{"id":1030,"date":"2015-02-02T06:00:00","date_gmt":"2015-02-02T04:00:00","guid":{"rendered":"http:\/\/www.glc.us.es\/~jalonso\/exercitium\/?p=1030"},"modified":"2022-03-26T12:13:19","modified_gmt":"2022-03-26T10:13:19","slug":"siguiente-elemento-en-una-lista","status":"publish","type":"post","link":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/siguiente-elemento-en-una-lista\/","title":{"rendered":"Siguiente elemento en una lista"},"content":{"rendered":"

Definir la funci\u00f3n<\/p>\n

\n   siguiente :: Eq a => a -> [a] -> Maybe a\n<\/pre>\n
tal que (siguiente x ys) es justo el elemento siguiente a la primera  ocurrencia de x en ys o Nothing si x no pertenece a ys. Por ejemplo,<\/p>\n
\n   siguiente 5 [3,5,2,5,7]                       ==  Just 2\n   siguiente 9 [3,5,2,5,7]                       ==  Nothing\n   siguiente 'd' \"afdegdb\"                       ==  Just 'e'\n   siguiente \"todo\" [\"En\",\"todo\",\"la\",\"medida\"]  ==  Just \"la\"\n   siguiente \"nada\" [\"En\",\"todo\",\"la\",\"medida\"]  ==  Nothing\n   siguiente 999999 [1..1000000]                 ==  Just 1000000\n   siguiente 1000000 [1..1000000]                ==  Nothing\n<\/pre>\n
Soluciones<\/h4>\n
\nimport Data.Maybe (listToMaybe) -- Para la 4\u00aa soluci\u00f3n\n\n-- 1\u00aa soluci\u00f3n (por recursi\u00f3n):\nsiguiente1 :: Eq a => a -> [a] -> Maybe a\nsiguiente1 x (y1:y2:ys) | x == y1   = Just y2\n                        | otherwise = siguiente1 x (y2:ys)\nsiguiente1 x _ = Nothing\n\n-- 2\u00aa soluci\u00f3n (por comprensi\u00f3n):\nsiguiente2 :: Eq a => a -> [a] -> Maybe a\nsiguiente2 x ys \n    | null zs   = Nothing\n    | otherwise = Just (snd (head zs))\n    where zs = [(u,v) | (u,v) <- zip ys (tail ys), u == x]  \n\n-- 3\u00aa soluci\u00f3n (con dropWhile)\nsiguiente3 :: Eq a => a -> [a] -> Maybe a\nsiguiente3 x = aux . drop 1 . dropWhile (\/=x)\n    where aux []    = Nothing\n          aux (y:_) = Just y\n\n-- 4\u00aa soluci\u00f3n (con dropWhile y listToMaybe):\nsiguiente4 :: Eq a => a -> [a] -> Maybe a\nsiguiente4 x = listToMaybe . drop 1 . dropWhile (\/=x)\n\n-- Nota: En la librer\u00eda Data.Maybe est\u00e1 definida la funci\u00f3n \n--    listToMaybe :: [a] -> Maybe a \n-- tal que (listToMaybe xs) es Nothing si la lista xs est\u00e1 vac\u00eda y es\n-- justo el primer elemento, en caso contrario, Por ejemplo,\n--    listToMaybe []       ==  Nothing\n--    listToMaybe [3,2,5]  ==  Just 3\n\n-- Comparaci\u00f3n de eficiencia\n-- =========================\n\n-- La comparaci\u00f3n es\n--    ghci> let n=10^6 in siguiente1 (n-1) [1..n]\n--    Just 1000000\n--    (1.34 secs, 277352616 bytes)\n--    \n--    ghci> let n=10^6 in siguiente2 (n-1) [1..n]\n--    Just 1000000\n--    (1.45 secs, 340836576 bytes)\n--    \n--    ghci> let n=10^6 in siguiente3 (n-1) [1..n]\n--    Just 1000000\n--    (0.26 secs, 84987544 bytes)\n--    \n--    ghci> let n=10^6 in siguiente4 (n-1) [1..n]\n--    Just 1000000\n--    (0.26 secs, 84987440 bytes)\n<\/pre>\n","protected":false},"excerpt":{"rendered":"
Definir la funci\u00f3n siguiente :: Eq a => a -> [a] -> Maybe a tal que (siguiente x ys) es justo el elemento siguiente a la primera ocurrencia de x en ys o Nothing si x no pertenece a ys. Por ejemplo, siguiente 5 [3,5,2,5,7] == Just 2 siguiente 9 [3,5,2,5,7] == Nothing siguiente ‘d’…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_kad_post_transparent":"","_kad_post_title":"","_kad_post_layout":"","_kad_post_sidebar_id":"","_kad_post_content_style":"","_kad_post_vertical_padding":"","_kad_post_feature":"","_kad_post_feature_position":"","_kad_post_header":false,"_kad_post_footer":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"footnotes":"","_jetpack_memberships_contains_paid_content":false},"categories":[5],"tags":[8,500,46,59,246,11,6,45,9],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1030"}],"collection":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/comments?post=1030"}],"version-history":[{"count":5,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1030\/revisions"}],"predecessor-version":[{"id":1798,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1030\/revisions\/1798"}],"wp:attachment":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/media?parent=1030"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/categories?post=1030"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/tags?post=1030"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}