Concat 2D Arrays in f#

Question

How can I combine two 2d arrays generally?

My assumption (I could obviously easily test this) is that they always have the same number of columns:

let concatArrays (arr1:obj[,]) (arr2:obj[,]) = 
    ([arr1; arr2]) |> Array2d.concat

This function doesnt exist though. Just to be clear, the result should produce a 2d array with length = sum of lengths and same number of columns as the original arrays2D and should be the same type as the input, here obj[,]. I could obviously do this in a looping construct but I was wondering about an f# way. Thanks.

I tried this:

let append2D (arr1:float[,]) (arr2:float[,]) = 
    let cls = arr1.GetLength 1
    let rows1 = arr1.GetLength 0
    let rows2 = arr2.GetLength 0
    Array2D.init (rows1+rows2) cls (fun i j -> match i with | a when a <= rows1 -> arr1.[i,j] | _ ->  arr2.[i,j])

But this comes back with index out of bounds error.

Update of last row:

 Array2D.init (rows1+rows2) cls (fun i j -> if i < rows1 then arr1.[i,j] else arr2.[i,j])  

Update working solution:

 Array2D.init (rows1+rows2) cls (fun i j -> if i < rows1 then arr1.[i,j] else arr2.[i-rows1,j])  

thanks all

Answer 1

Following this recommendation here is a concat function for two equal column size Array2D arguments of any type 'a:

let concat (a1: 'a[,]) (a2: 'a[,]) =
    let a1l1,a1l2,a2l1,a2l2 = (Array2D.length1 a1),(Array2D.length2 a1),(Array2D.length1 a2),(Array2D.length2 a2)
    if a1l2 <> a2l2 then failwith "arrays have different column sizes"
    let result = Array2D.zeroCreate (a1l1 + a2l1) a1l2
    Array2D.blit a1 0 0 result 0 0 a1l1 a1l2
    Array2D.blit a2 0 0 result a1l1 0 a2l1 a2l2
    result

You may check this experimentally, but it would have times better performance, than any variant based on Array2D.init because Array2D.zeroCreate and Array2D.blit implementations are highly optimized.