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Today's Topics:
1. Re: Unique integers in a list (Prasanna K Rao)
2. Re: Unique integers in a list (Ramesh Kumar)
----------------------------------------------------------------------
Message: 1
Date: Tue, 27 Mar 2012 23:55:40 -0700 (PDT)
From: Prasanna K Rao <[email protected]>
Subject: Re: [Haskell-beginners] Unique integers in a list
To: Ramesh Kumar <[email protected]>,
"[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Hi Ramesh,
Sorry. I was trying to solve a wrong problem. Please refer to solution provided
by AbdulSattar Mohammed.
Regards
________________________________
From: Ramesh Kumar <[email protected]>
To: Prasanna K Rao <[email protected]>; "[email protected]"
<[email protected]>
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 <[email protected]>
>To: Ramesh Kumar <[email protected]>; "[email protected]"
><[email protected]>
>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 <[email protected]>
>To: "[email protected]" <[email protected]>
>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
>
>
>
>
>
>_______________________________________________
>Beginners mailing list
>[email protected]
>http://www.haskell.org/mailman/listinfo/beginners
>
>
>
>
>
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------------------------------
Message: 2
Date: Wed, 28 Mar 2012 00:09:29 -0700 (PDT)
From: Ramesh Kumar <[email protected]>
Subject: Re: [Haskell-beginners] Unique integers in a list
To: AbdulSattar Mohammed <[email protected]>,
"[email protected]" <[email protected]>
Message-ID:
<[email protected]>
Content-Type: text/plain; charset="iso-8859-1"
Thanks Abdul Sattar. I'll try this.
>________________________________
> From: AbdulSattar Mohammed <[email protected]>
>To: [email protected]
>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 <[email protected]>
>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 <[email protected]>
>>To: "[email protected]" <[email protected]>
>>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
>>
>>
>>
>>
>>
>>_______________________________________________
>>Beginners mailing list
>>[email protected]
>>http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>>
>>_______________________________________________
>>Beginners mailing list
>>[email protected]
>>http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
>
>
>--
>Warm Regards,
>
>AbdulSattar Mohammed
>
>
>
>
>--
>Warm Regards,
>
>AbdulSattar Mohammed
>
>_______________________________________________
>Beginners mailing list
>[email protected]
>http://www.haskell.org/mailman/listinfo/beginners
>
>
>
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