Correct code from kiwitrader

M2-P6-Correct

def rabbitGrowth():
    """
   rabbitGrowth is called once at the beginning of each time step.
 
   It makes use of the global variables: CURRENTRABBITPOP and MAXRABBITPOP.
 
   The global variable CURRENTRABBITPOP is modified by this procedure.
 
   For each rabbit, based on the probabilities in the problem set write-up,
     a new rabbit may be born.
   Nothing is returned.
   """
    global MAXRABBITPOP
    global CURRENTRABBITPOP
 
    numRabbits = CURRENTRABBITPOP
    # print 'initial', numRabbits
    for r in range(CURRENTRABBITPOP):
        pRabbitReproduction = 1 - (numRabbits / float(MAXRABBITPOP))
        # if r < 10: print pRabbitReproduction,
        if pRabbitReproduction <= 0 or numRabbits == MAXRABBITPOP:
            CURRENTRABBITPOP = MAXRABBITPOP  # they may require numRabbits
            break
        if random.random() <= pRabbitReproduction:
            numRabbits += 1
    CURRENTRABBITPOP = numRabbits
    return
 
def foxGrowth():
    """
   foxGrowth is called once at the end of each time step.
 
   It makes use of the global variables: CURRENTFOXPOP and CURRENTRABBITPOP,
       and both may be modified by this procedure.
 
   Each fox, based on the probabilities in the problem statement, may eat
     one rabbit (but only if there are more than 10 rabbits).
 
   If it eats a rabbit, then with a 1/3 prob it gives birth to a new fox.
 
   If it does not eat a rabbit, then with a 1/10 prob it dies.
 
   Nothing is returned.
   """
    global MAXRABBITPOP
    global CURRENTRABBITPOP
    global CURRENTFOXPOP
    numRabbits = CURRENTRABBITPOP
    numFoxes = CURRENTFOXPOP
    pFoxReproduction = 1 / 3.0
    pFoxDeath = 0.10
    for r in range(CURRENTFOXPOP):
        pFoxEats = numRabbits / float(MAXRABBITPOP)
        if random.random() <= pFoxEats:
            if numRabbits > 10:
                numRabbits -= 1
            if random.random() <= pFoxReproduction:
                numFoxes += 1
        else:
            if random.random() <= pFoxDeath:
                numFoxes -= 1
    CURRENTRABBITPOP = numRabbits
    CURRENTFOXPOP = numFoxes
    return
 
def runSimulation(numSteps=200):
    """
   Runs the simulation for `numSteps` time steps.
 
   Returns a tuple of two lists: (rabbit_populations, fox_populations)
     where rabbit_populations is a record of the rabbit population at the
     END of each time step, and fox_populations is a record of the fox population
     at the END of each time step.
 
   Both lists should be `numSteps` items long.
   """
    r = []
    f = []
    for n in range(numSteps):
        # if (n)%5 == 0: print
        rabbitGrowth()
        # global CURRENTRABBITPOP
        # CURRENTRABBITPOP = 1000
        # print CURRENTRABBITPOP,
        foxGrowth()
        # print n, MAXRABBITPOP, CURRENTRABBITPOP, CURRENTFOXPOP, '    ',
        # print; print
        r.append(CURRENTRABBITPOP)
        f.append(CURRENTFOXPOP)
    return (r, f)