[Haskell-cafe] function unique

[Gregory Propf]

Well there's this approach.  Granted you need to split the input list into *both* args of the fold (i.e. the "input list" is really [1,4,5,3] but you can get this with head and tail).  I'm just learning about the fold family myself. - Greg

Prelude> foldl (\a b -> if (any (\x -> x == b) a) then a else b:a) [1] [4,5,3,3,4]
[3,5,4,1]



----- Original Message ----
From: Alexteslin <alexteslin at yahoo.co.uk>
To: haskell-cafe at haskell.org
Sent: Tuesday, July 10, 2007 1:40:56 PM
Subject: [Haskell-cafe] function unique


Hi, i am a beginner to Haskell and i have a beginner's question to ask.

An exercise asks to define function unique :: [Int] -> [Int], which outputs
a list with only elements that are unique to the input list (that appears no
more than once).  I defined a function with list comprehension which works
but trying to implement with pattern matching and primitive recursion with
lists and doesn't work.

unique :: [Int] -> [Int]
unique xs = [x | x <- xs, elemNum2 x xs == 1]


elemNum2 :: Int -> [Int] -> Int
elemNum2 el xs = length [x| x <- xs, x == el]

//This doesn't work, I know because the list shrinks and produces wrong
result but can not get a right //thinking

unique2 :: [Int] -> [Int]
unique2 [] = []
unique2 (x:xs)
    |elemNum2 x xs == 1 = x:unique2 xs
    |otherwise = unique2 xs


Any help to a right direction would be very appreciated, thanks.
-- 
View this message in context: http://www.nabble.com/function-unique-tf4058328.html#a11528933
Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe at haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe







      ____________________________________________________________________________________
Shape Yahoo! in your own image.  Join our Network Research Panel today!   http://surveylink.yahoo.com/gmrs/yahoo_panel_invite.asp?a=7 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20070710/4762bf82/attachment-0001.htm