Roulette Selection in Genetic Algorithms

Question

Can anyone provide some pseudo code for a roulette selection function? How would I implement this:

I don't really understand how to read this math notation. I never took any probability or statistics.

Answer 1

The pseudocode posted contained some unclear elements, and it adds the complexity of generating offspring in stead of performing pure selection. Here is a simple python implementation of that pseudocode:

def roulette_select(population, fitnesses, num):
    """ Roulette selection, implemented according to:
        <http://stackoverflow.com/questions/177271/roulette
        -selection-in-genetic-algorithms/177278#177278>
    """
    total_fitness = float(sum(fitnesses))
    rel_fitness = [f/total_fitness for f in fitnesses]
    # Generate probability intervals for each individual
    probs = [sum(rel_fitness[:i+1]) for i in range(len(rel_fitness))]
    # Draw new population
    new_population = []
    for n in xrange(num):
        r = rand()
        for (i, individual) in enumerate(population):
            if r <= probs[i]:
                new_population.append(individual)
                break
    return new_population