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MathGroup Archive 2006

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Re: Unexpected "Invalid comparison" error when plotting function defined with a Condition pattern

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69174] Re: [mg69165] Unexpected "Invalid comparison" error when plotting function defined with a Condition pattern
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Fri, 1 Sep 2006 06:40:21 -0400 (EDT)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

Clear[g];

g[x_?Positive] = x;

Plot[Abs[g[10^y]], {y, 1, 2}];

Clear[g];

g[x_] = x*UnitStep[x];

Plot[Abs[g[10^y]], {y, 1, 2}];


Bob Hanlon

---- Andrew Moylan <andrew.j.moylan at gmail.com> wrote: 
> Hi,
> 
> Please consider the following simple function, defined for positive x
> using a Condition pattern:
> 
>   g[x_ /; 0 < x] = x;
> 
> When I try to plot this function in the following way:
> 
>   Plot[Abs[g[10^y]], {y, 1, 2}];
> 
> I unexpectedly receive some "Less::nord : Invalid comparison with 10. +
> 0. I attempted" errors. Can anyone explain why this is, and how I
> should best prevent it?
> 
> Relevant note:
> Both of the following different plots of g succeed with no errors:
>   Plot[Abs[g[y]], {y, 1, 2}];
>   Plot[g[10^y], {y, 1, 2}];
> 
> Irrelevant note:
> In my actual application, the function g is more complicated and is
> complex-valued (hence my use of Abs); but the error is reproducable
> with the very simple real-valued function g as defined above.
> 
> Thanks for any insight you might be able to give me on this.
> 
> Cheers,
> 
> Andrew
> 


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