[Haskell-beginners] Unique integers in a list

[Prasanna K Rao]

Hi Ramesh,

Sorry. I was trying to solve a wrong problem. Please refer to solution provided by AbdulSattar Mohammed.

Regards



________________________________
 From: Ramesh Kumar <rameshkumar.techdynamics at ymail.com>
To: Prasanna K Rao <prasannakrao at yahoo.com>; "Beginners at haskell.org" <Beginners at haskell.org> 
Sent: Wednesday, March 28, 2012 7:53 AM
Subject: Re: [Haskell-beginners] Unique integers in a list
 

Thanks Prasanna.

Is there a base case for the solution with recursion? 

I'm trying with this code but I'm not getting the actual desired result:


isIn :: Integer -> [Integer] -> Bool
isIn _ [] = False
isIn n (x:xs) = if (n == x) then True else isIn n xs

unique :: [Integer] -> [Integer]
unique [] = []
unique (x:xs)   = if not(isIn x xs) then [x] ++ unique(xs) else unique(xs)



*Main> unique [1,2,3,4,5,4]
[1,2,3,5,4]         *** should have been  [1,2,3,5]



Thanks & Regards.







>________________________________
> From: Prasanna K Rao <prasannakrao at yahoo.com>
>To: Ramesh Kumar <rameshkumar.techdynamics at ymail.com>; "Beginners at haskell.org" <Beginners at haskell.org> 
>Sent: Wednesday, March 28, 2012 1:40 PM
>Subject: Re: [Haskell-beginners] Unique integers in a list
> 
>
>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
>
>
>and use it like this::
>
>
>unique (x:xs)   = if not(isin x xs) then [x] ++ unique(xs) else unique(xs)
>
>
>
>or like this::
>
>
>unique(x:xs)    = [x | x <- (x:xs), not(isin x xs)] ++ unique xs
>
>
>The later being the preferred one. HTH..
>
>
>Regards,
>
>
>
>
>
>
>________________________________
> From: Ramesh Kumar <rameshkumar.techdynamics at ymail.com>
>To: "Beginners at haskell.org" <Beginners at 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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