Random Drawing Simulation -- performance issue
David J. Braden
dbraden at invalid.add
Wed Sep 13 18:12:13 CEST 2006
Travis E. Oliphant wrote:
> Brendon Towle wrote:
>> I need to simulate scenarios like the following: "You have a deck of
>> 3 orange cards, 5 yellow cards, and 2 blue cards. You draw a card,
>> replace it, and repeat N times."
>>
>
> Thinking about the problem as drawing sample froms a discrete
> distribution defined by the population might help.
>
> For example, in SciPy you can define your own discrete random variable:
>
> var = scipy.stats.rv_discrete(name='sample',
> values=([0,1,2],[3/10.,5/10.,2/10.]))
>
> Then
>
> var.rvs(size=10000)
>
> would quickly return an array of "draws"
>
> If you didn't want to install SciPy, but were willing to install NumPy,
> then the crux of the algorithm is to generate an entire array of uniform
> random variables: numpy.random.rand(count) and then map them through
> the inverse cumulative distribution function to generate your samples.
> The inverse cumulative distribution function is just a series of steps
> whose width depends on the probablity distribution.
>
> Thus, the population defines the draw boundaries. To make it easy, just
> multiply the uniform random number by the total number of cards and then
> the boundaries are on the integers of the number of each kind of card.
>
> Here is an implementation. I find this version to be 2x - 5x faster
> depending on how many draws are used.
>
>
> import numpy as N
>
> def randomDrawing_N(count, population):
> probs = [x[0] for x in population]
> res = [[0, item[1]] for item in population]
> nums = N.random.rand(count)
> cprobs = N.cumsum([0]+probs)
> nums *= cprobs[-1]
> for k, val in enumerate(probs):
> res[k][0] += ((nums > cprobs[k]) & (nums < cprobs[k+1])).sum()
> return res
>
>
> -Travis Oliphant
>
In response to what Travis and Simon wrote -
(1) Where the heck can I find a description of scipy's stat functions?
Documentation on these seems sparse.
(2) How does one set up a good timer for Python as implemented for
Windows? (i.e., how can I make API calls to Windows from Python?)
Please bear with me, or not; I am /just/ starting off with Python.
Thanks,
Dave Braden
More information about the Python-list
mailing list