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

int main ( );
void timestamp ( );

/******************************************************************************/

int main ( )

/******************************************************************************/
/*
  Purpose:

    grid_surface() draws a surface from a table Z(X,Y).

  Discussion:

    Here, the grid is a 41x41 array of equally spaced points in [-2,2]x[-2,2].

    and Z(X,Y) is a function which can be evaluated by:

      Z = exp ( - ( X^2 + Y^2 ) ) * cos ( 0.25 * X )
        * sin ( Y ) * cos ( 2 * (X^2 + Y^2 ) )

  Licensing:

    This code is distributed under the MIT license.

  Modified:

    26 September 2026

  Author:

    John Burkardt
*/
{
  char command_filename[80];
  FILE *command;
  char data_filename[80];
  FILE *data;
  char header[] = "grid";
  int i;
  int j;
  int m;
  int n;
  double r2;
  double x;
  double y;
  double z;

  timestamp ( );
  printf ( "\n" );
  printf ( "grid_surface():\n" );
  printf ( "  C version\n" );
  printf ( "  Make a surface plot Z(X,Y) for a 41x41 table.\n" );
/*
  Set number of points in plot.
*/
  m = 41;
  n = 41;
/*
  Define the data.
*/
  strcpy ( data_filename, header );
  strcat ( data_filename, "_data.txt" );

  data = fopen ( data_filename, "wt" );

  for ( i = 0; i < m; i++ )
  {
    if ( 0 < i )
    {
      fprintf ( data, "\n" );
    }
    x = ( ( m - i - 1 ) * ( -2.0 ) 
        + (     i     ) * (  2.0 ) ) 
        / ( m     - 1 );
    for ( j = 0; j < n; j++ )
    {
      y = ( ( n - j - 1 ) * ( -2.0 ) 
          + (     j     ) * (  2.0 ) )
          / ( n     - 1 );
      r2 = x * x + y * y;
      z = exp ( - r2 ) * cos ( 0.25 * x ) * sin ( y ) * cos ( 2 * r2 );
      fprintf ( data, "  %g  %g  %g\n", x, y, z );
    }
  }

  fclose ( data );

  printf ( "\n" );
  printf ( "  Data stored in \"%s\".\n", data_filename );
/*
  Create the command file.
*/
  strcpy ( command_filename, header );
  strcat ( command_filename, "_commands.txt" );

  command = fopen ( command_filename, "wt" );

  fprintf ( command, "# %s\n", command_filename );
  fprintf ( command, "#\n" );
  fprintf ( command, "# Usage:\n" );
  fprintf ( command, "#  gnuplot < %s\n", command_filename );
  fprintf ( command, "#\n" );
  fprintf ( command, "set term png\n" );
  fprintf ( command, "set output '%s_surface.png'\n", header );
  fprintf ( command, "set xlabel 'X'\n" );
  fprintf ( command, "set ylabel 'Y'\n" );
  fprintf ( command, "set zlabel 'Z'\n" );
  fprintf ( command, "set title 'Grid surface'\n" );
  fprintf ( command, "set view 77, 77\n" );
  fprintf ( command, "set xyplane -0.5\n" );
  fprintf ( command, "set hidden3d\n" );
  fprintf ( command, "set timestamp\n" );
  fprintf ( command, "set grid\n" );
  fprintf ( command, "set style data lines\n" );
  fprintf ( command, "splot '%s' with lines\n", data_filename );
  fprintf ( command, "quit\n" );

  fclose ( command );

  printf ( "\n" );
  printf ( "  Plot commands stored in \"%s\".\n", command_filename );
/*
  Terminate.
*/
  printf ( "\n" );
  printf ( "grid_surface():\n" );
  printf ( "  Normal end of execution.\n" );
  printf ( "\n" );
  timestamp ( );

  return 0;
}
/******************************************************************************/

void timestamp ( )

/******************************************************************************/
/*
  Purpose:

    timestamp() prints the current YMDHMS date as a time stamp.

  Example:

    17 June 2014 09:45:54 AM

  Licensing:

    This code is distributed under the MIT license. 

  Modified:

    17 June 2014

  Author:

    John Burkardt
*/
{
# define TIME_SIZE 40

  static char time_buffer[TIME_SIZE];
  const struct tm *tm;
  time_t now;

  now = time ( NULL );
  tm = localtime ( &now );

  strftime ( time_buffer, TIME_SIZE, "%d %B %Y %I:%M:%S %p", tm );

  printf ( "%s\n", time_buffer );

  return;
# undef TIME_SIZE
}