[Haskell-cafe] function unique

[Alexteslin]

I'v got it - it produces the right output.
Thank you.


Brent Yorgey wrote:
> 
> The problem with your second implementation is that elements which occur
> more than once will eventually be included, when the part of the list
> remaining only has one copy. For example:
> 
> unique2 [1,1,2,4,1]
> = unique2 [1,2,4,1]
> = unique2 [2,4,1]
> = 2 : unique2 [4,1]
> = 2 : 4 : unique2 [1]
> = 2 : 4 : 1 : unique2 []   -- only a single 1 left, so it gets mistakenly
> included
> = [2,4,1]
> 
> When you determine that a certain number should not be included in the
> output, you need to delete all remaining occurrences of it from the list,
> so
> it won't get included later.
> 
> unique2 (x:xs)
>         |elemNum2 x xs == 1 = x:unique2 xs
>         |otherwise = unique2 (deleteElt x xs)
> 
> I'll let you figure out how to implement the deleteElt function.
> 
> hope this is helpful!
> -Brent
> 
> On 7/10/07, Alexteslin <alexteslin at yahoo.co.uk> wrote:
>>
>>
>> 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
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>>
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