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a question about pattern replacements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36163] a question about pattern replacements
  • From: Sidney Cadot <sidney at jigsaw.nl>
  • Date: Fri, 23 Aug 2002 21:34:55 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Hi all,

While playing arounf with patterns and substitutions, I came across the
following behavior which I didn't expect:

z := SomeHead[{{1, 2}, {3, 4}}]
z /. {SomeHead[q_] -> Flatten[q]}

While this _does_ yield the desired result {1,2,3,4}, Mathematica complains:

Flatten::"normal": Nonatomic expression expected at position 1 in
Flatten[q].

........as if it is trying to evaluate Flatten[q], with q not bound to
{{1,2},{3,4}}.

Could anybody explain why this happens?

Best regards,

  Sidney Cadot





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