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Today's Topics:
1. randmomR produces only even values (martin)
2. Re: randmomR produces only even values (Lyndon Maydwell)
3. Re: randmomR produces only even values (martin)
----------------------------------------------------------------------
Message: 1
Date: Fri, 30 Oct 2015 19:43:23 +0100
From: martin <martin.drautzb...@web.de>
To: beginners@haskell.org
Subject: [Haskell-beginners] randmomR produces only even values
Message-ID: <5633ba4b.7030...@web.de>
Content-Type: text/plain; charset=utf-8
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?
------------------------------
Message: 2
Date: Sat, 31 Oct 2015 09:40:24 +1100
From: Lyndon Maydwell <maydw...@gmail.com>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] randmomR produces only even values
Message-ID:
<CAM5QZtzeQcT0QOOQbQuexNAqKNgQUTkOHB4aGQwk=ffhrri...@mail.gmail.com>
Content-Type: text/plain; charset="utf-8"
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 <martin.drautzb...@web.de> wrote:
> 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
>
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Message: 3
Date: Sat, 31 Oct 2015 09:32:58 +0100
From: martin <martin.drautzb...@web.de>
To: The Haskell-Beginners Mailing List - Discussion of primarily
beginner-level topics related to Haskell <beginners@haskell.org>
Subject: Re: [Haskell-beginners] randmomR produces only even values
Message-ID: <56347cba.8030...@web.de>
Content-Type: text/plain; charset=windows-1252
I filed an issue on github (https://github.com/haskell/random/issues). Maybe
the authors can shed some light on this.
Am 10/30/2015 um 11:40 PM schrieb Lyndon Maydwell:
> 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 <martin.drautzb...@web.de
> <mailto:martin.drautzb...@web.de>> wrote:
>
> 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 <mailto:Beginners@haskell.org>
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
>
>
>
> _______________________________________________
> Beginners mailing list
> Beginners@haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners
>
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