# include <cstdlib>
# include <iostream>
# include <cmath>

using namespace std;

int main ( )
//
//  SURFACE_GRID.CPP evaluates a formula Z(X,Y) over a grid of (X,Y)
//  points -2.0 <= X <= 2.0, -2.0 <= Y <= 2.0.
//
//  N = 41 points are used in X and in Y.
//
//  This data is to be read by Gnuplot and displayed by the "splot" command,
//  which could be as simple as the single line:
//
//    splot "surface_grid_gnuplot.txt" with lines
//
//  Gnuplot requires that the data be listed in groups, with a blank line
//  between each group.  Within one group, the X values should be fixed,
//  while the Y values increase.
//
{
  float dx;
  float dy;
  int n;
  float x;
  float x1;
  float x2;
  float y;
  float y1;
  float y2;
  float z;
//
//  Use the same number of points for X and Y.
//
  n = 41;
//
//  Define the limits of X and Y, and the spacing.
//
  x1 = -2.0;
  x2 = +2.0;
  dx = ( x2 - x1 ) / ( n - 1 );

  y1 = -2.0;
  y2 = +2.0;
  dy = ( y2 - y1 ) / ( n - 1 );
//
//  Generate each (X,Y) point and evaluate Z(X,Y) and print it.
//
//  Even though our FOR loop is "correct", we might not generate the last
//  values where X = 2.0 or Y = 2.0, because of roundoff.
//
//  Using an integer counter is a safer way to guarantee you get 
//  exactly what you want!
//
  for ( x = -2.0; x <= 2.0; x = x + dx )
  {
    for ( y = -2.0; y <= 2.0; y = y + dy )
    {
      z = exp ( - ( x * x + y * y ) )
        * cos ( x / 4.0 ) * sin ( y )
        * cos ( 2 * ( x * x + y * y ) );
      cout << x << "  " << y << "  " << z << "\n";
    }
    cout << "\n";
  }
  cerr << "\n";
  cerr << "Created table of Z(X,Y) data for Gnuplot.\n";

  return 0;
}