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MathGroup Archive 2007

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Re: Replacement according to the pattern

  • To: mathgroup at smc.vnet.net
  • Subject: [mg77573] Re: Replacement according to the pattern
  • From: David Bailey <dave at Remove_Thisdbailey.co.uk>
  • Date: Wed, 13 Jun 2007 07:30:03 -0400 (EDT)
  • References: <f4lair$avv$1@smc.vnet.net>

tomuf at seznam.cz wrote:
> Hello,
> I wanted to parse a big expression and according to some rules
> integrate it term by term. For example, I wanted to replace symbol
> 
> HPL[{1, 1, 0}, u]
> 
> where HPL(list,variable) is function from package, with (note that
> this simplified example has no mathematical sense)
> 
> HPL[{1, 1, 0}, u] - D[HPL[{1, 1, 0}, u], u]
> 
> i.e. that function minus its derivative with respect to 'u'. I have
> loaded package which allows me to differentiate HPL. This works fine.
> But when I try to generalize this to handle all lists like {1,1,0} at
> the same time so that I don't have to type the rule for all
> combinations, the replacement doesn't work well. When I type:
> 
> in = HPL[{1, 1, 0}, u]
> in /. HPL[{1, 1, 0}, u] -> HPL[{1, 1, 0}, u] - D[HPL[{1, 1, 0}, u], u]
> in /. HPL[a_, u] -> HPL[a, u] - D[HPL[a, u], u]
> 
> the third line produces different result than the second line (the
> difference is in the term involving derivative). Does anyone know what
> could be wrong? Thanks in advance
> 
> Best regards,
> Tomas Prochazka
> 
> 
Use delayed rules (:>) rather than ordinary rules (->) otherwise you end 
up executing D[HPL[a,u],u] BEFORE the variable a has been replaced.

David Bailey
http://www.dbaileyconsultancy.co.uk


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