Java - Printing Array List of String Arrays

Question

I am pretty new to Java and had a question. This is homework, so I would not like any outright answers. Thanks!

I'm working on a genetic algorithm for playing poker. I wanted to create an arrayList of string arrays. The string arrays would hold moves for each possible hand during a round. What I want to do right now to make sure it is working is run my method and print out the results. Here are my practice classes (this is only a small part of the assignment):::

import java.util.ArrayList;
import java.util.Arrays;


public class Play 
{

    GeneticAlg start;
    ArrayList<Object> pop;

    public static void main(String[] args) 
    {
        GeneticAlg start = new GeneticAlg();
        ArrayList<String[]> pop = start.initializePopulation();

        for(String[] arr: pop)
        {
            System.out.println(Arrays.toString(arr));
        }

    }

}

import java.util.ArrayList;
import java.util.Random;


public class GeneticAlg 
{
    ArrayList<String[]> population;
    int[] populationScores;
    String[] chromosome;
    int generation;
    int index;

    public GeneticAlg()
    {

    }

    public ArrayList<String[]> initializePopulation()
    {
        ArrayList<String[]> population = new ArrayList<String[]>();

        for (int i = 0; i < 20; i++)
        {
            Random generator = new Random();
            String[] choices = {"bet","raise","call","check"};
            chromosome = new String[33];

            for (int j = 0; j < 33; j++)
            {
                if (j < 6) //first and second round possible hands)
                {
                    index = generator.nextInt((choices.length)-1); 
                    chromosome[j] += choices[index];
                }

                else //third, fourth, and fifth round possible hands)
                {   
                    index = generator.nextInt(choices.length);
                    chromosome[j] += choices[index];
                }
            }

            population.add(chromosome); 
        }

        return population;
    }

}

Right now, it's printing out the array, but each entry looks like this:

[nullcall, nullraise, nullbet, nullraise, nullcall, nullraise,....

I want it to just return the move without null on the front. Any advice is appreciated. Thanks!

ETA: I fixed the two lines with the concatenation error, but it is still printing "null" in front of each command. Any advice?

Code after repairing the error:

import java.util.ArrayList;
import java.util.Arrays;


public class Play 
{

    GeneticAlg start;
    ArrayList<Object> pop;

    public static void main(String[] args) 
    {
        GeneticAlg start = new GeneticAlg();
        ArrayList<String[]> pop = start.initializePopulation();

        for(String[] arr: pop)
        {
            System.out.println(Arrays.toString(arr));
        }

    }

}

import java.util.ArrayList;
import java.util.Random;


public class GeneticAlg 
{
    ArrayList<Object> population;
    String[] choices;
    int[] populationScores;
    String[] chromosome;
    int generation;
    int index;

    public GeneticAlg()
    {

    }

    public ArrayList<Object> initializePopulation()
    {
        population = new ArrayList<Object>();

        for (int i = 0; i < 20; i++)
        {
            Random generator = new Random();

            for (int j = 0; j < 24; j++)
            {
                if (j < 6) //first and second round possible hands)
                {
                    choices[0]= "bet";
                    choices[1]= "raise";
                    choices[3]= "call";

                    index = generator.nextInt(choices.length); 
                    chromosome[j] = choices[index];
                }

                else //third, fourth, and fifth round possible hands)
                {
                    choices[4] = "check";

                    index = generator.nextInt(choices.length);
                    chromosome[j] = choices[index];
                }
            }

            population.add(chromosome); 
        }

        return population;
    }

}

Pshemo · Accepted Answer · 2014-03-23 20:22:50Z

4

Arrays of objects (like Stirngs) are filled at start with null values, so doing

chromosome[j] += choices[index];

is the same as

chromosome[j]  = chromosome[j] + choices[index];

which is the same as

chromosome[j]  = null + choices[index];

So you are concatenating null with choices[index]; which gives you nullbet for instance.

To solve it just use = instead of +=.

answered Mar 23, 2014 at 20:22

Pshemo

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4 Comments

AbigailB Over a year ago

Thanks! I fixed the two lines that contained this error, but it is still giving me "nullbet" instead of "bet." Any ideas?

Pshemo Over a year ago

That is unusual. I don't see any nulls after fixing two lines with this problem. Did you add anything new to your code?

Pshemo Over a year ago

Code you posted doesn't compile because of <Object> and <String[]> incompatibility. When I corrected it it seem it throws NPE when you try to assign bet to choices[0] = "bet"; because you didn't initialized array yet.

Pshemo Over a year ago

Post code which we could use to actually reproduce your problem, because now I am just guessing that you didn't initialize rest of your choices arrays and they are still nulls there (your code sets up only indexes [0] [1] [3] and [4], while [5] [6] may be not changed so they will contain null values.

David Ehrmann · Accepted Answer · 2014-03-23 20:22:59Z

2

Since you don't want an outright answer, look at the code that populates your String[]'s. Your printing's doing the right thing, but the Strings in the array actually are "nullbet," "nullraise," etc.

answered Mar 23, 2014 at 20:22

David Ehrmann

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1 Comment

AbigailB Over a year ago

Thank you! This helped me make more sense of it.

Anthony · Accepted Answer · 2014-03-23 20:31:21Z

1

The error comes from this line here:

chromosome[j] += choices[index];

The += operator, when used with a Strings, will concatenate the right-hand string to the end of the left-hand string. In this case, it tacks on choices[index] to the existing contents of chromosome[j], which will null by default if chromosome is declared as an array of Objects.

You probably meant

chromosome[j] = choices[index];

And accidentally inserted the + because you use population.add() with your list below.

answered Mar 23, 2014 at 20:31

Anthony

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2 Comments

Anthony Over a year ago

Dang it, Pshemo, that was a quick response. I guess I need to type faster. ;)

AbigailB Over a year ago

Thanks! I fixed the two lines that contained this error, but it is still giving me "nullbet" instead of "bet." Any ideas?

MPeti · Accepted Answer · 2014-03-23 22:45:54Z

1

Your "fixed" code looks like it doesn't compile - you're probably still running the previous version. You don't initialize choices ("choices = new String[4]") and you're confusing its indices (0, 1, 3 and 4). Also, if you're trying to add a fourth element to the array later, don't, you can't do that with arrays. And you're assigning an ArrayList to an ArrayList without a cast. You only needed to swap += with = in your original code, it seemed fine otherwise.

answered Mar 23, 2014 at 22:45

MPeti

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1 Comment

AbigailB Over a year ago

Thanks. I was running the previous version on accident. It's fixed now!

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