{"id":974,"date":"2015-01-16T06:00:00","date_gmt":"2015-01-16T04:00:00","guid":{"rendered":"http:\/\/www.glc.us.es\/~jalonso\/exercitium\/?p=974"},"modified":"2015-04-26T20:04:25","modified_gmt":"2015-04-26T18:04:25","slug":"ordenacion-segun-una-funcion","status":"publish","type":"post","link":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/ordenacion-segun-una-funcion\/","title":{"rendered":"Ordenaci\u00f3n seg\u00fan una funci\u00f3n"},"content":{"rendered":"<p>Definir la funci\u00f3n<\/p>\n<pre lang=\"text\">\n   ordenaSegun :: Ord b => (a -> b) -> [a] -> [a]\n<\/pre>\n<p>tal que (ordenaSegun f xs) es la lista obtenida ordenando los elementos de xs seg\u00fan sus valores mediante la funci\u00f3n f. Por ejemplo,<\/p>\n<pre lang=\"text\">\n   ordenaSegun abs [-3,2,5,-2]                           ==  [2,-2,-3,5]\n   ordenaSegun abs [-3,-2,5,2]                           ==  [-2,2,-3,5]\n   ordenaSegun length [\"pq\",\"x\",\"mn\"]                    ==  [\"x\",\"pq\",\"mn\"]\n   [g 0 | g <- ordenaSegun (\\f -> f 4) [(+5),(+2),(+3)]] == [2,3,5]\n<\/pre>\n<p>Comprobar con QuickCheck que (ordenaSegun id) es equivalente a sort.<\/p>\n<h4>Soluciones<\/h4>\n<pre lang=\"haskell\">\nimport Data.List (sort, sortBy)\nimport Data.Ord (comparing)\nimport Test.QuickCheck\n\n-- 1\u00aa definici\u00f3n\n-- =============\n\nordenaSegun :: Ord b => (a -> b) -> [a] -> [a]\nordenaSegun _ []  = []\nordenaSegun f (x:xs) = insertaSegun f x (ordenaSegun f xs)\n\ninsertaSegun :: Ord b => (a -> b) -> a -> [a] -> [a]\ninsertaSegun _ x [] = [x]\ninsertaSegun f x (y:ys) | f x <= f y = x : y : ys\n                        | otherwise  = y : insertaSegun f x ys\n\n-- 2\u00aa definici\u00f3n\n-- =============\n\nordenaSegun2 :: Ord b => (a -> b) -> [a] -> [a]\nordenaSegun2 f xs = [xs!!i | (_,i) <- sort (zip [f x | x <- xs] [0..])]\n\n-- 3\u00aa definici\u00f3n\n-- =============\n\nordenaSegun3 :: Ord b => (a -> b) -> [a] -> [a]\nordenaSegun3 f xs = map ((xs!!) . snd) (sort (zip (map f xs) [0..]))\n\n-- 4\u00aa definici\u00f3n\n-- =============\n\nordenaSegun4 :: Ord b => (a -> b) -> [a] -> [a]\nordenaSegun4 = sortBy . comparing\n\n-- La propiedad es\nprop_ordenaSegun :: Ord a => [a] -> Bool\nprop_ordenaSegun xs =\n    ordenaSegun id xs == sort xs\n\n-- La comprobaci\u00f3n es\n--    ghci> quickCheck prop_ordenaSegun\n--    +++ OK, passed 100 tests.\n<\/pre>\n","protected":false},"excerpt":{"rendered":"<p>Definir la funci\u00f3n ordenaSegun :: Ord b => (a -> b) -> [a] -> [a] tal que (ordenaSegun f xs) es la lista obtenida ordenando los elementos de xs seg\u00fan sus valores mediante la funci\u00f3n f. Por ejemplo, ordenaSegun abs [-3,2,5,-2] == [2,-2,-3,5] ordenaSegun abs [-3,-2,5,2] == [-2,2,-3,5] ordenaSegun length [\u00abpq\u00bb,\u00bbx\u00bb,\u00bbmn\u00bb] == [\u00abx\u00bb,\u00bbpq\u00bb,\u00bbmn\u00bb] [g 0&#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":[244,8,63,10,11,6,16,14,159,146,9],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/974"}],"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=974"}],"version-history":[{"count":6,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/974\/revisions"}],"predecessor-version":[{"id":1374,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/974\/revisions\/1374"}],"wp:attachment":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/media?parent=974"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/categories?post=974"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/tags?post=974"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}