Collapsing a List<double[][]> to a double[][]

Question

I have a variable of type List<double[][]>

An example of what would be in the list is as follows (with each new line being a list element):

{ {1.0,0.0,0.0}, {0.0,0.0,0.0} }
{ {0.0,0.0,0.0}, {1.0,0.0,0.0}, {0.0,1.0,0.0} }

The outcome I need to achieve is a double[][] with all the elements from the list as such (using the same values as above):

{ {1.0,0.0,0.0}, {0.0,0.0,0.0}, {0.0,0.0,0.0}, {1.0,0.0,0.0}, {0.0,1.0,0.0} }

The size of the inner most array will always be the same. However, the outter array varies in size.

I'd really appreciate some help on this one, I seem to think the solution is easy but I just can't come up with it!

Answer 1

This is what the Streaming API's flatMap is for:

List<double[][]> list = Arrays.asList(
        new double[][]{{1.0, 0.0, 0.0}, {0.0, 0.0, 0.0}},
        new double[][]{{0.0, 0.0, 0.0}, {1.0, 0.0, 0.0}, {0.0, 1.0, 0.0}});

double[][] result = list.stream()
        .flatMap(Arrays::stream)
        .toArray(double[][]::new);

Produces { {1.0,0.0,0.0}, {0.0,0.0,0.0}, {0.0,0.0,0.0}, {1.0,0.0,0.0}, {0.0,1.0,0.0} }

Answer 2

List<double[][]> list;

//calculate the length of the array (sum of the length of all double[][]s in the list)
int resultLen = 0;
for(double[][] d : list)
    resultLen += d.length;

double[][] result = new double[resultLen][];

//copy all double[]s that are in the list (wrapped into double[][]s)
//into the new double[][]
int index = 0;
for(double[][] d : list)
    for(double[] a : d)
        result[index++] = a;

return result;