PROGRAM cool
! numerical solution of Newton's law of cooling
CALL initial(t,T_coffee,T_room,r,delta_t,tmax,nshow)
LET counter = 0                   ! initialize counter for clarity
DO while t <= tmax
   CALL Euler(t,T_coffee,T_room,r,delta_t)
   LET counter = counter + 1      ! number of iterations
   IF mod(counter,nshow) = 0 then
      CALL output(t,T_coffee,T_room)
   END IF
LOOP
END

SUB initial(t,T_init,T_room,r,delta_t,tmax,nshow)
    ! time is keyword in True BASIC so cannot be used as a variable
    LET t = 0                     ! time
    LET T_init = 82.3             ! initial coffee temperature (C)
    LET T_room = 17               ! room temperature (C)
    INPUT prompt "cooling constant r = ": r
    INPUT prompt "time step dt = ": delta_t
    INPUT prompt "duration of run = ": tmax      ! minutes
    INPUT prompt "time between output of data = ": tshow
    LET nshow = round(tshow/delta_t)
    CALL output(t,T_init,T_room)
END SUB

SUB Euler(t,T_coffee,T_room,r,delta_t)
    LET change = -r*(T_coffee - T_room)*delta_t
    LET T_coffee = T_coffee + change
    LET t = t + delta_t
END SUB

SUB output(t,T_coffee,T_room)
    IF t = 0 then
       PRINT
       PRINT "time (min)","T_coffee","T_coffee - T_room"
       PRINT
    END IF
    PRINT t,T_coffee,T_coffee - T_room      ! show results on screen
END SUB