#include <stdio.h>
#include <math.h>
/* besection : 1999 3 2 by Y.Yamazaki and T.Oguni */
double func(f, x1, x2, tol, itr)
  double(*f)(), x1, x2, tol;
  int itr;
{
  int i;
  double x3, a;
  if (f(x1)*f(x2)>=0.) exit(1);
  for (i=1; i<=itr; i++){
    x3=(x1+x2)/2.0;
    printf("i=%d x3=%f\n", i, x3);
    if (fabs(x2-x1)<tol) return x3;
    if ((*f)(x1)*(*f)(x3)<0.) x2=x3;
    else x1=x3;
  }
  return x3;
}
/* main */
double f(double x) {return x*x - 2.0;}
int main(){
  double a;
  a=func(f, 1.0, 2.0, 0.0001, 20);
  printf("a= %f\n", a);
}