! magnetic field from a straight wire
! Program to accompany "Computational Physics" by N. Giordano
! Copyright Prentice Hall 1997
program straight_wire
   option nolet
   library "sglib*","sgfunc*"
   dim r(0),field(0)
   lstart = .5
   lfinish = 10
   call initialize(r,field,max)
   length = lstart
   i = 1
   do 
      call calculate(r,field,max,length)
      if length = lstart then call display(r,field,max)
      if length > lstart then call adddatagraph(r,field,4,0,"black")
      i = i + 1
      length = 10 * length
   loop until length > lfinish
   call exact(r,field)
end
! initialize variables
sub initialize(r(),field(),max)
   input prompt "number of grid steps -> ": max
   mat redim field(max),r(max)
end sub
sub calculate(r(),field(),max,length)
   for i = 1 to max
      call calculate_field(field(i),i/max,max,length)
      r(i) = i/max
   next i
end sub
sub calculate_field(by,x,max,length)
   dz = 0.1
   by = 0
   for z = -length to length step dz
      r = sqr(x^2 + z^2)
      by = by + dz * x / r^3
   next z
end sub
sub display(r(),field(),max)
   call settitle("Magnetic field from a straight wire")
   call sethlabel("R")
   call setvlabel("B")
   call datagraph(r,field,3,0,"black")
end sub
sub exact(r(),field())
   max = 100
   mat redim r(max),field(max)
   for i = 1 to max
      r(i) = i / max
      field(i) = 2 / r(i)
   next i
   call adddatagraph(r,field,1,1,"black")
end sub