Am Samstag 30 Januar 2010 20:59:08 schrieb michael rice:
I'm not sure where I got this PICK function from, and don't understand why it's written as it is, so I wanted to test it for randomness. It seems random enough. But if I understand the algorithm correctly, instead of selecting one of the elements from the list, it eliminates all the elements but one and that's the value it returns.
Yep.
Seems like a roundabout way of doing it. Comments?
Indeed.
Also, is there a more direct way of printing an array?
Sure, printing immutable arrays: print arr ~> array (lo,hi) [(lo,arr!lo), ... , (hi,arr!hi)] print (assocs arr) ~> [(lo,arr!lo), ... , (hi,arr!hi)] print (elems arr) ~> [(arr!lo), ... , (arr!hi)] printing IO[U]Arrays: do immArr <- freeze arr print (immArr :: [U]Array ix el) do ass <- getAssocs arr print ass (getAssocs arr >>= print) do els <- getElems arr print els (getElems arr >>= print) or, to get output like below: getElems arr >>= mapM_ print Printing ST[U]Arrays would need an unsafeIOToST, but reasonably, you wouldn't want to print them before you've left ST.
Output below.
Michael
=================
import System.Random import Data.Array.IO
pick :: [a] -> IO a pick [] = undefined pick [x] = do return x pick (x:xs) = pick' x xs (2 :: Int)
pick' :: (Num p, Random p) => t -> [t] -> p -> IO t pick' curr [] _ = do return curr pick' curr (next:rest) prob = do r <- getStdRandom (randomR (1,prob)) let curr' = if r == 1 then next else curr pick' curr' rest (prob+1)
main = do arr <- newArray (1,9) 0 :: IO (IOArray Int Int) doLoop arr [1,2,3,4,5,6,7,8,9] 0 doLoop arr z k = do p <- pick z a <- readArray arr p writeArray arr p (a+1) if k > 10000 then do v <- readArray arr 1 print v v <- readArray arr 2 print v v <- readArray arr 3 print v v <- readArray arr 4 print v v <- readArray arr 5 print v v <- readArray arr 6 print v v <- readArray arr 7 print v v <- readArray arr 8 print v v <- readArray arr 9 print v else do doLoop arr z (k+1)
===============
[michael@localhost ~]$ runhaskell array1.hs 1110 1117 1080 1169 1112 1119 1137 1084 1074 [michael@localhost ~]$