Using two streams in Java lambda to compute covariance

Question

Let\'s say I have two arrays of double. I\'ve been experimenting with Stream from Java 8. I think I\'ve understood the main ideas but then I realised that I\'m not sure how

Answer 1

Other FP languages have the zip operation, which makes a stream of pairs from two streams of elements. This has not been made available in Java.

In case of arrays, however, you can easily make a stream of array indices and let your functions close over the arrays, accessing them by the index parameter.

I should also warn you that in this line

.mapToDouble(d -> d.doubleValue() - mean(xs))

you are making your program complexity O(n²) because mean is recalculated at each step. You should precalculate that and close over the result in the lambda.