Myujth

I have the following 2d list

[ [a1 ; a2 ; a3 ; a4] ;
 [b1 ; b2 ; b3 ; b4] ;
 [c1 ; c2 ; c3 ; c4] ]

and I have an input function F.
I have the following problem : Given a 2d list m and a function f, compute the i-th value in
the result column c by using the i-th value from each column in m.

[ f [a1; b1; c1]; f [a2; b2; c2]; f [a3; b3; c3]; f [a4; b4; c4]; f [a5; b5; c5] ]

How can I create a list of the first elements of each sublist? I was thinking of using the List.map function but I am not sure what function to pass to it so I can get those results.

Any help is appreciated

I am not sure to understand the question.

But, as I can see from your example, I would transpose your list of lists (see transpose of a list of lists)

example:

let rec transpose list = match list with
| -> 
| :: xss -> transpose xss
| (x::xs) :: xss ->
 (x :: List.map List.hd xss) :: transpose (xs :: List.map List.tl xss);;

//An example of int list list
let m=[ [11 ; 12 ; 13 ; 14] ;
 [21 ; 22 ; 23 ; 24] ;
 [31 ; 32 ; 33 ; 34] ];;

//An example of int list-> int function
let f list = List.fold_right (+) list 0;;

List.map f (transpose m);;

This gives [f [11;21;31]; f [12;22;32]; f [13;23;33]; f [14;24;34]]

Maybe you can try this :

open List;;

let map2d f l =
 let rec aux acc l =
 try (
 let l'=map hd l and
 l =map tl l in
 aux (f l'::acc) l
 ) with _ -> rev acc
 in
 if l= then else aux l
;;

Test

let f = fold_left (+) 0;;
let id x = x;;

let m=[ [11 ; 12 ; 13 ; 14] ;
 [21 ; 22 ; 23 ; 24] ;
 [31 ; 32 ; 33 ; 34] ];; 

# map2d id m;;
- : int list list = [[11; 21; 31]; [12; 22; 32]; [13; 23; 33]; [14; 24; 34]]

# map2d f m;;
- : int list = [63; 66; 69; 72]

# map2d id [[11;21];[12];[13]];;
- : int list list = [[11; 12; 13]]

# map2d id [;];;
- : 'a list list = 
# map2d f [;];;
- : int list =