Elemento más cercano que cumple una propiedad
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-- Definir la función -- cercano :: (a -> Bool) -> Int -> [a] -> Maybe a -- tal que (cercano p n xs) es el elemento de xs más cercano a n que -- verifica la propiedad p. La búsqueda comienza en n y los elementos se -- analizan en el siguiente orden: n, n+1, n-1, n+2, n-2,... Por ejemplo, -- cercano (`elem` "aeiou") 6 "Sevilla" == Just 'a' -- cercano (`elem` "aeiou") 1 "Sevilla" == Just 'e' -- cercano (`elem` "aeiou") 2 "Sevilla" == Just 'i' -- cercano (`elem` "aeiou") 5 "Sevilla" == Just 'a' -- cercano (`elem` "aeiou") 9 "Sevilla" == Just 'a' -- cercano (`elem` "aeiou") (-3) "Sevilla" == Just 'e' -- cercano (>100) 4 [200,1,150,2,4] == Just 150 -- cercano even 5 [1,3..99] == Nothing |
Soluciones
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import Data.List -- 1ª solución -- =========== cercano1 :: (a -> Bool) -> Int -> [a] -> Maybe a cercano1 p n xs | null ys = Nothing | otherwise = Just (head ys) where ys = filter p (ordenaPorCercanos xs n) -- (ordenaPorCercanos xs n) es la lista de los elementos de xs que -- ocupan las posiciones n, n+1, n-1, n+2, n-2... Por ejemplo, -- ordenaPorCercanos [0..9] 4 == [4,5,3,6,2,7,1,8,0,9] -- ordenaPorCercanos [0..9] 7 == [7,8,6,9,5,4,3,2,1,0] -- ordenaPorCercanos [0..9] 2 == [2,3,1,4,0,5,6,7,8,9] -- ordenaPorCercanos [0..9] (-3) == [0,1,2,3,4,5,6,7,8,9] -- ordenaPorCercanos [0..9] 20 == [9,8,7,6,5,4,3,2,1,0] ordenaPorCercanos :: [a] -> Int -> [a] ordenaPorCercanos xs n | n < 0 = xs | n >= length xs = reverse xs | otherwise = z : intercala zs (reverse ys) where (ys,(z:zs)) = splitAt n xs -- (intercala xs ys) es la lista obtenida intercalando los elementos de -- las lista xs e ys. Por ejemplo, -- intercala [1..4] [5..10] == [1,5,2,6,3,7,4,8,9,10] -- intercala [5..10] [1..4] == [5,1,6,2,7,3,8,4,9,10] intercala :: [a] -> [a] -> [a] intercala [] ys = ys intercala xs [] = xs intercala (x:xs) (y:ys) = x : y : intercala xs ys -- 2ª solución (usando find) -- ========================= cercano2 :: (a -> Bool) -> Int -> [a] -> Maybe a cercano2 p n xs = find p (ordenaPorCercanos xs n) |
Referencia
El ejercicio está basado en el problema del 12 de mayo de 1HaskellADay.