! Simulation of radioactive decay
! Program to accompany "Computational Physics" by N. Giordano and H. Nakanishi and H. Nakanishi
! Copyright Prentice Hall 1997, 2006, 2006
program decay
   option nolet                 ! can omit the let keyword
   library "sgfunc.trc"
   library "sglib.trc"  ! the graphics routines are found here
   dim n_uranium(100), t(100)   ! declare the arrays we will need
   call initialize(n_uranium,t,tau,dt)   ! use subroutines to do the work
   call calculate(n_uranium,t,tau,dt)
   call display(n_uranium,t,tau,dt)
   get key z
end
! initialize variables
sub initialize(nuclei(),t(),time_constant,dt)
   input prompt "initial number of nuclei -> ": nuclei(1)
   t(1) = 0
   input prompt "time constant -> ": time_constant
   input prompt "time step -> ": dt
end sub
sub calculate(n_uranium(),t(),tau,dt)
   for i = 1 to size(t)-1             ! now use the Euler method
      n_uranium(i+1) = n_uranium(i) - (n_uranium(i) / tau) * dt
      t(i+1) = t(i) + dt
   next i
end sub
sub display(n_uranium(),t(),tau,dt)
              ! first set up title and label axes for graph
   call settitle("Radioactive Decay    Number of nuclei versus time")
   call sethlabel("Time(s)")
   call setvlabel("Number of Nuclei")
   call datagraph(t,n_uranium,4,0,"black")  ! the graph is produced here
   set cursor 5,30                          ! reposition cursor
   print "time constant = ";tau
   set cursor 6,30
   print "time step = ";dt
end sub