{"id":1179,"date":"2015-03-10T06:00:16","date_gmt":"2015-03-10T04:00:16","guid":{"rendered":"http:\/\/www.glc.us.es\/~jalonso\/exercitium\/?p=1179"},"modified":"2016-05-02T09:06:39","modified_gmt":"2016-05-02T07:06:39","slug":"actualizacion-de-una-lista","status":"publish","type":"post","link":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/actualizacion-de-una-lista\/","title":{"rendered":"Actualizaci\u00f3n de una lista"},"content":{"rendered":"<p>Definir la funci\u00f3n<\/p>\n<pre lang=\"text\">\n   actualiza :: [a] -> [(Int,a)] -> [a]\n<\/pre>\n<p>tal que (actualiza xs ps) es la lista obtenida sustituyendo en xs los elementos cuyos \u00edndices son las primeras componentes de ps por las segundas. Por ejemplo,<\/p>\n<pre lang=\"text\">\n   actualiza [3,5,2,4] [(2,1),(0,7)]  ==  [7,5,1,4]\n   sum (actualiza [1..10^4] [(i,2*i) | i <- [0..10^4-1]]) == 99990000\n<\/pre>\n<h4>Soluciones<\/h4>\n<pre lang=\"haskell\">\nimport qualified Data.Vector as V\nimport Data.Array \n\n-- 1\u00aa soluci\u00f3n (por recursi\u00f3n)\n-- ===========================\n\nactualiza :: [a] -> [(Int,a)] -> [a]\nactualiza xs []         = xs\nactualiza xs ((n,y):ps) = actualiza (actualizaE xs (n,y)) ps\n\n-- (actualizaE xs (n,y)) es la lista obtenida sustituyendo el  elemento\n-- n-\u00e9simo de xs por y. Por ejemplo, \n--    actualiza [3,5,2,4] (2,1) ==  [3,5,1,4]\nactualizaE :: [a] -> (Int,a) -> [a]\nactualizaE xs (n,y) =\n    take n xs ++ y : drop (n+1) xs\n\n-- 2\u00aa soluci\u00f3n (por tablas)\n-- ========================\n\nactualiza2 :: [a] -> [(Int,a)] -> [a]\nactualiza2 xs ps = \n    elems (listArray (0,length xs - 1) xs \/\/ ps)\n\n-- 3\u00aa soluci\u00f3n (por vectores)\n-- ==========================\n\nactualiza3 :: [a] -> [(Int,a)] -> [a]\nactualiza3 xs ps = \n    V.toList (V.fromList xs V.\/\/ ps)\n\n-- Comparaci\u00f3n de eficiencia\n--    ghci> let n = 10^2 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]\n--    9900\n--    (0.02 secs, 4668984 bytes)\n--    ghci> let n = 10^3 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]\n--    999000\n--    (0.28 secs, 77454496 bytes)\n--    ghci> let n = 10^4 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]\n--    99990000\n--    (63.46 secs, 8769501704 bytes)\n--    \n--    ghci> let n = 10^2 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    9900\n--    (0.02 secs, 4147304 bytes)\n--    ghci> let n = 10^3 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    999000\n--    (0.02 secs, 4694784 bytes)\n--    ghci> let n = 10^4 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    99990000\n--    (0.05 secs, 12276800 bytes)\n--    \n--    ghci> let n = 10^2 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    9900\n--    (0.01 secs, 4147304 bytes)\n--    ghci> let n = 10^3 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    999000\n--    (0.02 secs, 4682680 bytes)\n--    ghci> let n = 10^4 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]\n--    99990000\n--    (0.04 secs, 12595224 bytes)\n<\/pre>\n","protected":false},"excerpt":{"rendered":"<p>Definir la funci\u00f3n actualiza :: [a] -> [(Int,a)] -> [a] tal que (actualiza xs ps) es la lista obtenida sustituyendo en xs los elementos cuyos \u00edndices son las primeras componentes de ps por las segundas. Por ejemplo, actualiza [3,5,2,4] [(2,1),(0,7)] == [7,5,1,4] sum (actualiza [1..10^4] [(i,2*i) | i [(Int,a)] -> [a] actualiza xs [] =&#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":[5],"tags":[46,245,347,28,72,42,6,47,260,53],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1179"}],"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=1179"}],"version-history":[{"count":6,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1179\/revisions"}],"predecessor-version":[{"id":2387,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/1179\/revisions\/2387"}],"wp:attachment":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/media?parent=1179"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/categories?post=1179"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/tags?post=1179"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}