[Haskell-beginners] System.Random

[Thomas Friedrich]

Hi,

thanks for the web-link.  The idea of putting the whole thing into a 
random shuffling framework is in deed a really neat idea.  And it 
perfectly suits my purpose.  I don't need it for QuickCheck, it was just 
an idea.

Cheers,
Thomas



Regis Saint-Paul wrote:
> Hi, 
>
> Maybe you'd like to check this page: 
> http://apfelmus.nfshost.com/random-permutations.html
> which gives also interesting pointers to other references. 
>
> Your sampling from a list can be seen as taking the first n elements of a
> permutation of the list. Of course, computing the full permutation of a long
> list to just take the first two or three elements would not be very wise. So
> there are other solutions depending on your setting (sampling from a finite
> list with or without replacement, sampling from an infinite list (a stream)
> as data comes in, etc...
> I just give some rough idea: 
> - for fixed size lists, a random number can be used to skip some elements of
> the list, if you start from the beginning of the list after reaching its
> end, then you sample with replacement. 
> - for infinite list, a list of the n sampled element is maintained. This
> list is first filled with the first n elements of the source list and then,
> for each new incoming element, you decide randomly if you want to keep it or
> not. If you decide to keep it, you decide, also randomly, which element of
> the sample list it replaces. You can find all the details in [1]. 
>
> Do you need that for performing tests with QuickCheck or was using
> QuickCheck just an idea on how to go about the problem?
>
> Best,
> Regis
>
> [1] Vitter, Jeffrey S. 1985. "Random sampling with a reservoir." ACM Trans.
> Math. Softw. 11(1):37-57.
>
>
>
>
>
>   
>> -----Original Message-----
>> From: beginners-bounces at haskell.org [mailto:beginners-bounces at haskell.org]
>> On Behalf Of Thomas Friedrich
>> Sent: Thursday, 28 May 2009 1:55 AM
>> To: beginners
>> Subject: [Haskell-beginners] System.Random
>>
>> Hi everyone,
>>
>> Using random numbers in Haskell is not entirely trivial (at least, still
>> not for me) and again I am bagging my head against the Gen-Monad.  I'd
>> like to write a kind of bootstrap function
>>
>> sample :: Int -> [a] -> [a]
>> sample n xs = ...
>>
>> that samples uniformly n elements from the list xs.  I am not sure how
>> to go about this.  Would you try something like
>>
>> sample1 :: StdGen -> Int -> [a] -> [a]
>>
>> and later use this in an IO Monad, something along the lines of:  do {g
>> <- mkStdGen; return $ sample g n xs}, or would you write it like
>>
>> sample2 :: Int -> [a] -> Gen [a]
>>
>> and then use a function from QuickCheck like `generate` to get your
>> samples?
>>
>> You see, I don't even know how to start thinking about the problem.
>>
>> If anyone got an idea, I'd be pleased if you could help me.
>>
>> Cheers,
>> Thomas
>>
>>
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>
>