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MathGroup Archive 2004

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Re: Optional Alternatives


On 11 Aug 2004, at 11:53, Selwyn Hollis wrote:

> *This message was transferred with a trial version of CommuniGate(tm) 
> Pro*
> Is it possible to require a function argument to match alternative
> patterns and be optional at the same time? As a toy example, suppose I
> define
>
>     f[x:_Real|_Rational]:= x
>
> Is there a way make the argument optional with a given default value,
> say 0? I've tried this:
>
>     f[Optional[x:_Real|_Rational, 0]]:= x
>
> and various other combinations with no success. (Of course I could do
> this: f[]:= 0, but I'm interested in the more general issue.)
>
> -----
> Selwyn Hollis
> http://www.appliedsymbols.com
> (edit reply-to to reply)
>
>
There is no problem as long as your optional value matches your 
pattern. Thus the following will work:

   In[1]:=
   f[Optional[x:_Real|_Rational, 0.]]:= x

In[2]:=
f[]

Out[2]=
0.

The same with 0 in place of 0 will not.

Andrzej Kozlowski



Andrzej Kozlowski
Chiba, Japan
http://www.mimuw.edu.pl/~akoz/


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