/* root.c       A simple iteration method for finding the zeroes of a quadratic function    */

#  include <stdio.h>
#  include <math.h>		/* necessary for use of abs() */

double func(double x, double a, double b, double c);     /* function prototype */

main()						/* header for function main()  */
{ 						/* start of main()  */
double  x1, x2, x3, f, f1, f2, a, b, c, min;
int step;	
 	printf("\nEnter values for a, b and c, separated by spaces or carriage returns:\n");
	scanf("%lf %lf %lf", &a, &b, &c);

	printf("Enter a value of x for which f(x) is negative:\n");
	scanf("%lf", &x1);
	while (func(x1, a, b, c) >=0) {
		printf("f(x) is not negative for this x.  Choose another value:\n");
		scanf("%lf", &x1);
	}
	printf("Enter a value of x for which f(x) is positive:\n");
	scanf("%lf", &x2);
	while(func(x2, a, b, c) <=0) {
		printf("f(x) is not positive for this x.  Choose another value:\n");
		scanf("%lf", &x2);
	}

	step=1;
 	min=0.001;
 	do{
		x3 = (x1 + x2)/2; 
		f=func(x3, a, b, c);
		if  (f>0){
			x2=x3;
			}
		else {
			x1=x3;
			}
		step++;
		}  while(fabs(f)>min); 	         /* stops loop when f is sufficiently close to 0 */

 	printf("\nf(x) is zero at x=%0.2lf\n", x3);        /* prints x to 2 places of decimals */
 	printf("Number of steps = %d\n", step);   
}						/* end of main()  */


double func(double x, double a, double b, double c)       /* function header */

{						/* start of func() */
double fvalue;
fvalue = a*x*x + b*x + c;
return fvalue;
}						/* end of func() */