Regarding finding the number of unique elements in a list.

Ketil's local user
18 Jan 2002 09:10:55 +0100

"Amrit K Patil ;012;VKSF6;" <> writes:

> I am not able to write a program to find the number of unique elments in a
> list in haskell.

Why not?  Are you able to find the number of unique elements yourself?
How?  I can think of a few ways to do it, for instance

        remove all repeated occurences of each letter in turn
        sort the list before you count (better time complexity?)

> I am also not able to write a porgram to find the elements in the
> innermost list in a list within lists.

There is a prelude function to concatenate a list of lists, returning
all the elements of the next level as a list.  Notice that you can't
really have an "innermost" list in Haskell, since all elements of a
list must be of the same type.  E.g. in Lisp you could have

        (((a b) (c d) ((e f) g) h) (i j k))

with (e f) as the innermost list.  But a list like this is
incompatible with the Haskell type system, and you'd need to declare a
data type something like

        data MyList a = Elem a | Nested [MyList a]

or something like that.

> Can anybody guide me as to how to go about it or better still send me
> the program.

Somebody did post a program, but will probably get nasty mail from
your tutor, so I shan't. :-)

If I haven't seen further, it is by standing in the footprints of giants