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

int main ( int argc, char *argv[] )
//
//  Purpose:
//
//    SQUARE_ROOT2 estimates the square root of a (nonnegative) number.
//
//  Usage:
//
//    square_root2 s r1
//
//    where S is the number whose square root is desired, and R1 is an
//    initial guess for the root.
//
//    If S and R1 are not given on the command line, the program will
//    remind the user to type them in.
//
{
  double e;
  double r1;
  double r2;
  double s;

  if ( argc < 2 )
  {
    printf ( "Enter a number whose square root is desired: " );
    scanf ( "%lf", &s );
    printf ( "S = %g\n", s );
  }
  else
  {
    s = atof ( argv[1] );
  }

  if ( s < 0.0 )
  {
    fprintf ( stderr, "The input number is unacceptable because it is negative.\n" );
    return 1;
  }

  if ( argc < 3 )
  {
    printf ( "Enter a guess for the square root of %g: ", s );
    scanf ( "%lf", &r1 );
  }
  else
  {
    r1 = atof ( argv[2] );
  }

  if ( r1 == 0.0 && s != 0.0 )
  {
    fprintf ( stderr, "A starting guess of 0 is not acceptable!\n" );
    return 2;
  }

  do
  {
    r2 = s / r1;
    e = s - r1 * r1;
    printf ( "  %24.16g  %24.16g  %g\n", r1, r2, e );
    r1 = ( r1 + r2 ) / 2.0;
  }
  while ( 0.0000001 < fabs ( e ) );

  return 0;
}