Inversion sum is not correct using Stream.reduce

Question

Inversion sum is not correct using Stream.reduce, what is going wrong here ?

double[] array = {1.0, 2.0};
double iunversionSum = Arrays.stream(array).reduce(0.0, (a, b) -> Double.sum(1.0 / a, 1.0 / b));

output is .5 but expected is 1.5 (1/1 + 1/2)

Answer 1

I think using map() it could be simplier.

double inversionSum = Arrays.stream(arr).map(val -> 1 / val).sum();

Answer 2

The error in your reduce is: Double.sum(1.0 / a, 1.0 / b), starting the series with 0.0. Now check why your outcome is .5.

Use Double.sum(a, 1.0 / b), if you want to use reduce.