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MathGroup Archive 2005

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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. 



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