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MathGroup Archive 1999

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Re: [Q] Extracting patterns

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18724] Re: [Q] Extracting patterns
  • From: dreeves at flip.eecs.umich.edu (Daniel Reeves)
  • Date: Sat, 17 Jul 1999 02:36:43 -0400
  • References: <7meikl$9gj@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

If you have a list, you can get the first element with Part or First.
Like  {x, y}[[1]]
But {x, y} is really List[x, y] and it turns out there's nothing special
about the head List.
You can get the first element from other things the same way, eg, 

Pattern[x, y][[1]]

--    --    --    --    --    --    --    --    --    --    --    -- 
Daniel Reeves               http://ai.eecs.umich.edu/people/dreeves/

It's not enough to be Hungarian; you must have talent too.
                -- Alexander Korda

On 13 Jul 1999, Kevin Jaffe wrote:

> 
> 
> 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
> 
> 
> 
> Get your FREE Email at http://mailcity.lycos.com
> Get your PERSONALIZED START PAGE at http://my.lycos.com
> .
> 


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