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Re: Bug in pattern matching?

  • To: mathgroup at
  • Subject: [mg8730] Re: [mg8722] Bug in pattern matching?
  • From: Olivier Gerard <jacquesg at>
  • Date: Sat, 20 Sep 1997 22:27:58 -0400
  • Sender: owner-wri-mathgroup at

At 08:48 +0200 97.09.19, Peter Str=96mbeck wrote:
> Doing this in a pattern matching statement produces a totally different
> result:
> In[5]:=3D
> ls/.{first_, middle___, last_}\[Rule] {{first},C {middle},{last}}
> Out[5]=3D
> {{a[0]},{C a[1] a[2] a[3] a[4]},{a[5]}}
> The middle list now contains a completely different result than Out[4]!
> Can anyone please explain this to me?
> /Peter Str=96mbeck

This is not a bug, Peter, this is a feature. When you define
rules, the right hand side is evaluated. This explain the result.

You should use RuleDelayed  (:>)  (This is not a smilie, or is it ?)
This way, the right hand side will be evaluated when the pattern is
applied so Times will be threaded on the whole middle list.

Hope this helps


(PS: curious you didn't have an answer from WRI support. I am sure
they could find this out. Are you registered ?)

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