Traversing matrix

Learning
1

Hello there,
I’m trying to write a function that checks if there exists an element that is strictly the smallest in it’s row and strictly the largest in it’s column.

For example:
# let a = [| [|3;2;6;4|]; [|4;5;6;1|]; [|0;3;2;5|] |] # min_max a;;- : bool = false

Should be of the following structure int array array -> bool

Any help is greatly appreciated and thanks in advance.

2

I gave it a try based on what I understood. The following code is in all likelihood neither good nor efficient.

let getcol a c =
  let nr = Array.length a in
  let col = Array.make nr 0 in
  for i = 0 to (nr-1) do
    col.(i) <- a.(i).(c)
  done;
  col

let get_val_index a cmp =
  let n = Array.length a in
  let m = ref a.(0) in
  let o = ref (Some a.(0), Some 0) in
  let duplicate = ref false in
  let res=ref 0 in
  for i = 1 to (n-1) do
    res := (cmp a.(i) !m);
    if !res < 0 then
    begin
      m := a.(i);
      o := (Some a.(i), Some i);
      duplicate := false;
    end
    else if !res = 0 then
      duplicate := true
    else
      ()
  done;
  if not !duplicate then !o else (None,None)

let mini a = get_val_index a (compare)

let maxi a = get_val_index a (fun x y -> (-1)*compare x y)

let minmax a =
  let r = Array.length a in
  let t = ref false in
  for i = 0 to (r-1) do
    let minr = mini a.(i) in
    if minr <> (None, None) then
      let maxc = maxi (getcol a (Option.get (snd minr))) in
      if maxc <> (None, None) then
        if Option.get (fst minr) = Option.get (fst maxc) then t := true;
  done;
  !t
3

After trying it out, I’m getting an Error: Unbound module option. The rest of the code so far seems alright, it’s just this part that’s bothering.

4

After compiling it online the error seems to be gone, it is probably due to the version I am using.

5

Maybe you have an older version of the OCaml compiler. As I understand, the Option module has been introduced with version 4.08: https://caml.inria.fr/pub/docs/manual-ocaml/libref/Option.html

6

That’s true, I’m on 4.05. The code is working well, I will take some time to study it through. Thank you so much, you are amazing!