Re: Returning an empty sequence
- To: mathgroup at smc.vnet.net
- Subject: [mg63303] Re: [mg63273] Returning an empty sequence
- From: Sseziwa Mukasa <mukasa at jeol.com>
- Date: Fri, 23 Dec 2005 05:08:28 -0500 (EST)
- References: <200512220504.AAA05673@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On Dec 22, 2005, at 12:04 AM, 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)?
Sequence[] always gets evaluated unless the function has the
attribute SequenceHold or HoldAllComplete which Switch does not. You
can unprotect Switch to set the SequenceHold attribute but I must ask
why you would want to do this? You can get the same effect by having
f take the values "foo",{},{"bar"} and writing Sequence[f[n]]
wherever it's appropriate.
Regards,
Ssezi
- References:
- Returning an empty sequence
- From: "Trevor Baca" <trevorbaca@gmail.com>
- Returning an empty sequence