How to convert a nested list to a 2d array

Question

I need to convert a nested list of Double to a double[][]. I have tried using the code below, but the issue is how to convert to the primitive double. Any help would be much appreciated.

double[][] matrix = new double[listReturns.size()][];
    int i = 0;
    for (List<Double> nestedList : listReturns) {
        matrix[i++] = nestedList.toArray(new Double[nestedList.size()]);
    }

Eran · Accepted Answer · 2019-06-20 10:37:13Z

1

You can use streams:

double[][] mat =
    listReturns.stream() // Stream<List<Double>>
               .map(list -> list.stream() 
                                .mapToDouble(Double::doubleValue)
                                .toArray()) // map each inner List<Double> to a double[]
               .toArray(double[][]::new); // convert Stream<double[]> to a double[][]

edited Jun 20, 2019 at 10:37

answered Jun 20, 2019 at 10:31

Eran

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2 Comments

Paul Nikonowicz Over a year ago

Just be warned that if this is for a college homework, and you give this answer, you better be prepared to know EVERYTHING about streams.

makasu Over a year ago

It solved the problem. This is not homework. Many thanks.

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