m (added a foldl solution)
Revision as of 22:21, 4 January 2012
Insert an element at a given position into a list.
insertAt :: a -> [a] -> Int -> [a] insertAt x xs (n+1) = let (ys,zs) = split xs n in ys++x:zs
insertAt :: a -> [a] -> Int -> [a] insertAt x ys 1 = x:ys insertAt x (y:ys) n = y:insertAt x ys (n-1)
As a note to the above solution - this presumes that the inserted argument will be a singleton type a inserted into a list [a]. The lisp example does not infer this intent. As a result, presuming the data to be inserted is likewise of type [a] (which we are tacitly inferring here to be String into String insertion), a solution is:
insertAt x xs n = take (n-1) xs ++ [x] ++ drop (n-1) xs
This solution, like many others in this quiz presumes counting element positions starts at 1, perhaps causing needless confusion.
A solution using foldl and a closure, also assumes lists are 1 indexed:
insertAt :: a -> [a] -> Int -> [a] insertAt el lst n = fst $ foldl helper (,1) lst where helper (acc,i) x = if i == n then (acc++[el,x],i+1) else (acc++[x],i+1)