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Re: Ask Mathematica to sub for a long expression

• To: mathgroup at smc.vnet.net
• Subject: [mg71921] Re: Ask Mathematica to sub for a long expression
• From: carlos at colorado.edu
• Date: Mon, 4 Dec 2006 06:38:47 -0500 (EST)
• References: <ekp4o4$2fq$1@smc.vnet.net><ekudat$86k$1@smc.vnet.net>

aaronfude at gmail.com wrote:
> Hi,
>
> Thanks for all the responses. It doesn't quite work for me, but maybe
> with a little extra expertise it will. The problem is that that pattern
> is not always obvious w/o extra algebra. The basic problem is
> summarized in the fact that the following doesn't work the way I'd like
> it to:
>
> -a-b/.a+b->c
>
> 2a+2b/.a+b->c
>
> Many thanks in advance!
>
> Aaron Fude

Replacing nontrivial patterns sometimes works, sometimes it doesnt.
It may depends on FullForm details.  This violates a basic tenet of
computer science: user-level results should not depend on internal
representation. Here is a simple example.

ClearAll[f,a,b,c,i];
f[b_,c_]:=1/(c+b^2);
v=Table[{i,f[b,i]/.b^2->16},{i,-6,6}];
ListPlot[v,PlotJoined->True];

Try to track the source of the error message and the plot gap.

• Follow-Ups:
  • Re: Re: Ask Mathematica to sub for a long expression
    • From: Andrzej Kozlowski <akoz@mimuw.edu.pl>
  • Re: Re: Ask Mathematica to sub for a long expression
    • From: Daniel Lichtblau <danl@wolfram.com>