How to find array column sum using Java streams

Question

I have a 2D integer array:

int arr[][] = new int[rows][columns];

The nth Row sum can be found using:

int rSum=Arrays.stream(arr[n]).sum();

How can I find the pth Column sum?

int cSum=Arrays.stream(arr[][p]).sum();

The above line does not work.

Answer 1

You can map every row on the p-th element:

int cSum = Arrays.stream(arr).mapToInt(row -> row[p]).sum();

This works as follows: first we construct a stream from the arr. This stream will contain the rows of the "matrix". Then for every such row, we mapToInt it to an int: the p-th element of that row. Then we sum(..) the stream of ints together to the sum.