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Re: Validating functions input

• To: mathgroup at smc.vnet.net
• Subject: [mg58795] Re: Validating functions input
• From: dh <dh at metrohm.ch>
• Date: Tue, 19 Jul 2005 04:10:04 -0400 (EDT)
• References: <dbflkk$n0p$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

Hi Nilton,
You want a function that returns unevaluated if the input fullfills some 
condition and does something if the condition is not fulfilled.
Well, this calls for negation: !

f[x_, y_] /; ! x <= 2^y := (Print["invalid Input"]; Null)

This will print an error message and return Null if the condition: x <= 
2^y is not (!) fulfilled.

sincerely, Daniel
Nilton wrote:
> I have a function that must have its input validated.
> 
> f[x,y]
> 
> I want to validate if x <= 2^y. If it's valid, then just return the
> symbol f[x,y] unevaluated. If it's not valid, I want to return some
> symbol, Null for instance, and issue an error message.
> 
> The following is a try, but it doesn't work, because it's definition is
> recursive:
> 
> f::invalid = "Invalid arguments"
> f[x_,y_] := If[x <= 2^y, f[x,y], Message[f::invalid]]
> 
> Is there any way to implement this?
> 
> Thanks,
> -- Nilton
>