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MathGroup Archive 2004

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Re: Question on pattern matching

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47801] Re: [mg47765] Question on pattern matching
  • From: "Roman Green" <rgreen at mail.ru>
  • Date: Tue, 27 Apr 2004 04:46:56 -0400 (EDT)
  • References: <NDBBJGNHKLMPLILOIPPOCEIGEAAA.djmp@earthlink.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi David,

Thanks for reply.

I have to say I still do not understand exactly how Mathematica performs 
pattern matching in that case.

I supposed the following:
When processing

expression /. rule

Mathematica considers all forms of expression that are invariant 
relatively to attributes set for the components of the expression until 
a form that matches the rule is found.
For ex. in the simple case of expression = k[a, b, b] when k carries 
Orderless attribute, Mathematica should consider forms
k[a,b,b]; k[b,a,b]; k[b,b,a].
And the third form should match pattern f_[x_, x_, y_].
But it does not work...

Here are some more tests I made:

In[1]:= SetAttributes[k, Orderless]

In[2]:= k[b,b,a] /. f_[x_, x_, y_]->0

Out[2]= k[a, b, b]

In[4]:= ClearAttributes[k, Orderless]

In[5]:= k[b,b,a] /. f_[x_, x_, y_]->0

Out[5]= 0

So it seems that f_ matches k without any special attributes but does 
not matches k with attribute Orderless...
Why ? I thought f_[x_, x_, y_] should be structurally equivalent to <any 
symbol with any attributes>[<any  another symbol with any attributes>, 
<the same symbol as previous>, <any  another symbol with any 
attributes>].

In any case, what is a pattern that matches structure <any symbol with 
any attributes>[<any  another symbol with any attributes>, <the same 
symbol as previous>, <any  another symbol with any attributes>] ?

Will appreciate any explanation.


---
Cheers

Roman Green



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