I've replicated this and it does seem very strange, and possibly even a bug.
I would guess that most people don't encounter this issue as a generator is
usually only seeded once, then threaded throughout generation. Not seeded
for once for every random output. The other common practice is that
generators are usually seeded on far more random input values than a list
of ascending ints.
Seed behaviour:
main = mapM_ print (map p l)
p x = length $ filter even $ map (\s -> fst $ randomR (1::Int,10)
(mkStdGen s)) x
l = map ls [1..10]
ls x = map (\y -> x * y * 10) [1..1000]
1000
1000
1000
1000
1000
894
766
670
596
536
Still, I am very surprised by this behaviour. I couldn't find any reference
to this behaviour in[1], which is supposedly what the System.Random
implementation is based on.
Does anyone else have an explanation?
- Lyndon
[1] - https://en.wikipedia.org/wiki/Linear_congruential_generator
On Sat, Oct 31, 2015 at 5:43 AM, martin
Hello all
When I read:
mkStdGen :: Int -> StdGen
The function mkStdGen provides an alternative way of producing an initial generator, by mapping an Int into a generator. Again, distinct arguments should be likely to produce distinct generators.
I thought, that
fst $ R.randomR (1,10) (R.mkStdGen s)
should get me a random value between 1 and 10 and that I get different values depending on the seed s. But this
length $ filter even $ map (\s -> fst $ randomR (1::Int,10) (mkStdGen s))[1..1000]
gives my 1000, i.e. all random numbers are even numbers.
However, when I use random instead of randomR
length $ filter even $ map (\s -> fst $ random (mkStdGen s) ::Int)[1..1000]
I get 499 (okay)
Why is that so? _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners