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Re: Don't understand replacement rule for some functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg89559] Re: Don't understand replacement rule for some functions
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 12 Jun 2008 06:31:05 -0400 (EDT)
  • References: <g2qhhr$8or$1@smc.vnet.net>


Hi Mac,

arguments of functions without special attributes like HoldFirst,.. are 

evaluated before beeing fed to the function. Therefore, 

RandomReal[GammaDistribution[L, 1/L], 2] is evaluated before beeing fed 

to ReplaceAll. You may prevent this by wrapping it into Hold or HoldForm.

Hope this helps, Daniel





Mac 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.

> 

> Many thanks again for your help.

> 

> Mac

> 





-- 



Daniel Huber

Metrohm Ltd.

Oberdorfstr. 68

CH-9100 Herisau

Tel. +41 71 353 8585, Fax +41 71 353 8907

E-Mail:<mailto:dh at metrohm.com>

Internet:<http://www.metrohm.com>




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