/* 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
*/
// eqstring.c: Solution of  wave equation using time stepping           
//  comment: Output data is saved in 3D grid format used by gnuplot 

#include <stdio.h>
#include <math.h>
#define rho 0.01                                // density per length
#define ten 40.0                                           // tension
#define max 100                                         // time steps

main()  {
	
   int i,k;
   double x[101][3];
	 
   FILE *out;                              // save data in string.dat
   out = fopen("eqstring.dat","w");
	                                                  // initial config
   for (i = 0; i<81; i++) x[i][0]= 0.00125*i;             
   for (i = 81; i<101; i++) x[i][0]= 0.1-0.005*(i-80); 
	                                                        // 1st step
   for (i = 1; i<100; i++)                                      
		   {x[i][1]= x[i][0]+0.5*(x[i+1][0]+x[i-1][0]-2.0*x[i][0]);} 
	                                            // all later time steps
   for (k = 1; k<max; k++) {                       
     for (i = 1; i<100; i++)     x[i][2]= 2.*x[i][1]-x[i][0]
	                                +(x[i+1][1]+x[i-1][1]-2.*x[i][1]);
      for (i = 0; i<101; i++) { 
        x[i][0]= x[i][1];    
        x[i][1]= x[i][2];     
      }
      if ((k%5) ==  0) {                     // print every 5th point
        for (i = 0; i<101; i++) fprintf(out, "%f\n",x[i][2]);  
        fprintf(out, "\n");                 // empty line for gnuplot
      }  
   }
   printf("data stored in eqstring.dat\n");
   fclose(out);
}