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Re: bug in RandomChoice if weight is zero?

• To: mathgroup at smc.vnet.net
• Subject: [mg106037] Re: [mg105976] bug in RandomChoice if weight is zero?
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Thu, 31 Dec 2009 03:14:02 -0500 (EST)
• References: <200912290617.BAA02582@smc.vnet.net>

Matthias Greiff wrote:
> If I use RandomChoice with weights of zero the corresponding elements will never be selected.
> 
> In[62]:= RandomChoice[{0, 0, 3} -> {3, 2, 1}, 5]
> Out[62]= {1, 1, 1, 1, 1}
> 
> Why is it not the same if I use RandomSample?
> The following command
> 
> RandomSample[{1, 2, 0} -> Range[3], 2]
> 
> returns either {1,2} or {2,1}. But when I change the command to sample size 3  I get the following.
> 
> In[60]:= RandomSample[{1, 2, 0} -> Range[3], 3]
> Out[60]= {2, 1, 3}
> 
> Why is the third element selected? Shouldn't Mathematica return an error message because the sample size is larger than the population size?
> 
> Appreciate your answers. 
> 
> Merry Christmas!
> 
> Matthias

I see your bug, and raise you another.

In[25]:= TimeConstrained[
   RandomSample[UnitVector[100,1]->Range[100], 2], 10] (* hangs *)

Actually that's two more bugs, because it is both in some infinite 
cycling and also not responding to aborts.

I'll look into repair today. Alternatively, I might go home and have a 
good sob.

Daniel Lichtblau
Wolfram Research

• References:
  • bug in RandomChoice if weight is zero?
    • From: Matthias Greiff <greiff@mac.com>