Hi all, I've been working hard at the programming, but have come to a stand still.

Anyway, I'm having trouble with lists. This is just one of many many many questions I have, so I will keep it down to the minimum.

I have to state whether a list is true or false, based on the integers in the list.

[1;2;3;4;5;6;7;8] would come up as TRUE#
[1;2;3;4;5;6;7;8;8;7] would come up as FALSE as this list contains two 7's and two 8's.

I have had a few ideas as to how to do this problem, and one of them is as a follows:-

If I could somehow subtract every value in the TL of the list from the HD, this would mean that if the list were as follows:-

[1;2;3;4;1], then:

1-2 = -1
1-3 = -2
1-4 = -3
1-1= 0

so if the value of the calculation is 0, then FALSE is displayed as a result.

Now I would need to do thsi recursivly, in order to take the hd(tl(tl(tl(tl(x))))


So it would subtract every number from a 'HD', and so even if the following list was used, the recursive function would work it out:-


I hope that makes sense. Basically I'm having alot of trouble coding it, but I thinik that the method of solving the problem I have come up with will work.

Can anyone give me some advice and/or help?




Ok, this is a very old thread, and I guess you have found a solution already.
Anyway, here is how I would do it (I hope I understood you right, you just want to know if the list contains only unique elements or not, right?):

let l = [1;2;3;4];;

let rec check l =
  match l with
  | [] -> true
  | h::t -> if (List.mem h t) then false else (check t)

if (check l) then 
  print_endline "no doubles" 
  print_endline "doubles found"

Regards, mawe