{"id":3536,"date":"2017-12-19T06:00:12","date_gmt":"2017-12-19T04:00:12","guid":{"rendered":"http:\/\/www.glc.us.es\/~jalonso\/exercitium\/?p=3536"},"modified":"2022-03-26T12:10:58","modified_gmt":"2022-03-26T10:10:58","slug":"vecino-en-lista-circular","status":"publish","type":"post","link":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/vecino-en-lista-circular\/","title":{"rendered":"Vecino en lista circular"},"content":{"rendered":"<p>En la lista circular [3,2,5,7,9]<\/p>\n<ul>\n<li>el vecino izquierdo de 5 es 2 y su vecino derecho es 7,<\/li>\n<li>el vecino izquierdo de 9 es 7 y su vecino derecho es 3,<\/li>\n<li>el vecino izquierdo de 3 es 9 y su vecino derecho es 2,<\/li>\n<li>el elemento 4 no tiene vecinos (porque no est\u00e1 en la lista).<\/li>\n<\/ul>\n<p>Para indicar las direcciones se define el tipo de datos<\/p>\n<pre lang=\"text\">\n   data Direccion = I | D deriving Eq\n<\/pre>\n<p>Definir la funci\u00f3n<\/p>\n<pre lang=\"text\">\n   vecino :: Eq a => Direccion -> [a] -> a -> Maybe a\n<\/pre>\n<p>tal que (vecino d xs x) es el vecino de x en la lista de elementos distintos xs seg\u00fan la direcci\u00f3n d. Por ejemplo,<\/p>\n<pre lang=\"text\">\n   vecino I [3,2,5,7,9] 5  ==  Just 2\n   vecino D [3,2,5,7,9] 5  ==  Just 7\n   vecino I [3,2,5,7,9] 9  ==  Just 7\n   vecino D [3,2,5,7,9] 9  ==  Just 3\n   vecino I [3,2,5,7,9] 3  ==  Just 9\n   vecino D [3,2,5,7,9] 3  ==  Just 2\n   vecino I [3,2,5,7,9] 4  ==  Nothing\n   vecino D [3,2,5,7,9] 4  ==  Nothing\n<\/pre>\n<h4>Soluciones<\/h4>\n<pre lang=\"haskell\">\ndata Direccion = I | D deriving Eq\n\n-- 1\u00aa soluci\u00f3n\n-- ===========\n\nvecino1 :: Eq a => Direccion -> [a] -> a -> Maybe a\nvecino1 d xs x = busca x (vecinos d xs)\n\n-- (vecinos d xs) es la lista de elementos de xs y sus vecinos seg\u00fan la\n-- direccio\u0144 d. Por ejemplo,\n--    vecinos I [1..5]  ==  [(2,1),(3,2),(4,3),(5,4),(1,5)]\n--    vecinos D [1..5]  ==  [(1,2),(2,3),(3,4),(4,5),(5,1)]\nvecinos :: Direccion -> [a] -> [(a,a)]\nvecinos I xs = zip (tail (cycle xs)) xs\nvecinos D xs = zip xs (tail (cycle xs))\n\n-- (busca x ps) es el la segunda componente de primer par de ps cuya\n-- primera componente es igual a x. Por ejemplo, \n--    busca 3 [(4,1),(3,2),(3,7)]  ==  Just 2\n--    busca 7 [(4,1),(3,2),(3,7)]  ==  Nothing\nbusca :: Eq a => a -> [(a,b)] -> Maybe b\nbusca x ps\n  | null zs   = Nothing\n  | otherwise = Just (head zs)\n  where zs = [z | (x',z) <- ps, x' == x]\n\n-- 2\u00aa soluci\u00f3n\n-- ===========\n\nvecino2 :: Eq a => Direccion -> [a] -> a -> Maybe a\nvecino2 d xs x = lookup x (vecinos d xs)\n\n-- 3\u00aa soluci\u00f3n\n-- ===========\n\nvecino3 :: Eq a => Direccion -> [a] -> a -> Maybe a\nvecino3 I xs x = lookup x (zip (tail (cycle xs)) xs) \nvecino3 D xs x = lookup x (zip xs (tail (cycle xs))) \n<\/pre>\n","protected":false},"excerpt":{"rendered":"<p>En la lista circular [3,2,5,7,9] el vecino izquierdo de 5 es 2 y su vecino derecho es 7, el vecino izquierdo de 9 es 7 y su vecino derecho es 3, el vecino izquierdo de 3 es 9 y su vecino derecho es 2, el elemento 4 no tiene vecinos (porque no est\u00e1 en la&#8230;<\/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":[4],"tags":[8,166,500,71,414,411,141,45,9],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/3536"}],"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=3536"}],"version-history":[{"count":2,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/3536\/revisions"}],"predecessor-version":[{"id":3571,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/3536\/revisions\/3571"}],"wp:attachment":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/media?parent=3536"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/categories?post=3536"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/tags?post=3536"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}