# Difference between revisions of "99 questions/Solutions/2"

From HaskellWiki

< 99 questions | Solutions

(Added if-based solution) |
|||

Line 29: | Line 29: | ||

if length xs == 1 then x |
if length xs == 1 then x |
||

else myButLast''''' xs |
else myButLast''''' xs |
||

+ | |||

+ | myButLast'''''' = head . reverse . init |
||

</haskell> |
</haskell> |
||

## 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
```