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[Caml-list] "List.index" or "List.unique" functions?
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Date: 2004-04-30 (17:54)
From: Rahul Siddharthan <rsidd@o...>
Subject: [Caml-list] "List.index" or "List.unique" functions?
I just discovered OCaml a week or so ago, and it really seems to be
the "holy grail" -- more concise and elegant that python, almost as
fast as C.  I wish I'd known of it a year ago.  Now I just need to get
used to the functional way of thinking...

I have a question: suppose I have a list l1, and I want to create a new
list l2 with only one copy of any repeated members of the first list
(eg, l1=[1;2;3;4;3;4;5;6;5] -> l2=[1;2;3;4;5;6])

In python, I can define a function to do this quite concisely, eg:

unique = lambda l: [l[n] for n in range(len(l)) if l.index(l[n])==n]

How do I do it in OCaml?  Are there functions equivalent to index
(return the position of the first matching element in the list) and
range (range n = [0;1;...;n-1]), or is there a cleaner way to do it?
The best I can come up with is:

let unique l = 
  let range n = 
    let rec rangen n lacc =
      if n<0 then lacc else rangen (n-1) (n::lacc)
    in rangen (n-1) []
  let index a l =
    let rec indexn a l n =
      if n==(List.length l) then -1
      else if (List.nth l n) =a then n
      else indexn a l (n+1)
    in indexn a l 0
  in (fun n -> List.nth l n) (List.filter 
                                      (fun n -> n=(index (List.nth l n) l))   
                                      (range (List.length l)));;

(it would be more concise if range and index already exist, but even
then, the last line looks rather ugly to me...)



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