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From: AbdulSattar Mohammed <codingtales@gmail.com>
To: beginners@haskell.org
Sent: Wednesday, March 28, 2012 2:30 PM
Subject: [Haskell-beginners] Unique integers in a list

On Wed, Mar 28, 2012 at 11:10 AM, Prasanna K Rao <prasannakrao@yahoo.com> wrote:

Hi,

One way is to define a 'isin' function like this::

isin x (a:[])  = if (x == a) then True else False

isin x (a:as)  = if (x == a) then True else isin x as

Unnecessary. We have elem for that.

 

and use it like this::

unique (x:xs)   = if not(isin x xs) then [x] ++ unique(xs) else unique(xs)

It removes duplicates. Does not remove the elements that have duplicates (which was asked by OP). It also needs the empty list check to terminate. It'll fail with Non-exhaustive patterns. 

or like this::

unique(x:xs)    = [x | x <- (x:xs), not(isin x xs)] ++ unique xs

 

The not(isin x xs) will definitely fail for all the elements except the first one because those elements are being taken from xs.

 

The later being the preferred one. HTH..

If you see correctly, the former is the preferred one (giving different solution).

To OP, 

When we pass x to isSingle x xs, we know that there is at least one x in xs. If we remove that x and check for the existence of x in the remainder of the list, we know if there is more than one x.

isSingle x xs = x `notElem` (delete x xs)

delete is in Data.List. 

Regards,

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From: Ramesh Kumar <rameshkumar.techdynamics@ymail.com>
To: "Beginners@haskell.org" <Beginners@haskell.org>
Sent: Wednesday, March 28, 2012 3:03 AM
Subject: [Haskell-beginners] Unique integers in a list

Hi,

I've just started learning Haskell a couple of weeks ago using Simon Thompson's "Haskell: Craft of Functional Programming".

There is an exercise in chapter 7 of the book which goes something like this:

Define a function of the type:     unique :: [Integer] -> [Integer]

which if given a list of integers, should return a list of those integers which occur only once in the input list.

Example:

   unique [5,2,4,2,3,1,5,2] should result in [4,3,1]

*** The questions assumes we know only of list comprehensions and recursion.

I am guessing the solution must include something like this:

unique :: [Integer] -> [Integer]

unique xs = [ x | x <- xs, isSingle x ]

My problem is in defining the function 'isSingle'.

I would greatly appreciate any pointers on this.

Many thanks.

Ramesh

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--
Warm Regards,

AbdulSattar Mohammed

--
Warm Regards,

AbdulSattar Mohammed

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