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MathGroup Archive 2005

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Re: easy question about random numbers

  • To: mathgroup at smc.vnet.net
  • Subject: [mg53413] Re: easy question about random numbers
  • From: Mark Fisher <mark at markfisher.net>
  • Date: Tue, 11 Jan 2005 01:30:44 -0500 (EST)
  • References: <crqb7f$cak$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

With 5.1, we can use Piecewise as follows:

MakeDiscreteRandomFunction[probs_] :=
   MakeDiscreteRandomFunction[probs, Range[Length[probs]]]

MakeDiscreteRandomFunction[probs_, vals_] :=
   Block[{x},
     Compile @@ {x, Piecewise @
       Transpose[{vals, x <= # & /@ Rest@FoldList[Plus, 0, probs]}]}
   ]

li2= {1/6, 1/6, 1/6, 1/6, 9/60, 11/60}
ranfun = MakeDiscreteRandomFunction[li2]

Usage:

ranfun[Random[]]

--Mark



Pedrito wrote:

> Hi everybody!
> 
> I wanted to obtain a discrete random number generator that I needed for
> a project.
> 
> On the library Statistics`DiscreteDistributions` I could find the DiscreteUniformDistribution
> function. But I wanted to specify the probability for each one of the
> states.
> 
> 
> For instance:
> If we need to simulate an unfair dice, we could have this probabilities
> for each one of the sides:
> {1/6, 1/6, 1/6, 1/6, 9/60, 11/60}
> 
> So I wrote:
> li2 = {1/6, 1/6, 1/6, 1/6, 9/60, 11/60}
> li3=FoldList[Plus,0,li2]
> Module[{i = 1, r = Random[]}, While[ !li3[[i]] < r < li3[[i + 1]], i++]; i]
> 
> It works ok but I don't know if there is another (better) way of doing
> this.
> Any suggestion?
> 


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