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MathGroup Archive 2005

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Re: Problem using Evaluate[] inside a Plot command. 5.1

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54212] Re: Problem using Evaluate[] inside a Plot command. 5.1
  • From: Nasser Abbasi <nma at 12000.org>
  • Date: Sun, 13 Feb 2005 22:17:01 -0500 (EST)
  • References: <cu78ub$me9$1@smc.vnet.net> <cumor4$3rc$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

D Herring wrote:

> Hi Nasser,
> 
> Maybe Evaluate[OutputResponse[sys,UnitStep[t], t]] is returning a value 
> which needs to be Evaluate[]d again before it can be plotted?
> 
> I've never had your problem (don't even have the control toolbox), but 
> maybe
> 
> Plot[{t2, Evaluate[Evaluate[OutputResponse[sys,UnitStep[t], t]]]},
>        {t, 0, 3}, PlotRange -> All]
> 
> might work?
> 
> Your error message, "plot::plnr: .... is not a machine-size real
> number at t=...." usually indicates the need to Evaluate[] one of the 
> Plot[] arguments.
> 
> Good luck,
> Daniel
> 

Daniel;

I tried your command above (You had a typo, 't2' should
be 't^2' or just 't'), any way, it does not work either.

I am sure this is a bug in Mathematica, either the kernel or the
ControlSystems toolbox or the parser.

It does not make sense for this command to work:

Plot[ Evaluate[OutputResponse[sys, UnitStep[t], t]] ,
       {t, 0, 3}, PlotRange -> All]

But if I now simply combine another function in the plot command,
so to plot 2 functions together, it will now fail:

Plot[ {t^2, Evaluate[OutputResponse[sys, UnitStep[t], t]] },
       {t, 0, 3}, PlotRange -> All]
"plot::plnr: .... is not a machine-size real


Why would the second example above fail but the first not fail?

btw, the output from the OutputResponse[] is simply a list that
contains a function of t, such as { Exp[t/2] Cos[t] }

So, nothing too complicated here.

bye,
Nasser





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