Re: Returning an empty sequence
- To: mathgroup at smc.vnet.net
- Subject: [mg63306] Re: Returning an empty sequence
- From: "Jean-Marc Gulliet" <jeanmarc.gulliet at gmail.com>
- Date: Fri, 23 Dec 2005 05:08:30 -0500 (EST)
- Organization: The Open University, Milton Keynes, U.K.
- References: <dode3g$62d$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
"Trevor Baca" <trevorbaca at gmail.com> a écrit dans le message de news:
dode3g$62d$1 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.
|
Hi Trevor,
You could add the attribute *SequenceHold* to the built-in function *Switch*
as in the following:
In[1]:= f[n_]:=Switch[n,1,"foo",2,Sequence[],3,{"bar"}];
In[2]:= Attributes[Switch]
Out[2]= {HoldRest,Protected,ReadProtected}
In[3]:= Unprotect[Switch];
SetAttributes[Switch,SequenceHold];
Protect[Switch];
In[6]:= Attributes[Switch]
Out[6]= {HoldRest,Protected,ReadProtected,SequenceHold}
In[7]:= f[2]
Out[7]= Sequence[]
Best regards,
/J.M.