[Haskell-beginners] Palindromic solution??

[Miguel Pignatelli]

Yes, the index too large exception is a bug in my original code, it  
should check for "i >= j" instead of "i==j"

Thanks,

M;


El 16/02/2009, a las 17:26, Alan Cameron escribió:

>> Date: Mon, 16 Feb 2009 16:32:23 +0100
>> From: Miguel Pignatelli <miguel.pignatelli at uv.es>
>> Subject: [Haskell-beginners] Indentation of local functions
>> To: beginners at haskell.org
>> Message-ID: <90C28A9E-A67D-47CA-8416-5F1C5C27A1F9 at uv.es>
>> Content-Type: text/plain; charset="us-ascii"
>>
>> Hi all,
>>
>> This is my first post in this forum, I'm pretty new to Haskell  
>> (although I
>> have some previous experience in functional programming with OCaml)
>>
>> I'm trying to write the typical function that determines if a list  
>> is a
>> palindrome.
>> the typical answer would be something like:
>>
>> isPalindrome xs = xs == (reverse xs)
>>
>> But I find this pretty inefficient (duplication of the list and  
>> double of
>> needed comparisons).
>> So I tried my own version using just indexes:
>>
>> isPalindrome xs =
>>  	isPalindrome' 0 (length xs)
>>  	where isPalindrome' i j =
>>            if i == j   -- line 43
>>            then True
>>            else
>>            	if (xs !! i) == (xs !! (j-1))
>>            	then isPalindrome' (i+1) (j-1)
>>            	else False
>>
>> But, when trying to load this in ghci it throws the following error:
>>
>> xxx.hs:43:12: parse error (possibly incorrect indentation) Failed,  
>> modules
>> loaded: none.
>> (Line 43 is marked in the code)
>>
>> I seems that the definition of isPalindrome' must be in one line.  
>> So, this
>> works as expected:
>>
>> isPalindrome xs =
>>  	isPalindrome' 0 (length xs)
>>  	  where isPalindrome' i j = if i == j then True else if (xs !! i)  
>> ==
> (xs !! (j-1)) then isPalindrome' (i+1) (j-1) else False
>>
>> Is there any way to make the local definition of isPalindrome' more
>> readable?
>>
>> Any help in understanding this would be appreciated
>>
>> Thanks in advance,
>>
> I have found one solution to your problem
>
> isPalindrome xs =
>    isPalindrome' 0 (length xs)
>        where
>            isPalindrome' i j =
>               if i == j   -- line 43
>               then True
>               else
>             	   if (xs !! i) == (xs !! (j-1))
>             	   then isPalindrome' (i+1) (j-1)
>             	   else False
>
> This loads without error but poses a second problem it generates an  
> index
> too large exception.
>
>
>
>
> Alan Cameron
>
>
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