warg] + 1;
    t2 = FARG;
    op = (t2 > t1) ? 0.001 : -0.001;
    while(1) {
		if(FARG > goal) a[warg] = a[warg] - op;
		if(FARG < goal) a[warg] = a[warg] + op;
		if(fabs(FARG - goal) < 0.0001) return a[warg];
    }
}