Elementwise multiplication of arrays in F#

Question

Is there a simple way to multiply the items of an array in F#?

So for example of I want to calculate a population mean from samples I would multiply observed values by frequency and then divide by the sample numbers.

     let array_1 = [|1;32;9;5;6|];;

     let denominator = Array.sum(array_1);;


     denominator;;

     let array_2 = [|1;2;3;4;5|];;

     let productArray = [| for x in array_1 do
                             for y in array_2 do
                                 yield x*y |];;


     productArray;;

     let numerator = Array.sum(productArray);;

     numerator/denominator;;

Unfortunately this is yielding a product array like this:-

     val it : int [] =
     [|1; 2; 3; 4; 5; 32; 64; 96; 128; 160; 9; 18; 27; 36; 45; 5; 10; 15; 20; 25;
        6; 12; 18; 24; 30|]

Which is the product of everything with everything, whereas I am after the dot product (x.[i]*y.[i] for each i).

Unfortunately adding an i variable and an index to the for loops does not seem to work.

What is the best solution to use here?

Related: http://stackoverflow.com/questions/7421527/how-do-i-add-two-numeric-arrays-in-f — Daniel, Oct 17 '12 at 15:01

score 8 · Accepted Answer · answered Oct 17 '12 at 14:35

8

Array.zip array_1 array_2
    |> Array.map (fun (x,y) -> x * y)

As the comment points out, you can also use Array.map2:

Array.map2 (*) array_1 array_2

answered Oct 17 '12 at 14:35

Lee

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Brilliant. Hadn't quite got my head around the Array.zip stuff. Will accept when the time limit has passed. – Simon Hayward Oct 17 '12 at 14:36
10

You could also do `Array.map2 (*) array_1 array_2`. – kvb Oct 17 '12 at 14:37

score 7 · Answer 2 · answered Oct 17 '12 at 18:49

7

Like this:

Array.map2 (*) xs ys

answered Oct 17 '12 at 18:49

J D

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Hey! John Harrop. Just been reading F# for scientists on the train, looking at the database interops. – Simon Hayward Oct 17 '12 at 19:41

score 3 · Answer 3 · answered Oct 17 '12 at 14:35

3

Something like

[| for i in 0 .. array_1.Length - 1 ->
    array_1.[i] * array_2.[i] |]

should work.

answered Oct 17 '12 at 14:35

kvb

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Elementwise multiplication of arrays in F#

3 Answers3