How to apply recursion to an algorithm?

Question

I am trying to write a predator-prey model where I take the input for the amount of rabbits, foxes, and years. Then output the final number of rabbits and foxes after that amount of years. I am able to return an amount of rabbits and foxes, but I am not getting the correct values. The algorithm for the foxes and rabbits after one year are:

F1yr = F0yr – Floor(F0yr * (G-S * R0yr))

R1yr = R0yr + Floor( R0yr * (A-B * F0yr))

Where A = 0.04, B = 0.0005, G = 0.2, and S=0.00005

For a starting input of 5891 rabbits and 16 foxes after 99 years, it should return 6484 rabbits and 144 foxes, but I am getting 4682 rabbits and 189 foxes.

This is the code that I have so far, I feel like I am close to the answer, but not fully there:

def bunnies(rabbits,foxes,years):
    if __name__ == '__main__':
        if years == 0:
            tot = []
            tot.append(rabbits)
            tot.append(foxes)
            return tot
        else:
            a = 0.04
            b = 0.0005
            g = 0.2
            s = 0.00005
            rabbits = rabbits + math.floor(rabbits * (a-b * foxes))
            foxes = foxes - math.floor(foxes * (g-s * rabbits))
            return bunnies(rabbits,foxes,years-1)

rabbits = int(input('Enter Initial Rabbit Population:\n'))
foxes = int(input('Enter Initial Fox Population:\n'))
years = int(input('Enter Number of Years to Simulate:\n'))

print(bunnies(rabbits,foxes,years))

ruohola · Accepted Answer · 2019-04-12 13:02:03Z

Your code was almost correct, here is a fixed and cleaned up version:

import math


def bunnies(rabbits, foxes, years):
    A = 0.04
    B = 0.0005
    G = 0.2
    S = 0.00005
    if years == 0:
        return rabbits, foxes
    else:
        rabbits_last, foxes_last = rabbits, foxes

        foxes = foxes_last - math.floor(foxes_last * (G - S * rabbits_last))
        rabbits = rabbits_last + math.floor(rabbits_last * (A - B * foxes_last))

        return bunnies(rabbits, foxes, years - 1)


if __name__ == '__main__':
    rabbits = int(input('Enter Initial Rabbit Population: '))
    foxes = int(input('Enter Initial Fox Population: '))
    years = int(input('Enter Number of Years to Simulate: '))
    print(bunnies(rabbits, foxes, years))

The problem ocurred when you used the already changed value of the rabbit population for the new fox population count.

You also used wrong variable names when calling print(bunnies(rab,fox,yrs)), but I think that was just a copying mistake since you didn't get error messages.

Lastly your if __name__ == '__main__' shouldn't have been inside the function but on the module scope like I have it.

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