how to add a matrix to an ArrayList in java

Question

I need to add a number of matrces to an arraylist or some sort of collection in order to recall them at a later stage

I have tried the arraylist and arraycopy

List<Double> al = new ArrayList<>();
double [][] k = new double [d.length][d[0].length];

System.arraycopy (d,0,k,0,d.length);

for (int i1 =0; i1 < d.length; i1++)
    k[i1] = k.add(D[i1]);
al.add(k[i1]);
for (Integer x : k)
    System.out.print(x + " ");
print2D(k);

I need an array of say 4x4 matrices If I do say al.add(d); I get error: cannot find suitable method to add double even when al.add(Matrix)

Zabuzard · Accepted Answer · 2019-07-24 21:44:27Z

1

You forgot the array as type for your generics. What you want is List<double[][]> and not List<Double>. Then you will be able to add your matrices to the list.

The addition is simple, just

list.add(matrix);
// or in your case:
al.add(k);

no need to copy anything around or access the individual entries in the matrices.

You can retrieve matrices via get for example:

double[][] firstMatrix = list.get(0);

edited Jul 24, 2019 at 21:44

answered Jul 24, 2019 at 21:39

Zabuzard

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2 Comments

Phillip Oosthuizen Over a year ago

Thanks Butiri & Zabuza I have corrected the List<Double [][]> but still get this error: no suitable method found for add(double[][]) al.add(k); and also this error: incompatible types: Double[][] cannot be converted to double[][], please explain double[][] firstMatrix = al.get(0);

Zabuzard Over a year ago

As I said, it is double[][] and not Double[][].

Ramiro Alarcón · Accepted Answer · 2019-07-24 22:05:50Z

0

The list must be a list of matrices, and you are declaring a list of Doubles. Try something like this:

List<double[][]> list = new ArrayList<>();
    double[][] matrix = {{1D,1D},{2D,2D}};
    list.add(matrix);

Hope that helps!

answered Jul 24, 2019 at 22:05

Ramiro Alarcón

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