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MathGroup Archive 2006

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Re: PiecewiseExpand bug?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68476] Re: PiecewiseExpand bug?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Mon, 7 Aug 2006 01:40:30 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <eb45vn$n7p$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Mark Fisher wrote:
> This produces what I expect:
> 
> Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}] /.
> 	{x[1] -> 0, x[2] -> 1, x[3] -> 1}
> 
> But this does not:
> 
> PiecewiseExpand[Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}]] /.
> 	{x[1] -> 0, x[2] -> 1, x[3] -> 1}
> 
> It appears PiecewiseExpand does something it shouldn't here. Any thoughts?

No, it does not: you are not computing the same expressions because of 
the location of the transformation rules. You can clearly see what's 
going on with the Trace function if needed.

In[1]:=
<< "Statistics`DiscreteDistributions`"

In[2]:=
Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}] /. {x[1] -> 0, x[2] 
-> 1, x[3] -> 1}

Out[2]=
(1 - p)*p^2

In[3]:=
PiecewiseExpand[Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}]] /. 
{x[1] -> 0, x[2] -> 1, x[3] -> 1}

Out[3]=
0

In[4]:=
PiecewiseExpand[Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}] /. 
{x[1] -> 0, x[2] -> 1, x[3] -> 1}]

Out[4]=
(1 - p)*p^2

In[5]:=
PiecewiseExpand[(1 - p)*p^2]

Out[5]=
(1 - p)*p^2

In[6]:=
Trace[Product[PDF[BernoulliDistribution[p], x[i]], {i, 3}] /. {x[1] -> 
0, x[2] -> 1, x[3] -> 1}]

In[7]:=
Trace[PiecewiseExpand[Product[PDF[BernoulliDistribution[p], x[i]], {i, 
3}]] /. {x[1] -> 0, x[2] -> 1, x[3] -> 1}]

Regards,
Jean-Marc


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