99 questions/Solutions/2: Difference between revisions
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myButLast'''' = head . tail . reverse | 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 | |||
</haskell> | </haskell> | ||
[[Category:Programming exercise spoilers]] | [[Category:Programming exercise spoilers]] | ||
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