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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: [mg69264] Re: Unexpected "Invalid comparison" error when plotting function defined with a Condition pattern
  • From: "Andrew Moylan" <andrew.j.moylan at gmail.com>
  • Date: Mon, 4 Sep 2006 04:47:04 -0400 (EDT)
  • References: <ed68td$jgk$1@smc.vnet.net><ed92v5$p46$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Jens,

Does Mathematica give a small complex part for 10^y for any y in the
range {1,2}? I can't find any y for which it does.

Of course there are various ways I can modify my expression until it
"works"; my question is really about understanding why Mathematica's
Plot function has this behaviour, so that I can predict it and design
for it in the future.

Cheers,

Andrew


Jens-Peer Kuska wrote:
> Hi,
>
> just read the error message -- 10^y gaves a small
> imaginary part and
>
> g[x_?NumericQ /; 0 < Chop[x]] := x
>
> will fix the problem.
>
> Regards
>   Jens
>
> "Andrew Moylan" <andrew.j.moylan at gmail.com>
> schrieb im Newsbeitrag
> news:ed68td$jgk$1 at smc.vnet.net...
> | 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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