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MathGroup Archive 2003

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RE: Pattern matches only when named

  • To: mathgroup at smc.vnet.net
  • Subject: [mg43913] RE: [mg43873] Pattern matches only when named
  • From: "Wolf, Hartmut" <Hartmut.Wolf at t-systems.com>
  • Date: Sat, 11 Oct 2003 01:33:32 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

>-----Original Message-----
>From: Ken Morgan [mailto:kemorgan at vt.edu]
To: mathgroup at smc.vnet.net
>Sent: Thursday, October 09, 2003 7:55 AM
>To: mathgroup at smc.vnet.net
>Subject: [mg43913] [mg43873] Pattern matches only when named
>
>
>Just a curiosity:
>
>In[1]:=   f + g /. u_ + _ -> h
>Out[1]:=  h
>
>but
>
>In[2]:=   f + g /. _ + _ -> h
>Out[2]:=  f + g
>
>Why does the pattern match only when one of the Blanks has a name?
>
>
>Thanks,
>Ken
>

Quite simple: because f + g does not match 2 _  !!

Clearly this is not what you meant, but to avoid evaluating the pattern in
Rule use Hold Pattern:

In[3]:= f + g /. HoldPattern[_ + _] -> h
Out[3]= h

--
Hartmut Wolf


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