Re: [Haskell-cafe] Generating random graph
Hi Mitar, > 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 Others have already remarked that you could implement this as a pure function. However, the mistake is the use of getStdGen here, which is (almost?) never what you need: two consecutive valls of getStdGen will return the same generator. You should call newStdGen instead. Best regards, Bertram ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generating random graph
Hi! On Mon, Apr 11, 2011 at 7:36 AM, Steffen Schuldenzucker wrote: > 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. I thought it would be interleaved. Thanks. Mitar ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generating random graph
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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Generating random graph
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-Random.html 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Generating random graph
Hi! 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 return $ zip3 (repeat node) others weights return $ mkGraph ns es 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
