Question

I have the first matrix which should account for each users (in lines) which products (in columns) they like.
Let’s take 3 users and 5 products.
No user liked a product, so my matrix ILike equals a nul matrix :

let matrixILike = [[0.; 1.;2.;3.]
                   [1.;0.;0.;0.]
                   [2.;0.;0.;0.]
                   [3.;0.;0.;0.]
                   [4.;0.;0.;0.]
                   [5.;0.;0.;0.]]

Now user 1 likes product 2 and user 3 likes product 5 which can be summarized in the following matrix:

let matrixAction = [[1.;2.]
                    [3.;5.]]

So I would like to implement the matrix ILike thanks to the matrixAction to obtain a new updated matrixILike like this :

let matrixILike = [[0.; 1.;2.;3.]
                   [1.;0.;0.;0.]
                   [2.;1.;0.;0.]
                   [3.;0.;0.;0.]
                   [4.;0.;0.;0.]
                   [5.;0.;0.;1.]] 

I try to do this with a “match with” code but it is not working.

for k = 0 to matrixAction.NumRows - 1 do
            match  (matrixAction.[k,0] , matrixAction.[k,1]) with
            | (matrixILike.[x,0] , matrixILike.[0,y]) -> (matrixILike.[x,y] <- 1.)
            | _ -> (matrixILike.[x,y] <- 0.)
matrixILike

If you have any suggestions I take it.

Answer 1

Without changing your input parameters, this function will do the job.

let update actions = 
    let mapiTail f = function 
                     | []   -> [] 
                     | h::t -> h :: List.mapi (f h) t
    mapiTail (fun matHead _ ->
        mapiTail (fun rowHead i x -> 
            if List.exists ((=) [matHead.[i+1];rowHead]) actions then 1. else x))

Usage:

update matrixAction matrixILike

It uses List.mapi which is the same as List.map but with additional parameter: the index.