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RE: Returning an empty sequence


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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