CODEWITHSUNDEEP
cgenetic-algorithmroulette-wheel-selection
user3345867
user3345867

Reputation: 25

Implementation of Roulette wheel Selection

I'm coding a genetic algorithm from scratch in C, I'm stuck in the roulette wheel implementation, here how I code it:

void selection(Chromosome * chromosome){

double totalFitness=0;
double totalProbability=0;
double probability=0;
double rndNumber;
double min,max;
int i;

min=0.0;
max=1.0;

for(i=0;i<POPULATION_SIZE;i++){
 totalFitness += chromosome[i].fitness;
  }

  for(i=0;i<POPULATION_SIZE;i++){
   chromosome[i].probability = (chromosome[i].fitness)/totalFitness;
   printf("Chromosome %d with probability %f\n",i, chromosome[i].probability);
  }
 srand((unsigned)time(NULL));
 for(i=0;i<POPULATION_SIZE;i++){
  rndNumber = ((double)rand()/(double)RAND_MAX);
  if(chromosome[i].probability >= rndNumber){
  printf("Chromosome %d selected \n",i);




}}}

The output return one or no chromosome selected, which isn't the expected result. I'm wondring if this is the wright way to dot it??

Thanks in advance.

Upvotes: 1

Views: 7348

Answers (1)

M Oehm
M Oehm

Reputation: 29116

When you pick a chromosome, you should determine the random number only once and then accumulate the offset, so that you consider all ranges:

                       | random number 
                       |
                       X
    +--------------+-----+----+--+--++---+-----+
    |              |     |    |  |  ||   |     |
    +--------------+-----+----+--+--++---+-----+
   0.0            p0   p0+p1                  1.0 == sum(p[i])

In code:

double rndNumber = rand() / (double) RAND_MAX;
double offset = 0.0;
int pick = 0;

for (i = 0; i < POPULATION_SIZE; i++) {
    offset += chromosome[i].probability;
    if (rndNumber < offset) {
        pick = i;
        break;
    }
}

printf("Chromosome %d selected.\n", pick);

Upvotes: 6

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