Re: Returning an empty sequence
- To: mathgroup at smc.vnet.net
- Subject: [mg63293] Re: Returning an empty sequence
- From: dh <dh at metrohm.ch>
- Date: Fri, 23 Dec 2005 05:08:20 -0500 (EST)
- References: <dode3g$62d$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Hi Trevor,
you must prevent "Sequence[]" to be evaluated before Switch is called.
This can be done by "Unevaluated":
f[n_] := Switch[n,
1, "foo",
2, Unevaluated[Sequence[]],
3, {"bar"}];
Daniel
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.
>