How to multiply two arrays element-wise

“Zipping” the two arrays gives a sequence of tuples (a_i, b_i)
which can then be multiplied element-wise:

let A = [1,2,3,4]
let B = [2,3,4,5]

let C = zip(A, B).map { $0 * $1 }

print(C) // [2, 6, 12, 20]

(If the arrays have different length then zip silently ignores the extra elements of the longer array.)

As @appzYourLife correctly said, you can also pass the multiplication
operator directly as an argument to map instead of a closure expression:

let C = zip(A, B).map(*)