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hashes4merkle
hashes4merkle

Reputation: 404

Haskell-- Searching List of List for element

I would like to search a list of a list of Chars, [[Char]] determine whether a Char is within the list, returning true or false. So far I have this,

    scanBoard :: [[Char]] -> Bool
    scanBoard board 
        | 'r' `elem` board = True
        | 'i' `elem` board = True
        | otherwise = False

Thanks!

Upvotes: 0

Views: 355

Answers (2)

stew
stew

Reputation: 11366

we can create a predicate to test on a character: 

\c -> (c == 'r') || (c == 'i')

then we can check that any sublist has any such char

scanBoard :: [[Char]] -> Bool
scanBoard = any $ any pred where
    pred = \c -> (c == 'r') || (c == 'i')

Upvotes: 6

shree.pat18
shree.pat18

Reputation: 21757

For the purpose of searching, you can convert your list of lists to a single list using concat, and then search within the derived list using elem, like so:

scanBoard :: [[Char]] -> Bool
scanBoard board 
        | elem 'r' $ concat board = True
        | elem 'i' $ concat board = True
        | otherwise = False

Alternatively, we can do this recursively as shown below:

scanBoard :: [[Char]] -> Bool
scanBoard [[]] = False
scanBoard [x] = (elem 'r' x || elem 'i' x)
scanBoard board@(x:xs) = (elem 'r' x || elem 'i' x) || scanBoard xs

This will eliminate the need for the concat operation, and allow use to work directly with the list of lists using pattern matching.

Upvotes: 2

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