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Re: Optional Alternatives

• To: mathgroup at smc.vnet.net
• Subject: [mg50036] Re: Optional Alternatives
• From: Bill Rowe <readnewsciv at earthlink.net>
• Date: Thu, 12 Aug 2004 05:44:00 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

On 8/11/04 at 5:53 AM, sh2.7183 at misspelled.erthlink.net (Selwyn
Hollis) wrote:

>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

The only way I know to achieve this is as follows:

f[x_Real]:=x
f[x_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

The following might do for you

In[1]:= g[x_, s_:0] := x + s

In[2]:= g[2]
Out[2]= 2

In[3]:= g[2, 2]
Out[3]= 4
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