CODEWITHSUNDEEP
listparsingocamlml
Ana Palea
Ana Palea

Reputation: 5

Ocaml function parsing list of lists

I am trying to build a function in Ocaml which parses a list of lists, eg from [[0;1];[3;4;8]] to [0;1;3;4;8]. I tried to do something like:

#let rec parse listoflists= 
   match listoflists with 
 [[]]->[]
|[h::t]->h::parse [t];;

but it doesn't work... I also need an explanation, because I don't understand how the lists of lists actually work...

I don't have to use the Ocaml library functions.

Upvotes: 0

Views: 506

Answers (1)

Jeffrey Scofield
Jeffrey Scofield

Reputation: 66808

If you can understand a list, then I claim you already know about lists of lists. This is the beauty of recursion.

The only real difficulty (as I see it) is keeping track of which list you're talking about. Your code needs to work with the list of lists itself, which consists of one list (call it h) followed by some other lists. It also needs to work with the list h, which consists of some element (call it hh) followed by some other elements.

It seems to me there are three interesting cases: (a) the list of lists is empty; (b) the first element of the list of lists h is empty (c) neither the list of lists nor h is empty.

You are not handling all three of these cases. That's one way to see that your code probably wouldn't work.

Here is a match that matches the three cases, which might help a little:

match listoflists with
| [] -> ...              (* List of lists is empty *)
| [] :: t -> ...         (* First list h is empty *)
| (hh :: ht) :: t -> ... (* Neither is empty *)

Upvotes: 1

Related Questions