Hi Toby,
Thanks for the helpful comments. I'd gotten used to arithmetic operator
sections (+2), (*2), etc. but hadn't picked up on the generality of using them
with *any* infix function. I can also see the benefit of using List.Group.
However, I'm uncertain about how to import just fromList and ! from with the
imports I'm using
import Data.Map (Map) (fromList,!) ???
import qualified Data.Map as Map (fromList,!) ???
Michael
--- On Mon, 6/8/09, Toby Hutton <toby.hut...@gmail.com> wrote:
From: Toby Hutton <toby.hut...@gmail.com>
Subject: Re: [Haskell-cafe] Applying Data.Map
To: "michael rice" <nowg...@yahoo.com>
Cc: haskell-cafe@haskell.org
Date: Monday, June 8, 2009, 8:57 PM
Although in this example using Data.Map is overkill, if the alphabet was very
large then Data.Map probably would be the way to go. In that case I'd use:
map head . group . sort instead of nub . sort
since it's noticeably quicker for large lists. This is because nub needs to
preserve the order of input, removing redundancies, but you're sorting it
anyway.
Also, in map (\c -> m Map.! c) s you can use the 'section' (m Map.!) instead.
e.g., map (m Map.!) s
The Map.! is ugly though. As you're only using fromList and (!) from Data.Map,
I'd just import those explicitly since they don't clash with Prelude. Then
you'd have map (m !) s
Toby.
On Tue, Jun 9, 2009 at 4:59 AM, michael rice <nowg...@yahoo.com> wrote:
I wrote a Haskell solution for the Prolog problem stated below. I had written a
function SQUISH before discovering that NUB does the same thing. While the
solution works, I thought maybe I could apply some functions in the Data.Map
module, and so wrote a second version of SERIALIZE, one no longer needing
TRANSLATE. Using the Data.Map module is probably overkill for this particular
problem, but wanted to familiarize myself with Map type. Suggestions welcome.
Prolog code also included below for those interested.
Michael
===========
{-
From "Prolog By Example", Coelho, Cotta, Problem 42, pg. 63
Verbal statement:
Generate a list of serial numbers for the items of a given
list,
the members of which are to be numbered in alphabetical order.
For example, the list [p,r,o,l,o,g] must generate [4,5,3,2,3,1]
-}
{-
Prelude> :l
serialize
[1 of 1] Compiling Main ( serialize.hs, interpreted )
Ok, modules loaded: Main.
*Main> serialize "prolog"
[4,5,3,2,3,1]
*Main>
-}
===========Haskell code==========
import Data.Char
import Data.List
import Data.Map (Map)
import qualified Data.Map as
Map
{-
translate :: [Char] -> [(Char,Int)] -> [Int]
translate [] _ = []
translate (x:xs) m = (fromJust (lookup x m)) : (translate xs m )
-}
{-
serialize :: [Char] -> [Int]
serialize s = let c = nub $ sort s
n = [1..(length c)]
in translate s (zip c n)
-}
serialize :: [Char] -> [Int]
serialize s = let c = nub $ sort s
n = [1..(length c)]
m = Map.fromList $ zip c n
in map (\c -> m Map.! c) s
============Prolog code============
serialize(L,R) :- pairlists(L,R,A),arrange(A,T),
numbered(T,1,N).
? <- typo?
pairlists([X|L],[Y|R],[pair(X,Y)|A]) :- pairlist(L,R,A).
pairlists([],[],[]).
arrange([X|L],tree(T1,X,T2)) :- partition(L,X,L1,L2),
arrange(L1,T1),
arrange(L2,T2).
arrange([],_).
partition([X|L],X,L1,L2) :- partition(L,X,L1,L2).
partition([X|L],Y,[X|L1],L2) :- before(X,Y),
partition(L,Y,L1,L2).
partition([X|L],Y,L1,[X|L2]) :- before(Y,X),
partition(L,Y,L1,L2).
partition([],_,[],[]).
before(pair(X1,Y1),pair(X2,Y2)) :- X1<X2.
numbered(tree(T1,pair(X,N1),T2),N0,N) :- numbered(T1,N0,N1),
N2 is N1+1,
numbered(T2,N2,N).
numbered(void,N,N).
============Prolog examples========
Execution:
?- serialize([p,r,o,l,o,g]).
[4,5,3,2,3,1]
?- serialize ([i,n,t,.,a,r,t,i,f,i,c,i,a,l]).
[5,7,9,1,2,8,9,5,4,5,3,5,2,6]
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