#include <stdio.h>
#include <math.h>

#define JMAX 20

void funcd(double,double *,double *);
double rtnewt(void (*)(double,double *,double *),double,double,double);
              
int main() {

 double x1,x2,xacc;
 double sol;
 
 do {
  printf("\nZadejte interval (x1,x2) obsahujici hledanou hodnotou reseni.");
  printf("\nMusí platit x1<x2, x1=x2 ukonci zadavani.");
  printf("\nx1=");
  scanf("%lf",&x1);
  printf("x2=");
  scanf("%lf",&x2);
  if (x1!=x2 && x1<x2) {
   printf("\nZadej presnost xacc.");
   printf("\nxacc=");
   scanf("%lf",&xacc);
   sol=rtnewt(funcd,x1,x2,xacc);
   printf("\nx=%1.8e\n",sol);
  } 
 } while (x1!=x2 && x1<x2);
 
 return 0;
}

void funcd(double x,double *f, double *df) {

 double a=1.;
 
 *f=a*sin(x);
 *df=a*cos(x);
}

double rtnewt(void (*fce)(double,double *,double *),double x1,double x2, double xacc) {
 
 int j;
 double df,dx,f,rtn;
 
 rtn=0.5*(x1+x2);
 for (j=1;j<=JMAX;j++) {
  fce(rtn,&f,&df);
  dx=f/df;
  rtn-=dx;
  if ((x1-rtn)*(rtn-x2)<0.0) {
   printf("\nMimo zvolený interval v rtnewt\n");
   return 0.0;
  } 
  if (fabs(dx)<xacc)
   return rtn;
 }  
 printf("\nPrekrocen pocet iteraci v rtnewt\n");
 return 0.0;
}