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Re: Returning an empty sequence
*To*: mathgroup at smc.vnet.net
*Subject*: [mg63290] Re: [mg63273] Returning an empty sequence
*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>
*Date*: Fri, 23 Dec 2005 05:08:17 -0500 (EST)
*References*: <200512220504.AAA05673@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
On 22 Dec 2005, at 14:04, Trevor Baca wrote:
> 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.
>
Yes.
f[n_] :=
Switch[n,
1, "foo",
2, Unevaluated[ Sequence[ ]],
3, {"bar"}];
f[1]
foo
f[2]
Sequence[]
Andrzej Kozlowski
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