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MathGroup Archive 2005

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RE: Returning an empty sequence

  • To: mathgroup at smc.vnet.net
  • Subject: [mg63301] RE: [mg63273] Returning an empty sequence
  • From: "David Park" <djmp at earthlink.net>
  • Date: Fri, 23 Dec 2005 05:08:26 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Trevor,

Use Unevaluated.

f[n_] :=
  Switch[n,
    1, "foo",
    2, Unevaluated[Sequence[ ]],
    3, {"bar"}];

f /@ {1, 2, 3}
{foo, {bar}}

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


From: Trevor Baca [mailto:trevorbaca at gmail.com]
To: mathgroup at smc.vnet.net


What's the right way to have an expression evaluate to an empty
sequence?

I'd like ...

f[n_] :=
  Switch[n,
    1, "foo",
    2, Sequence[ ],
    3, {"bar"}];

... to work, but instead I get the message

Switch::"argct": "Switch called with 6 arguments. Switch must be called
with an odd number of arguments."

meaning that the Sequence[ ] is flattened in the rhs definition of f[
].

Is there a way to get f[ ] to evaluate to the empty Sequence[ ] (rather
than Null)?

Trevor.



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