Am Dienstag 10 November 2009 22:09:36 schrieb legajid:
Hello, i'm trying to manipulate lists with a "complex" (two-level) structure : a list containing tuples, containing a list containing tuples. The object is to find a specific value according to 2 criterias.
Here is my code
tabmul= [ ( a, [ ( n, n*a ) | n <- [1..9] ] ) | a <- [2,3,4] ]
{- gives the following list [ (2, [(1,2), (2,4), ....(9,18)]), (3, [(1,3), (2,6), ....(9,27)]), (4, [(1,4), (2,8), ....(9,36)]) ] -}
{- Get the result for a=3 and n=5 -}
-- select level 1 tuple for a=3 s1_flt (x,_) = (x==3) s1=filter (s1_flt) tabmul -- get the list [(3, [(), () .......])]
filter ((== 3) . fst) tabmul
s1_liste =head(s1) -- get the tuple (3, [.....])
You might want to use Data.List.lookup :: (Eq a) => a -> [(a,b)] -> Maybe b lookup 3 tabmul ~> Just [(1,3),...]
-- extract the level 2 list of tuples s1_tup (x,y)=y
That's Prelude.snd (fst (a,b) = a; snd (a,b) = b)
-- then tuple for n=5 s1_flt2 (x, y) = x==5
(== 5) . fst
s1_soltup = filter (s1_flt2) (s1_tup s1_liste) -- [(3,15)]
--finally result for 3 * 5 s1_sol1 (x, y)= y s1_sol = s1_sol1 (head s1_soltup)
lookup 3 tabmul >>= lookup 5 or do listetrois <- lookup 3 tabmul lookup 5 listetrois ~> Just 15
Perhaps the structure is not the most efficient for this example, but it may simulate records in a database. Getting the result seems really hard. Do you know a shorter way to implement this search? It probably would be simpler when i had triples (a, n ,a*n) ?
Not necessarily, there are fst :: (a,b) -> a and snd :: (a,b) -> b in the Prelude, but not the analogous functions for triples or larger tuples. head . filter ((== val) . fst) is pretty simple, but if you want to use association lists as maps, lookup is even more handy.
Another question : the values of criterias are hard-coded. What if i would like to type in s1_sol 3 5; how to put these parameters in the expressions for filters; the filters must get the parameters of the function, in other words the function should return or generate filters ?
résultat x y = lookup x tabmul >>= lookup y résultat x y = do liste <- lookup x tabmul lookup y liste résultat x y = head . filter ((== y) . fst) . snd . filter ((== x) . fst) $ tabmul In my book, the first two are clear winners.
Thanks for helping Didier.