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
(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.