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Re: Using a Named Pattern in a Rule

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36288] Re: Using a Named Pattern in a Rule
  • From: Tom Burton <tburton at brahea.com>
  • Date: Fri, 30 Aug 2002 01:19:13 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

David,

On 8/28/02 10:53 PM, in article akkd5n$82d$1 at smc.vnet.net, you wrote:

> I would like to match a named pattern in an expression and then square the
> result. But my attempt fails.
> 
> Clear[f, g, x, y, a]
> expr = 3*f[x]*g[y] + 2 + x^2;
> 
> expr /. a:(f[_]*g[_]) :> a^2
> 2 + x^2 + 3*f[x]*g[y] ...

Well, I see that 

expr /. a:Times[f[_],g[_],h___]:> a^2

works, and I think I understand why your form above does not work (examine
FullForm of expr--your form does not account for "3"), but I don't
understand why eliminating "a:" from your form allows the pattern to match.

Tom Burton



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