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Re: Don't understand replacement rule for some functions

• To: mathgroup at smc.vnet.net
• Subject: [mg89551] Re: [mg89538] Don't understand replacement rule for some functions
• From: "Szabolcs HorvÃt" <szhorvat at gmail.com>
• Date: Thu, 12 Jun 2008 06:29:36 -0400 (EDT)
• References: <200806120658.CAA08881@smc.vnet.net>

On Thu, Jun 12, 2008 at 09:58, Mac <mwjdavidson at googlemail.com> wrote:
> Can somebody explain why the following replacement rule works
>
> In[15]:= f[x] /. x -> y
>
> Out[15]= f[y]
>
> and this one complains
>
> In[14]:= RandomReal[GammaDistribution[L, 1/L], 2] /. L -> 3
>
> During evaluation of In[14]:= RandomReal::parpos: The parameter L in \
> position 1 of GammaDistribution[L,1/L] should be positive >>
>
> Out[14]= {1.89311, 1.11748}
>
> I find it quite elegant to use replacements for complicated functions
> but I'm never quite sure whether the replacement is actually carried
> out because of such behavior.
>

Break up your expression and test the fragments separately.
RandomReal[GammaDistribution[L, 1/L], 2] will give an error because it
should not contain any symbolic expressions like L.

• References:
  • Don't understand replacement rule for some functions
    • From: Mac <mwjdavidson@googlemail.com>