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Re: Using a Named Pattern in a Rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36314] Re: Using a Named Pattern in a Rule
  • From: Rainer Gruber <rainer.gruber at gmx.at>
  • Date: Sat, 31 Aug 2002 01:25:58 -0400 (EDT)
  • Organization: Johannes Kepler Universitaet Linz
  • References: <akkd5n$82d$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

David Park wrote:

> I would like to match a named pattern in an expression and then square the
> result. But my attempt fails.
> 
> [...]

 >

> David Park
> djmp at earthlink.net
> 
> 


I simplified your example to see whats the problem. I only take the 
multiplication of three symbols a, b, c.


In[1]:= Clear[a, b, c, p, x, y]

If you use the "unnamed" pattern 'a c' in Replace you get

In[2]:= a b c /. a c :> x
Out[2]= b x

the replacement will be done because all possibilites of the 
multiplication are tested and the right one will be found.

But if you take a named pattern 'p : a c' your are searching for a 
multiplication of exactly two variables called 'a' and 'c' and you are 
not successful with the replacement:

In[3]:= a b c /. p : a c :> x
Out[3]= a b c

In this case you have to take into account that the multiplication can 
consist of more than two terms, e. g.

In[4]:= a b c /. p : a c z___ :> x z
Out[4]= b x¨

 
Hope that will help you,


-- 
Rainer Gruber




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