Python 3.4 Genetic Algorithm, Dividing dropping the remainder

Question

I am writing a simple genetic algorithm in python, and when I try to find the average fitness of a set of haploids, it gives it to me with the remainder dropped, I can't figure out why it is doing this. Below is the full source

import random

def generate_random_haploid(haploid_length):
    haploid = []
    for x in range(haploid_length):
        haploid.append(random.randint(0,1))

    return haploid

def crossover_haploid(haploid_1, haploid_2):
    locus = random.randint(1, len(haploid_1))

    for x in range(locus - 1):
        haploid_1[x] = haploid_2[x]

    for x in range(len(haploid_2) - locus):
        haploid_2[x + locus] = haploid_1[x + locus]

    return [haploid_1, haploid_2]

def crossover_diploid(diploid_1, diploid_2):
    children_1, children_2 = crossover_haploid(diploid_1[0], diploid_1[1]), crossover_haploid(diploid_2[0], diploid_2[1])
    return crossover_haploid(children_1[0], children_2[1])

def flipbit(bit):
    if bit == 1:
        bit = 0
    elif bit == 0:
        bit = 1
    return bit

def mutate_haploid(haploid, mutate_prob):
    for x in haploid:
        if random.randint(0, mutate_prob) <= mutate_prob:
            haploid[x] = flipbit(haploid[x])

    return haploid

def average_fitness(haploid_list):
    return sum(haploid_list[0]) / len(haploid_list)

def fitness(haploid):
    fitness = 0
    for x in range(len(haploid)):
        if haploid[x] == 1:
            fitness += 1
    return fitness

def print_haploid(haploid):
    print(haploid, "Fitness: ", fitness(haploid))

x = generate_random_haploid(4)
y = generate_random_haploid(4)
print_haploid(x)
print_haploid(y)
print("-------------------------------")
children = crossover_haploid(x, y)
print_haploid(children[0])
print_haploid(children[1])
print("-------------------------------")
print("Parent Fitness: ", average_fitness([x, y]) )
print("-------------------------------")
print("Children Fitness: ", average_fitness([children[0], children[1]]) )

Answer 1

This is because python integer is meant to truncate down to the lowest value. You have a bunch of options to get around this:

>>> 5 / 2
2

Option 1

Cast one of your values as float and python will automatically up-cast all other int types

>>> 5 / 2
2

>>> float(5) / 2
2.5

Correction to your code:

def average_fitness(haploid_list):
    return float(sum(haploid_list[0])) / len(haploid_list)

Option 2

Add this to the top of your script

from __future__ import division

Now, 5 / 2 will yield 2.5 and you don't need to change your average_fitness method as shown in option 1. The __future__ refers to python3 in which the / operator by default performs float divisions. By import that feature, you will now use the float division operator / everywhere in your code instead of python2's int division operator

Option 3

You can replace the / with the // operator

>>> 5 / 2
2

>>> 5 // 2
2.5

Correction to your code:

def average_fitness(haploid_list):
    return float(sum(haploid_list[0])) // len(haploid_list)