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Indexing ArrayLists of Vectors (AL[i] = new vector();)

Posted on 2007-04-01
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Last Modified: 2010-03-31
Is it possible to index ArrayLists?  I want to have a number of ArrayLists each consisting of a vector....

ArrayList al = new ArrayList();
for (int k=0; k<Bins;k++)
{
   al[k] = new vector();
   for (int l=k*Binsize;l<(k*Binsize)+Binsize;l++)
   {
      al[k].add(Entry[l]);
   }
}

To attempt to clarify the above, Entry[] is an array of integers.  I want to split that array up into 'Bins', each of which consists of a vector of entries from the array.  So, if Entry[6] = {1 2 3 4 5 6), and I want 3 Bins, I want to create 3 ArrayLists consisting of vectors so that al[0] is a vector {1 2}, al[1] is a vector {3 4} and al[2] is a vector {5 6}.  I want to use vectors because I'll be doing a lot of mathematical comparison and merging and vectors/ArrayLists seem like the friendliest animals to manipulate in this way.  

If I haven't confused you to the point of causing you to scream at your monitor and throw things at your pets, I'd greatly appreciate any help.  Thanks.

Jason
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Question by:cdromes
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5 Comments
 
LVL 10

Expert Comment

by:ADSLMark
ID: 18833045
Yes, that's possible. Here is an example with Integers. It uses generics pretty intensive, that's because it's easier to write. I can write the same without generics...

import java.util.*;

public class Example
{
    public static void main(String[] args)
    {
        int[] values = new int[]{1,2,3,4,5,6};

        //Number of bins and bin size
        int binCount = 3;
        int binSize = 2;

        //Bin holder
        ArrayList<Vector<Integer>> all = new ArrayList<Vector<Integer>>();

        //Create bins
        for(int i=0;i<binCount;i++)
        {
            //Create vector
            Vector<Integer> vec = new Vector<Integer>();

            //Fill vector
            for(int k=0;k<binSize;k++)
                vec.add(values[i*binSize+k]);

            //Add new vector
            all.add(vec);
        }

        //Print result
        for(Vector<Integer> vec : all)
        {
            System.out.print("{ ");
            for(Integer val : vec)
                System.out.print(val+" ");
            System.out.println("}");
        }
    }
}

Mark
0
 

Author Comment

by:cdromes
ID: 18833128
Thanks for the quick reply.

In your example, how would I go about accessing the 3rd entry in the 2nd vector in the ArrayList all?  I need to do element-by-element comparisons of neighboring vectors...would something like the following be kosher:

 for (k = 0; k < binCount; k++)
{
      myArrayList.add(new Vector());
      myArrayList.get(k);
      for (l =k*Binsize;l<(k*Binsize)+Binsize;l++)
            myArrayList.get(k).add(work[l]);    
}
Then once you've filled the vectors in the ArrayList, can you compound the .get, to specify vector elements, i.e. myArrayList.get(k).get(vectorElement); ?  
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LVL 10

Accepted Solution

by:
ADSLMark earned 175 total points
ID: 18833142
Not sure if the code you posted works, but if it does the job, then it's kosher. :-)

>> Can you compound the .get, to specify vector elements, i.e. myArrayList.get(k).get(vectorElement); ?  

Yes.

Mark
0
 

Author Comment

by:cdromes
ID: 18834054
Spec-friekin'-tacular.  I threw the code in a tested it a couple little files and it worked like a champ.  Thanks, Mark.

Finally, how do I test for the existence of a vector in the ArrayList prior to doing comparison actions?  I'm thinking of something along the lines of

for (int g=0;g<AL.size();g++)
{
      //If this IF is false, then you're at the end of the List
      //You'll have already computed the Chi2 value for this vector, i, with its neighbor, i-1
      if (AL.get(g+1) != null)
      {
      ....
      }
}
0
 
LVL 10

Expert Comment

by:ADSLMark
ID: 18834653
This will only loop over the vectors inside the AL list.

for (int g=0;g<AL.size();g++)
{
AL.get(g);
}

You can also use an Iterator here:
Iterator iAL = AL.iterator();
while(iAL.hasNext())
{
Vector vec = (Vector) AL.next();
}

Mark
0

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