Difference between revisions of "99 questions/Solutions/2"
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m (Add description to foldl solution.) |
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myButLast'''' = head . tail . reverse |
myButLast'''' = head . tail . reverse |
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| − | </haskell> |
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| + | lastbut1 :: Foldable f => f a -> a |
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| − | We can also use a pair to keep the last two elements processed from the list. We take advantage of lazy evaluation, such that if the list is too small, the initial elements in the pair get evaluated, resulting in an error: |
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| ⚫ | |||
| − | <haskell> |
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| + | where |
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| ⚫ | |||
| − | + | err1 = error "lastbut1: Empty list" |
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| − | + | err2 = error "lastbut1: Singleton" |
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| + | |||
| + | lastbut1safe :: Foldable f => f a -> Maybe a |
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| + | lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing) |
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| + | |||
| + | myButLast''''' [] = error "Empty list" |
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| + | myButLast''''' [x] = error "Too few elements" |
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| + | myButLast''''' (x:xs) = |
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| + | if length xs == 1 then x |
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| + | else myButLast''''' xs |
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| + | |||
| + | myButLast'''''' = head . reverse . init |
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</haskell> |
</haskell> |
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Latest revision as of 14:30, 25 October 2017
(*) Find the last but one element of a list.
myButLast :: [a] -> a
myButLast = last . init
myButLast' x = reverse x !! 1
myButLast'' [x,_] = x
myButLast'' (_:xs) = myButLast'' xs
myButLast''' (x:(_:[])) = x
myButLast''' (_:xs) = myButLast''' xs
myButLast'''' = head . tail . reverse
lastbut1 :: Foldable f => f a -> a
lastbut1 = fst . foldl (\(a,b) x -> (b,x)) (err1,err2)
where
err1 = error "lastbut1: Empty list"
err2 = error "lastbut1: Singleton"
lastbut1safe :: Foldable f => f a -> Maybe a
lastbut1safe = fst . foldl (\(a,b) x -> (b,Just x)) (Nothing,Nothing)
myButLast''''' [] = error "Empty list"
myButLast''''' [x] = error "Too few elements"
myButLast''''' (x:xs) =
if length xs == 1 then x
else myButLast''''' xs
myButLast'''''' = head . reverse . init