On 30 Jan 2010, at 20:59, michael rice wrote:
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. Seems like a roundabout way of doing it. Comments?
Below is a function draw() that shuffles an interval, and prints it out. Hans ---- import Random getRandomIndex :: [a] -> IO(Int) getRandomIndex ls = getStdRandom(randomR(0, (length ls) - 1)) remove :: Int -> [a] -> [a] remove 0 (x:xs) = xs remove n (x:xs) | n>0 = x: remove (n-1) xs remove _ (_:_) = error "remove: negative argument" remove _ [] = error "remove: too large argument" shuffle :: [a] -> IO [a] shuffle [] = return [] shuffle ls = do i <- getRandomIndex ls do l <- shuffle (remove i ls) return ((ls !! i) : l) draw ls = do k <- shuffle ls putStr (show k) ----