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 >