/* 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
*/
// decay.c: Spontaneous radioactive decay simulation  

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

// if you don't have drand48 uncomment the following two lines 
//    #define drand48 1.0/RAND_MAX*rand 
//    #define srand48 srand                   
#define lambda 0.01                             // the decay constant
#define max 1000                            // number of atoms at t=0
#define time_max 500                                   // time range
#define seed 68111                       // seed for number generator

main()  {
	
  int atom, time, number, nloop;
  double decay;
	
  FILE *output;                             // save data in decay.dat
  output = fopen("decay.dat","w");
  number = nloop = max;                              // initial value
	                                          // seed number generator
  srand48(seed);                               
	                                                       // time loop
  for (time=0; time<=time_max; time++) {                       
		                                                     // atom loop
    for (atom=1; atom<=number; atom++) {                     
      decay = drand48();
      if (decay < lambda) nloop--;                  // an atom decays
    }     
    number = nloop;
    fprintf(output, "%d\t%f\n", time, (double)number/max);
  }
  printf("data stored in decay.dat\n");
  fclose(output);
}