F# Sum of products of an array with element position

Question

Coming of a C# background and trying to learn F#.
I'm trying to iterate over an array of size 256, so the total sum of it be the product of the element position and the element, like this:

float sum = 0.0;
for (int i = 0; i < 256; i++) {
    sum += i * arr[i];
}

I made this but I don't know if this is the best way to do it in F#, probably not.

let mutable sum = 0
for i in 0 .. 255 do
    sum <- sum + i * arr.[i]
done

I don't know if it's possible to use Array.fold or Array.iteri to solve this in a better way.

Answer 1

您可以使用mapi和sum ：

let f s = s |> Seq.mapi (fun i j -> i * j) |> Seq.sum

Answer 2

One way of doing this with only one iteration through the array:

let f s = snd (Array.fold (fun (i, sum) x -> (i + 1, sum + x * float i)) (0, 0.0) s)

Although I prefer Lee's solution as being much easier to follow.

An alternative would be to write your own Array.foldi function and use that.