Construct ArrayList of smaller ArrayLists

Question

I have a matrix that has 223 rows and 11 columns.I want to split this matrix into packets(each packet will have 10 rows from the matrix) so at the end I should have 22 packets (I will dismiss the last 3 rows). After I have the packets I want to perform a cross validation algorithm on my packets and I was thinking to put all my packets in a HaskMap that will have as key one packet and value the rest of them. So it would look like this

key = pack1, value = pack2,...,pack22
key = pack2, values = pack1, pack3,...pack22
.............................................
key = pack22, values = pack1,...,pack21

Before creating the map I have problems in spliting the matrix into packets Here is my code:

        int k = 0;
        ArrayList<ArrayList<double[]>> allPacks = new ArrayList<ArrayList<double[]>>();
        ArrayList<double[]> batch = new ArrayList<double[]>();
        for (int i=0;i<matrix.length;i+=10) {
            System.out.println(i);
            batch.add(matrix[i]);
            k++;
            if (k % 10 == 0) {
                allPacks.add(batch);
                batch.clear();
            }
        }

If I print the size of allPacks is 2. I don't get it.Any help?

Could you add all relevant code (minimal working example) -- what is matrix, for example? — Mick Mnemonic
– Mick Mnemonic, Commented May 1, 2015 at 12:20
I said matrix has 223 rows and 11 columns. I hardcoded it so it would be aburde to paste in here 223 line of code. Basically it has double values — GeR
– GeR, Commented May 1, 2015 at 12:23

OldCurmudgeon · Accepted Answer · 2015-05-01 12:38:39Z

2

Your main problem is that you are adding the same batch every time.

This should fix it:

    ArrayList<ArrayList<double[]>> allPacks = new ArrayList<ArrayList<double[]>>();
    for (int i = 0; i < matrix.length - 10; i += 10) {
        System.out.println(i);
        ArrayList<double[]> batch = new ArrayList<double[]>();
        for (int j = 0; j < 10; j++) {
            batch.add(matrix[i + j]);
        }
        allPacks.add(batch);
    }

edited May 1, 2015 at 12:38

answered May 1, 2015 at 12:31

OldCurmudgeon

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6 Comments

GeR Over a year ago

I get ArrayIndexOutOfBoundException

OldCurmudgeon Over a year ago

@Georgiana - Sorry - edited - now using i < matrix.length - 10. You could also use for (int j = 0; j < 10 && i + j < matrix.length; j++) { but that would make your last batch small.

GeR Over a year ago

Thanks, now it works! Any idea on how I could create that HashMap about I was talking at the begining?

GeR Over a year ago

@Flash is that a solution on creating the HashMap?

OldCurmudgeon Over a year ago

@Georgiana - You could make allPacks a Map and use allPacks.put(batch.get(0),batch) or something like that.

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