mattia wrote:
> Hi everyone, I'm new to python and I want to create some simple code in
> order to code the classical genetic algorithm example: given a population
> of chromosomes, encoded using 1 and 0, find the chromosome with the
> maximum number of 1s. Now, despite all the code used to implement the
> solution, I'm wondering if there is a better way to use the so-called
> roulette wheel selection in this problem. Here I paste the code of my
> solution, any advice will be helpful:
Your code looks good to me.
> from random import randint, random
>
> def create_chromosome(min, max, length):
> chromosome = []
> for i in range(length):
> chromosome.append(randint(min, max))
> return chromosome
>
> def fitness(chrm, ffunc=sum):
> return ffunc(chrm)
fitness = sum
has the same effect, without the extra indirection.
> def create_population(nelem, min, max, length):
> return [create_chromosome(min, max, length) for i in range(nelem)]
>
> def get_fitness_and_population(population):
> return [(fitness(x), x) for x in population]
>
> def get_roulette_wheel(population):
> roulette_wheel = []
> index = 0
>
> for x in get_fitness_and_population(population):
> for j in range(x[0]):
> roulette_wheel.append(index)
> index += 1
Make that
for index, x in enumerate(get_fitness_and_population(population)):
...
I'd also pass the the fitness function explicitly around instead of making
it a global.
> return roulette_wheel
>
> pop = create_population(5, 0, 1, 10)
> rw = get_roulette_wheel(pop)
> print(rw)
> print(len(rw))
> ri = randint(0, len(rw) - 1)
> print("Random index:", rw[ri], ", value:", pop[rw[ri]])
But these are minor nits :)
Here's a slightly different approach:
from random import randint, choice
def create_chromosome(min, max, length):
return [randint(min, max) for i in range(length)]
def create_population(nelem, min, max, length):
return [create_chromosome(min, max, length) for i in range(nelem)]
def get_fitness_and_population(population, fitness):
return [(fitness(x), x) for x in population]
def get_roulette_wheel(weight_value_pairs):
roulette_wheel = []
for weight, value in weight_value_pairs:
roulette_wheel += [value]*weight
return roulette_wheel
if __name__ == "__main__":
pop = create_population(5, 0, 1, 10)
fap = get_fitness_and_population(pop, sum)
rw = get_roulette_wheel(fap)
print("Random value:", choice(rw))
Note how get_roulette_wheel() is now completeley independent of the concrete
problem you are using it for.
Peter
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