Hello. I don't know if that is the reason for the strange behaviour, but On 04/11/2011 03:03 AM, Mitar wrote:

I have made this function to generate a random graph for Data.Graph.Inductive library:

generateGraph :: Int -> IO (Gr String Double) generateGraph graphSize = do when (graphSize< 1) $ throwIO $ AssertionFailed $ "Graph size out of bounds " ++ show graphSize let ns = map (\n -> (n, show n)) [1..graphSize] es<- fmap concat $ forM [1..graphSize] $ \node -> do nedges<- randomRIO (0, graphSize) others<- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ -> randomRIO (1, graphSize) gen<- getStdGen let weights = randomRs (1, 10) gen

^ this use of randomRs looks wrong.

return $ zip3 (repeat node) others weights return $ mkGraph ns es

http://hackage.haskell.org/packages/archive/random/latest/doc/html/System-Ra... tells me: randomRs :: RandomGen g => (a, a) -> g -> [a] Plural variant of randomR, producing an infinite list of random values instead of returning a new generator. So when using randomRs, the state of the global random number generator is not updated, but it is used again in the next iteration of the toplevel forM [1..graphSize] loop. Try:

weights <- replicateM (length others) $ randomRIO (1, 10)

instead. -- Steffen

But I noticed that graph has sometimes same weights on different edges. This is very unlikely to happen so probably I have some error using random generators. Could somebody tell me where?

Mitar

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On Mon, 11 Apr 2011, Mitar wrote:

generateGraph :: Int -> IO (Gr String Double) generateGraph graphSize = do when (graphSize < 1) $ throwIO $ AssertionFailed $ "Graph size out of bounds " ++ show graphSize let ns = map (\n -> (n, show n)) [1..graphSize] es <- fmap concat $ forM [1..graphSize] $ \node -> do nedges <- randomRIO (0, graphSize) others <- fmap (filter (node /=) . nub) $ forM [1..nedges] $ \_ -> randomRIO (1, graphSize) gen <- getStdGen let weights = randomRs (1, 10) gen return $ zip3 (repeat node) others weights return $ mkGraph ns es

Just a note on style: This function can perfectly be written without IO. http://www.haskell.org/haskellwiki/Avoiding_IO#State_monad