Problems with input values to a list

May 13, 2022

I’m quite new at this and I have a problem. I want to read an input and store it at a list, for example:

2 -> number of trees that will be formed after this all
2 -> number of nodes of the first tree
1 -> node 1
2 -> node 2
1 -> number of nodes of the second tree
2 -> node 1

The code below doesn’t work as I wanted. That means for example, using the input before:

2 -> number of trees that will be formed after this all
2 -> number of nodes of the first tree
1 -> number of nodes of the second tree
2 -> node 1 of the second tree
1 -> node 1 of the first tree
2 -> node 2 of the first tree

Output that the program shows for the input above is:
1 2 2

But I expected to have this:
1 2 1
How I can fix this?

let rec inputs number_of_nodes =
  match number_of_nodes with 
  | 0 -> []
  | _ -> let a = read_int() in a :: (inputs (number_of_nodes - 1))


let rec fill_tree_values number_of_trees = 
  match number_of_trees with
  | 0 -> []
  | _ -> let number_of_nodes = read_int() in 
    (inputs number_of_nodes) :: fill_tree_values (number_of_trees - 1)


let number_of_trees = read_int()
let trees = fill_tree_values number_of_trees

let printlist l = List.iter (Printf.printf "%d ") l

let () = List.iter (fun ll -> printlist ll) trees

>Solution :

You wrote:

    (inputs number_of_nodes) :: fill_tree_values (number_of_trees - 1)

but the evaluation order between both operands of :: is unspecified. So it might very well be the case — and unless I’m mistaken, it is effectively the case — that they are evaluated right-to-left, i.e. fill_tree_values (number_of_trees - 1) is performed before inputs number_of_nodes. So you’re reading inputs in a broken order. You can test this right-to-left behavior:

let _ = print_int 1 :: print_int 2 :: print_int 3 :: []
(* with OCaml 4.12, this prints me "321" *)

You must enforce the evaluation order by rewriting your code as:

    let trees = inputs number_of_nodes in
    trees :: fill_tree_values (number_of_trees - 1)

Note: I believe the evaluation order around :: might change in cutting-edge versions of OCaml, with the introduction of the “tail-call-modulo-constructor” optimisation. In any case, you should not rely on it.