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RE: a question about pattern replacements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg36170] RE: [mg36163] a question about pattern replacements
  • From: "David Park" <djmp at earthlink.net>
  • Date: Mon, 26 Aug 2002 04:15:47 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Sidney,

With a direct rule Mathematica tries to Flatten the symbol q immediately.
You want a delayed rule to avoid the error message.

z /. SomeHead[q_] :> Flatten[q]

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/

From: Sidney Cadot [mailto:sidney at jigsaw.nl]
To: mathgroup at smc.vnet.net

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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