Error when convert to Boolean matrix - ocaml

Question

I have this problem still bother me a lots. I have a (string * string list) list and I want to convert it to Boolean matrix.
I have a special condition when transform it. For example I have this list:
let entries = [("name", ["string"; "label"]); ("label", ["int"; "name"]);
("symbol", ["string"])]
where "string" and "int" are undefined type, undefined type because in my real data, I don't have a definition describe this type. So I built a list of undefined type.
let undefined = ["string"; "int"]
And the first position in the list ("name", "label", "symbol") are defined type, defined type is the type I have definition in my data.
let defined = ["name"; "label"; "symbol"]
I am trying to do this: from entries, there position should be:
name: 2; string: 0; label: 3; int: 1; symbol: 4
And when showing the depend relation from the list entries, it doesn't change their position. For example: name(2) link to string(0) and label(3), and label (3) has an edge to int(1) and name (2),` and so on...
I have these functions return a position(num_of_name) and element (name_of_num) in a list.
let rec position x = function
| [] -> raise Not_found
| y :: ys -> if x = y then 0 else 1 + position x ys
let len_undefined = List.length undefined
let num_of_name defined undefined len_undefined s =
try (position s defined) + len_undefined;
with Not_found -> position s undefined
let name_of_num defined undefined len_undefined k =
if k < len_undefined then
List.nth undefined k else
List.nth defined (k - len_undefined)
So from the entries list I want to build a boolean matrix show there relation using the function num_of_name. So I write my function:
let matrix =
let len = List.length defined + len_undefined in
let boolmat = Array.make_matrix len len false in
List.iter (fun (s, strs) ->
let pos1 = num_of_name defined undefined len_undefined s in
List.iter (fun t ->
let pos2 = num_of_name defined undefined len_undefined t in
boolmat.(pos1).(pos2) <- true) strs) entries;
boolmat
let print_mat m =
for i = 0 to Array.length m - 1 do
for j = 0 to Array.length m.(0) - 1 do
print_string (string_of_bool m.(i).(j));
Printf.printf " ";
done;
Printf.printf " \n";
done;
;;
let test_print = print_mat matrix
It return an error "Fatal error: exception Not_found"
I need your help.
Thank you very much!!

Answer 1

As I said in the comment, your num_of_name function is fragile since it throws Not_found exception when its input is not an element of either defined or undefined. One way to fix is using Option type:
let num_of_name defined undefined len_undefined s =
try
let p = position s defined in
Some (p + len_undefined)
with Not_found ->
try
let p = position s undefined in
Some p
with Not_found -> None
and matrix is calculated as:
let matrix =
let len = List.length defined + len_undefined in
let boolmat = Array.make_matrix len len false in
List.iter (fun (s, strs) ->
match num_of_name defined undefined len_undefined s with
| Some pos1 -> List.iter (fun t ->
match num_of_name defined undefined len_undefined t with
| Some pos2 -> boolmat.(pos1).(pos2) <- true
| None -> ()) strs
| None -> ()
) entries;
boolmat
Of course, if you enforce your program by extracting defined and undefined from entries, your code is correct.