将 2D ArrayList 转换为 2D Array

Question

I've looked at several answers already, and I am getting errors with the answers that I've found. I'm trying to convert an ArrayList of Doubles[] to a normal double 2D array. My code:

    public ArrayList<Double[]>  ec;
    public double[][]           ei;
    ...
    encogCorpus = new ArrayList<Double[]>();
    ...
    ec.add(inputs);
    ...
    ei = new double[ec.size()][];

    for (int i = 0; i < ec.size(); i++) {
        ArrayList<Double> row = ec.get(i);
        ei[i] = row.toArray(new double[row.size()]);
    }

I'm getting errors that say

Type mismatch: cannot convert from Double[] to ArrayList

And

The method toArray(T[]) in the type ArrayList is not applicable for the arguments (double[])

Answer 1

Problems

1. First of all, ec here is of type ArrayList<Double[]> , which means ec.get(i) should return Double[] and not ArrayList<Double> .
2. Second, double and Double are completely different types. You can't simply use row.toArray(new double[row.size()]) on your code.

---

Solutions

1.

If you want a true 2D ArrayList of Doubles then the type of ec should be ArrayList<ArrayList<Double>> . But because we can't use toArray() , we manually loop instead.

public ArrayList<ArrayList<Double>> ec;  // line changed here
public double[][]                   ei;
...
encogCorpus = new ArrayList<ArrayList<Double>>(); // and here also
...
ec.add(inputs); // `inputs` here should be of type `ArrayList<Double>`
...
ei = new double[ec.size()][];

for (int i = 0; i < ec.size(); i++) {
    ArrayList<Double> row = ec.get(i);

    // Perform equivalent `toArray` operation
    double[] copy = new double[row.size()];
    for (int j = 0; j < row.size(); j++) {
        // Manually loop and set individually
        copy[j] = row.get(j);
    }

    ei[i] = copy;
}

2.

But if you insist of using ArrayList<Double[]> , we only need to change the main part:

public ArrayList<Double[]>  ec;
public double[][]           ei;
...
encogCorpus = new ArrayList<Double[]>();
...
ec.add(inputs);
...
ei = new double[ec.size()][];

for (int i = 0; i < ec.size(); i++) {
    // Changes are only below here

    Double[] row = ec.get(i);
    double[] copy = new double[row.length];

    // Still, manually loop...
    for (int j = 0; j < row.length; j++) {
        copy[j] = row[j];
    }

    ei[i] = copy;
}

3.

Finally, if you could change Double[] to double[] , solution 2 would become,

public ArrayList<double[]>  ec; // Changed type
public double[][]           ei;
...
...
for (int i = 0; i < ec.size(); i++) {
    // Simpler changes here
    ei[i] = ec.get(i).clone();
}
...

Answer 2

Basically all you need to do is this:

for (int i = 0; i < ec.size(); i++) {
    Double[] boxedRow = ec.get(i);
    double[] unboxedRow = new double[boxedRow.length];
    for (int j = 0; j < boxedRow.length; j++)
        unboxedRow[j] = boxedRow[j];
    ei[i] = unboxedRow;
}

You're having trouble because of boxing / unboxing. Manually unboxing the Doubles to doubles allows us to convert the array to the right type.

An alternate solution would be this:

public ArrayList<Double[]>  ec;
public Double[][]           ei; // no need to unbox

// ...

for (int i = 0; i < ec.size(); i++) {
    ei[i] = ec.get(i);
}

I'll add that your current solution is not a 2D ArrayList ; it is an ArrayList of Double arrays. It looks like what you might be trying to do is something like this:

public ArrayList<ArrayList<Double>>  ec;
public double[][]                    ei;

// ...

for (int i = 0; i < ec.size(); i++) {
    ArrayList<Double> row = ec.get(i);
    Double[] rowArray = row.toArray(new Double[row.size()]);
    double[] unboxedRow = new double[rowArray.length];
    for (int j = 0; j < rowArray.length; j++)
        unboxedRow[j] = rowArray[j];
    ei[i] = unboxedRow;
}

Which, again, could potentially be this:

public ArrayList<ArrayList<Double>>  ec;
public Double[][]                    ei;

// ...

for (int i = 0; i < ec.size(); i++) {
    ArrayList<Double> row = ec.get(i);
    ei[i] = row.toArray(new Double[row.size()]);
}

Finally, please note that when you instantiate a new Double[] , the arrays initialize to null and not to 0 . If you try the following, you'll get a NullPointerException , though it will compile.

Double[] boxedArray = new Double[1];
double unboxed = boxedArray[0]; // equiv to "double unboxed = null;"

You need to be careful when unboxing, and make sure that you are handling nulls correctly.

Answer 3

Error is in the following lines:

for (int i = 0; i < ec.size(); i++) {
    ArrayList<Double> row = ec.get(i);
    ei[i] = row.toArray(new double[row.size()]);
}

The first error will be resolved by replacing ArrayList<Double> with Double[] :

for (int i = 0; i < ec.size(); i++) {
    Double[] row = ec.get(i);
    ei[i] = row.toArray(new double[row.size()]);
}

Answer 4

List<double[]> list = Arrays.asList(new double[]{1.1, 2.0}, new double[]{3.0, 5.8});
double[][] res = list.toArray(new double[list.size()][]);
// System.out.println(Arrays.deepToString(res));

Same approach to convert ArrayList to 2d int array.