[Haskell-beginners] Unique integers in a list

[Ramesh Kumar]

Thanks Abdul Sattar. I'll try this.




>________________________________
> From: AbdulSattar Mohammed <codingtales at gmail.com>
>To: beginners at 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 at 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,
>>
>>
>>
>>
>>
>>
>>________________________________
>> 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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>
>
>
>-- 
>Warm Regards,
>
>AbdulSattar Mohammed
>
>
>
>
>-- 
>Warm Regards,
>
>AbdulSattar Mohammed
>
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