Beginner: Assigning values to arrays in Java

Question

I am attempting to write a simple Genetic Algorithm in Java after reading a book on Machine Learning and have stumbled on the basics. I'm out of practice with Java so I'm probably missing something extremely simple.

Individual

public class Individual {

    int n;
    int[] genes = new int[500];
    int fitnessValue;

    public int getFitnessValue() {
        return fitnessValue;
    }

    public void setFitnessValue(int fitnessValue) {
        this.fitnessValue = fitnessValue;
    }

    public int[] getGenes() {
        return genes;
    }

    public void setGenes(int index, int gene) {
        this.genes[index] = gene;
    }

    public int getN() {
        return n;
    }

    public void setN(int n) {
        this.n = n;
    }

    // Constructor
    public Individual() {


    }

}

Population

import java.util.Random;

public class Population {

    public Population() {

    }

    public static void main(String[] args) {
        Random rand = new Random();
        int p = rand.nextInt(10);
        int n = rand.nextInt(10);

        Individual pop[] = new Individual[p];

        System.out.println("P is: " + p + "\nN is: " + n);

        for(int j = 0; j <= p; j++) {
            for(int i = 0; i <= n; i++) {
                pop[j].genes[i] = rand.nextInt(2);
            }
        }
    }

    public void addPopulation() {

    }
}

The aim of this code is to populate the Population and the Genes with a random number. Could someone please take a look at my code to see where I'm going wrong?

Huh. I did just read "beginner" and "genetic algorithm" in the same question. — Jed Smith
– Jed Smith, Commented Oct 14, 2009 at 15:10
What you're currently doing is to populate a random portion of genes of a random amount of Individuals. What result did you get and what did you expect? — msp
– msp, Commented Oct 14, 2009 at 15:14
Sorry for the throwaway count and inability now to give a correct answer. I wanted to create an initial population so I could run a routine-wheel fitness method on them. Lou Franco has answered the question perfectly, so thank you for your help! — Mike B
– Mike B, Commented Oct 14, 2009 at 15:16
I think: j<=p in your for loop is off by one. Should be: j<p. Also nextInt can return 0 which could mess up your array (i.e. if p was 0) — barrowc
– barrowc, Commented Oct 15, 2009 at 2:28

Lou Franco · Accepted Answer · 2009-10-14 15:13:08Z

4

before

pop[j].genes[i] = rand.nextInt(2);

add

pop[j] = new Individual();

the elements of the array are null.

answered Oct 14, 2009 at 15:13

Lou Franco

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Comments

stian · Accepted Answer · 2009-10-14 15:13:19Z

0

I believe you need to initialize pop[j] before doing pop[j].genes[i] = rand.nextInt();

    Individual pop[] = new Individual[p];

This just initializes the array, not the individual elements. Try to put pop[j] = new Individual() between your two loops.

answered Oct 14, 2009 at 15:13

stian

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1 Comment

stian Over a year ago

And you should really access the object properties through accessor methods like getGenes() instead of direct access (.genes).

jrsconfitto · Accepted Answer · 2009-10-14 15:15:08Z

0

What they said...

Also, do you mean to call your setGenes method, or do you just want to directly access the gene array.

answered Oct 14, 2009 at 15:15

jrsconfitto

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Comments

bruno conde · Accepted Answer · 2009-10-14 15:17:15Z

0

From what I understand of your code I think you need to do this:

for(int j = 0; j <= p; j++) {
    pop[j] = new Individual();
    for(int i = 0; i <= n; i++) {
        pop[j].setGenes(i, rand.nextInt(2));
    }
}

answered Oct 14, 2009 at 15:17

bruno conde

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