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MathGroup Archive 1999

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[Q] Extracting patterns

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18607] [Q] Extracting patterns
  • From: "Kevin Jaffe" <kj0 at mailcity.com>
  • Date: Tue, 13 Jul 1999 01:01:32 -0400
  • Organization: MailCity (http://www.mailcity.lycos.com:80)
  • Sender: owner-wri-mathgroup at wolfram.com


How can I extract the first argument of expressions having the form
Pattern[x, y]?  I know how to do something like this in other
situations, e.g.:


In[1]:= Plus[x, y] /. Plus[a_, b_] -> a

Out[1]= x


But this strategy fails when the head of the lhs expression is Pattern:

In[2]:= Pattern[x, y] /. Pattern[a_, b_] -> a

Pattern::patsym: First element in pattern Pattern[a_, b_] is not a symbol.

Out[2]= x:y


In a last-gasp effort, I also tried:

In[3]:= Pattern[x, y] /. HoldPattern[Pattern[a_, b_]] -> a

Pattern::patsym: First element in pattern Pattern[a_, b_] is not a symbol.

Out[3]= x:y

to no avail.  Is there a way to do this?

Thanks,

kj0 at mailcity.com



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