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Scripting Games in Haskell
- February 21st, 2008
I saw the Cerebrate solve the first Scripting Games challenge: Pairing off. And immediately thought “I can do that in Haskell too”.
So, here it is.
import Data.List cards = [(1,7),(0,5),(3,7),(2,7),(2,13)] countpairs [] = 0 countpairs [a] = 0 countpairs (a:as) = length . filter (((snd a)==) . snd) $ as pairingOff = sum . map countpairs . tails
And that’s that. Alas, the actual competition only takes Perl, VBScript and PowerShell, so I won’t be submitting this.
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Michi is a recent PhD working in homological algebra and applied algebraic topology. This blog is his outlet for texts with some manner of thought put into them. Over at his LiveJournal intimate details and streams of consciousness might be found.
Not all here is mathematics. All here, though, are my personal thoughts and opinions. Please read the about page (linked above) for more details.
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4 People had this to say...
I also like the following:
import Data.List (sort, group)
cards :: [(Int, Int)]
cards = [(1,7),(0,5),(3,7),(2,7),(2,13)]
pairs :: [(Int, Int)] -> Int
pairs = sum . map ((`choose` 2) . length) . group . sort . map snd
-- | Binomial coefficient
-- http://en.wikipedia.org/wiki/Binomial_coefficient
choose :: (Integral a) => a -> a -> a
n `choose` k | n >= k && k >= 0 = fac n `div` (fac k * fac (n - k))
| otherwise = 0
fac :: (Num t) => t -> t
fac 0 = 1
fac n = n * fac (n-1)
(’choose’ and ‘fac’ don’t actually belong in this script but should be part of a larger mathematical library.)
Just for reference, here is my python solution
cards = [(1,7),(0,5),(3,7),(2,7),(2,13)]
print sum([(n*(n-1)/2) for n in [[l for _,l in cards].count(x) for x in xrange(1,14)] if n>1])
I have to poke around for functional language ports to the .NET Framework some more. They make the code so much more short and elegant.
Alistair: You might want to look into F#. I haven’t yet, but it’s a seriously functional language, integrated in the .NET Framework. Its development group is backed by some of the Great Minds behind Haskell.
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