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Re: Optional Alternatives

• To: mathgroup at smc.vnet.net
• Subject: [mg50034] Re: [mg50010] Optional Alternatives
• From: DrBob <drbob at bigfoot.com>
• Date: Thu, 12 Aug 2004 05:43:54 -0400 (EDT)
• References: <200408101002.GAA19525@smc.vnet.net> <200408110953.FAA04123@smc.vnet.net>
• Reply-to: drbob at bigfoot.com
• Sender: owner-wri-mathgroup at wolfram.com

A missing value can't really fit a pattern, but you can do this:

Clear[f]
f[x:_Real | _Rational] := x
f[x_Integer] := 2
f[] := f[GoldenRatio]
f /@ {0, 1/2, 0.3, Pi}
{2, 1/2, 0.3, f[Pi]}

f[]
f[GoldenRatio]

Or...

Clear[f]
Default[f] = GoldenRatio;
f[x:_Real | _Rational] := x
f[x_Integer] := 2
f[x_Symbol] := Symbol
f[x_.] := f[Floor[x]]^2
f /@ {0, 1/2, 0.3, Pi}
{2, 1/2, 0.3, Symbol}

f[]
4

In both cases, I've made GoldenRatio the default value.

Bobby

On Wed, 11 Aug 2004 05:53:08 -0400 (EDT), Selwyn Hollis <sh2.7183 at misspelled.erthlink.net> 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
>
> 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)
>
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net

• References:
  • 50,000th Message
    • From: "Steven M. Christensen" <steve@smc.vnet.net>
  • Optional Alternatives
    • From: Selwyn Hollis <sh2.7183@misspelled.erthlink.net>