Genetic Algorithm Tournament Selection

Question

I'm writing a genetic algorithm and I plan to move from roulette wheel selection to tournament selection, but I suspect my understanding may be flawed.

If I'm only selecting the n/2 best solutions in the population, surely I run out of population quite quickly?

My understanding of the algorithm is:

for(Member m in currentPopulation){
    Member randomMember1 = random member of currentPopulation which is then removed from currentPopulation
    Member randomMember2 = as above;
    //Mutate and crossover

    if(randomMember1.getScore() > randomMember2.getScore()){
        nextGeneration.add(randomMember1);
    } else {
        nextGeneration.add(randomMember2);
    }
}

Am I understanding this correctly?

Answer 1

In tournament selection the selected individuals are not removed from the population. You may select the same individuals to take part in multiple tournaments.

Having looked at your code a little closer, I see you do have another misunderstanding. You would not typically mutate/crossover all members of the tournament. Instead, you perform a tournament, with the winner of that tournament being select as an individual to undergo mutation/crossover. This means that for mutation your tournament size must be at least 2, and for crossover the size must be at least 3 with the best 2 winning (or you can perform 2 separate tournaments to choose each of the parents to crossover).

Some pseudo-code might help:

while (nextPopulation too small) {
    Members tournament = randomly choose x members from currentPopulation

    if(crossover){
        Member parents = select best two members from tournament
        Member children = crossover(parents)
        nextPopulation.add(children);
    } else {
        Member parent = select best one member from tournament
        Member child = mutate(parent)
        nextPopulation.add(child);
    }
}