// Function to compute square root using Newton's method.
// Michael Rothstein
// 2/3/2014

const double TOLERANCE = 1.0E-7;

double absval(double);

double sqrt(double A){
  if ( A< 0 ) return -1;
  double newguess,oldguess;
  for(oldguess = 0.0,newguess = 1.0;
      absval(oldguess-newguess)> A*TOLERANCE;
      oldguess = newguess,
	newguess = (oldguess + A/oldguess)/2.0);
  return newguess;
}

double absval(double x){
  return (x<0?-x:x);
}

#ifdef TEST
#include <iostream>
using std::cin; using std::cout; using std::endl;

int main(){
  while(1){
    cout << "Input a number to test: ";
    double x;
    cin >> x;
    if ( x < 0 ) break;
    cout << "Square root of " << x << " is " << sqrt(x) << endl;
  }
}
#endif