#include <iostream>
#include <fstream>
#include <cstdlib>

using namespace std;

void newton(double x0, double er, int nmax, double xr, double ea, double fr)
{
//--  initial values
	int niter=0;
	double xold = x0;
	fr=FF(xold);
//
//___start of iterations __________
//
	cout<<" niter xr ea fr\n";
	for(;;)
	{
		niter++;
		double fgr = FD1(xold);
		if(fgr!=0)
		{
			xr = xold - fr/fgr;
		}
		else
		{
			cout<<"error!  zero derivative"<<endl;
			return;
		}
//-- xr is a roote?
		fr=FF(xr);
		if(fr==0)
		{
			ea=0;
			return;
		}
//-- relatice (or absolute) error
		if(xr!=0)
		{
			ea = fabs((xr-xold)/xr);
		}
		else
		{
			ea = fabs(xr-xold);
		}
//-- monitoring (optional)
		cout<<niter<<" "<<xr<<" "<<ea<<" "<<fr<<"\n";
//-- exit checks
		if(ea<=er)	return ;
		if(niter>=nmax)
		{
			cout<<"warning! iterations limit"<<endl;
			return;
		}
		xold=xr;
	}
	return ;
}