# 99 questions/Solutions/21

### From HaskellWiki

Remi.berson (Talk | contribs) (Add a solution with foldr along with some explanation.) |
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</haskell> | </haskell> | ||

+ | The use of foldl impose the use of concatenation. With a foldr we can use (:) instead, which is faster (O(n) vs. O(n²) I guess ?). The use of zip [1..] doesn't seem to had overhead compared to the same solution with the index stored in the accumulator. | ||

+ | <haskell> | ||

+ | insertAt :: a -> [a] -> Int -> [a] | ||

+ | insertAt elt lst pos = foldr concat' [] $ zip [1..] lst | ||

+ | where | ||

+ | concat' (i, x) xs | ||

+ | | i == pos = elt:x:xs | ||

+ | | otherwise = x:xs | ||

+ | </haskell> | ||

+ | Compared to the simple recursive definition, the fold version visit every elements of the list, whereas we could just stop after insertion of the element. | ||

[[Category:Programming exercise spoilers]] | [[Category:Programming exercise spoilers]] |

## Revision as of 21:28, 8 May 2014

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

or

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)

The use of foldl impose the use of concatenation. With a foldr we can use (:) instead, which is faster (O(n) vs. O(n²) I guess ?). The use of zip [1..] doesn't seem to had overhead compared to the same solution with the index stored in the accumulator.

insertAt :: a -> [a] -> Int -> [a] insertAt elt lst pos = foldr concat' [] $ zip [1..] lst where concat' (i, x) xs | i == pos = elt:x:xs | otherwise = x:xs

Compared to the simple recursive definition, the fold version visit every elements of the list, whereas we could just stop after insertion of the element.