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
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