# 99 questions/Solutions/6

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< 99 questions | Solutions(Difference between revisions)

Dan.krejsa (Talk | contribs) (Add a more efficient palindrome checker) |
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where |
where |
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input = zip xs (reverse xs) |
input = zip xs (reverse xs) |
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+ | </haskell> |
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+ | |||

+ | Here's one that does half as many compares: |
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+ | |||

+ | <haskell> |
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+ | palindrome :: (Eq a) => [a] -> Bool |
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+ | palindrome xs = p [] xs xs |
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+ | where p rev (x:xs) (_:_:ys) = p (x:rev) xs ys |
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+ | p rev (x:xs) [_] = rev == xs |
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+ | p rev xs [] = rev == xs |
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</haskell> |
</haskell> |

## Revision as of 20:21, 20 May 2011

(*) Find out whether a list is a palindrome. A palindrome can be read forward or backward; e.g. (x a m a x).

isPalindrome :: (Eq a) => [a] -> Bool isPalindrome xs = xs == (reverse xs)

isPalindrome' [] = True isPalindrome' [_] = True isPalindrome' xs = (head xs) == (last xs) && (isPalindrome' $ init $ tail xs)

Here's one to show it done in a fold just for the fun of it. Do note that it is less efficient then the previous 2 though.

isPalindrome'' :: (Eq a) => [a] -> Bool isPalindrome'' xs = foldl (\acc (a,b) -> if a == b then acc else False) True input where input = zip xs (reverse xs)

Here's one that does half as many compares:

palindrome :: (Eq a) => [a] -> Bool palindrome xs = p [] xs xs where p rev (x:xs) (_:_:ys) = p (x:rev) xs ys p rev (x:xs) [_] = rev == xs p rev xs [] = rev == xs