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MathGroup Archive 2005

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Re: Validating functions input

  • To: mathgroup at
  • Subject: [mg58800] Re: Validating functions input
  • From: David Bailey <dave at>
  • Date: Tue, 19 Jul 2005 04:10:08 -0400 (EDT)
  • References: <dbflkk$n0p$>
  • Sender: owner-wri-mathgroup at

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

All you need to do is pick off the case that requires an error message 
and leave the others:

f[x_,y_] := Message[f::invalid]/;x>2^y;

David Bailey

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