Definir la funci\u00f3n<\/p>\n
\n nParejas :: Ord a => [a] -> Int\n<\/pre>\ntal que (nParejas xs) es el n\u00famero de parejas de elementos iguales en xs. Por ejemplo,<\/p>\n
\n nParejas [1,2,2,1,1,3,5,1,2] == 3\n nParejas [1,2,1,2,1,3,2] == 2\n nParejas [1..2*10^6] == 0\n nParejas2 ([1..10^6] ++ [1..10^6]) == 1000000\n<\/pre>\nEn el primer ejemplos las parejas son (1,1), (1,1) y (2,2). En el segundo ejemplo, las parejas son (1,1) y (2,2).<\/p>\n
Comprobar con QuickCheck que para toda lista de enteros xs, el n\u00famero de parejas de xs es igual que el n\u00famero de parejas de la inversa de xs.<\/p>\n
Soluciones<\/h4>\n\nimport Test.QuickCheck\nimport Data.List ((\\\\), group, sort)\n\n-- 1\u00aa soluci\u00f3n\nnParejas :: Ord a => [a] -> Int\nnParejas [] = 0\nnParejas (x:xs) | x `elem` xs = 1 + nParejas (xs \\\\ [x])\n | otherwise = nParejas xs\n\n-- 2\u00aa soluci\u00f3n\nnParejas2 :: Ord a => [a] -> Int\nnParejas2 xs =\n sum [length ys `div` 2 | ys <- group (sort xs)]\n\n-- 3\u00aa soluci\u00f3n\nnParejas3 :: Ord a => [a] -> Int\nnParejas3 = sum . map (`div` 2). map length . group . sort\n\n-- 4\u00aa soluci\u00f3n\nnParejas4 :: Ord a => [a] -> Int\nnParejas4 = sum . map ((`div` 2) . length) . group . sort\n\n-- Equivalencia\nprop_equiv :: [Int] -> Bool\nprop_equiv xs =\n nParejas xs == nParejas2 xs &&\n nParejas xs == nParejas3 xs &&\n nParejas xs == nParejas4 xs \n\n-- Comprobaci\u00f3n:\n-- \u03bb> quickCheck prop_equiv\n-- +++ OK, passed 100 tests.\n\n-- Comparaci\u00f3n de eficiencia\n-- \u03bb> nParejas [1..20000]\n-- 0\n-- (2.54 secs, 4,442,808 bytes)\n-- \u03bb> nParejas2 [1..20000]\n-- 0\n-- (0.03 secs, 12,942,232 bytes)\n-- \u03bb> nParejas3 [1..20000]\n-- 0\n-- (0.02 secs, 13,099,904 bytes)\n-- \u03bb> nParejas4 [1..20000]\n-- 0\n-- (0.01 secs, 11,951,992 bytes)\n\n-- Propiedad:\nprop_nParejas :: [Int] -> Bool\nprop_nParejas xs =\n nParejas xs == nParejas (reverse xs)\n\n-- Comprobaci\u00f3n:\n-- \u03bb> quickCheck prop_nParejas\n-- +++ OK, passed 100 tests.\n<\/pre>\nPensamiento<\/h4>\n\nToda la imaginer\u00eda
\nque no ha brotado del r\u00edo,
\nbarata bisuter\u00eda.<\/p>\n
Antonio Machado\n<\/p><\/blockquote>\n","protected":false},"excerpt":{"rendered":"
Definir la funci\u00f3n nParejas :: Ord a => [a] -> Int tal que (nParejas xs) es el n\u00famero de parejas de elementos iguales en xs. Por ejemplo, nParejas [1,2,2,1,1,3,5,1,2] == 3 nParejas [1,2,1,2,1,3,2] == 2 nParejas [1..2*10^6] == 0 nParejas2 ([1..10^6] ++ [1..10^6]) == 1000000 En el primer ejemplos las parejas son (1,1), (1,1) y…<\/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":[8,30,26,13,28,6,32,14,40,146],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/4313"}],"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=4313"}],"version-history":[{"count":6,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/4313\/revisions"}],"predecessor-version":[{"id":4597,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/posts\/4313\/revisions\/4597"}],"wp:attachment":[{"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/media?parent=4313"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/categories?post=4313"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.glc.us.es\/~jalonso\/exercitium\/wp-json\/wp\/v2\/tags?post=4313"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}