CODEWITHSUNDEEP
listhaskell
Slo
Slo

Reputation: 13

Haskell - Find and remove element from a list of list

I need to find an element l in a list of list, removing each list containing the element, and then return the list of list, but i don't know how to parsing list of list and, at the same time, removing the list and keeping parsing the rest of the list.

To be more understandable, i have : 

data Object = Pos Char | Neg Char
     deriving (Show, Eq)

type Box = [Object]

type Formula = [Box]

findAndDelete :: Formula -> Object -> Formula
findAndDelete cs l
findAndDelete (c:cs) l

edit : At the same time, I need to remove all x element from the Formula,x is returned by a function named negative with l in parameter.

negative :: Object -> Object
negative (Pos v) = Neg v
negative (Neg v) = Pos v

Upvotes: 1

Views: 1192

Answers (2)

moondaisy
moondaisy

Reputation: 4491

Since you want to remove the Box that contains the element you could use elem :: Eq a => a -> [a] -> Bool to check just that:

findAndDelete :: Formula -> Object -> Formula
findAndDelete [] o = []
findAndDelete (b:bs) o = case (elem o b) of {
                            True -> findAndDelete bs o;
                            False -> b:(findAndDelete bs o)
                       };

So to remove the negative objects during the findAndDelete function you can use filter :: (a -> Bool) -> [a] -> [a] this will give you a Box where all the objects satisfy the predicate:

findAndDelete :: Formula -> Object -> Formula
findAndDelete [] o = []
findAndDelete (b:bs) o = case (elem o b) of {
                             True -> findAndDelete bs o;
                             False -> (filter ((/=) (negative o)) b):(findAndDelete bs o)
                          };

Upvotes: 0

chi
chi

Reputation: 116139

If you want to remove the boxes, you can filter for all the boxes who do not have the given object as their element.

import Data.List

findAndDelete :: Formula -> Object -> Formula
findAndDelete f o = filter (not . elem o) f

Upvotes: 1

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