Reputation: 48644
I have two lists. One list contains some random data and other list contains the index of first list which needs to be deleted.
For example, let us consider two lists:
let a = [3,4,5,6,6,7,8]
let b = [1,3]
Then, the resultant output should be [3,5,6,7,8]
. The number 4 and 6 are deleted since they are on index positions 1 and 3 respectively.
I'm new to Haskell, so finding it difficult to find the solution.
Update: Following code makes it work
import Data.List
dele :: Eq a => [a] -> [Int] -> [a]
dele [] _ = []
dele x [] = x
dele x (y:ys) = dele (delete (x !! y) x) ys
I was just wondering, is there a way to solve it through map/fold way ?
Upvotes: 0
Views: 1790
Reputation: 7444
An alternative answer using the lens library which has received considerable attention recently
import Control.Lens
>let a = [3,4,5,6,6,7,8]
>let b = [1,3]
>a^..elements (`notElem`b)
[3,5,6,7,8]
(^..) is jus the infix for of toListOf which can be used to traverse a structure and make a list out of its parts. The elements function just lets you choose which ones to include.
Other options are 'traverse' to traverse a traversables, 'both' to traverse a (,) and they compose together with (.) so traverse.both would traverse [(1,2), (3,4)] for example.
[(1,2), (3,4)]^..traverse.both [1,2,3,4]
Upvotes: 1
Reputation: 3375
Off the top of my head:
removeByIndex :: [Integer] -> [a] -> [a]
removeByIndex indices = map snd . filter notInIndices . zip [0..]
where notInIndices (i,_) = i `notElem` indices
Upvotes: 2
Reputation: 624
deleteByIndex :: (Enum a, Eq a, Num a) => [a] -> [b] -> [b]
deleteByIndex r = map snd . filter (\(i, _) -> notElem i r) . zip [0..]
[0..]
produces an infinite list [0, 1, 2, 3, ...]
zip
constructs a list of pairs with the values of this list and your input list in the form [(0,x), (1, y), ...]
filter
takes a function a -> Bool
. The lambda checks if the index (first element of the pair) is in your input list r
.
map snd
returns the second element of each pair of the zip list.
zip
,filter
, map
and notElem
are documented here
Upvotes: 2