/* From: "COMPUTATIONAL PHYSICS, 2nd Ed" 
   by RH Landau, MJ Paez, and CC Bordeianu 
   Copyright Wiley-VCH, 2007.
   Electronic Materials copyright: R Landau, Oregon State Univ, 2007;
   MJ Paez, Univ Antioquia, 2007; & CC Bordeianu, Univ Bucharest, 2007
   Support by National Science Foundation
*/
// laplace.c:  Solution of Laplace equation with finite differences 
// Output data is saved in 3D grid format used by gnuplot     
#include <stdio.h>
#define max 40                              // number of grid points

main()  {
	
  double x, p[max][max];
  int i, j, iter, y;
	
  FILE *output;                           // save data in laplace.dat
  output= fopen("laplace.dat","w");
  for (i = 0; i<max; i++) for (j = 0; j<max; j++) p[i][j]= 0;// clear
  for (i = 0; i<max; i++) p[i][0]= 100.0;          // p[i][0]= 100 V
  for (iter = 0; iter<1000; iter++)  {                  // iterations
    for (i = 1; i<(max-1); i++)  {                    // x-direction
      for (j = 1; j<(max-1); j++)                     // y-direction
         { p[i][j]= 0.25*(p[i+1][j]+p[i-1][j]+p[i][j+1]+p[i][j-1]); }
    }
  }
  for (i = 0; i<max ; i++)   {       // write data gnuplot 3D format
    for (j = 0; j<max; j++) fprintf(output, "%f\n",p[i][j]);
    fprintf(output, "\n");                  // empty line for gnuplot
  }
  printf("data stored in laplace.dat\n");
  fclose(output);
}