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MathGroup Archive 2001

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Re: Parametric Plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27088] Re: Parametric Plot
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sun, 4 Feb 2001 02:58:15 -0500 (EST)
  • References: <95gma2$8ir@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Winston,
You are getting listed values like

xd[4]

        {1.24463}

Plot lets us get away with this (it flattens the input), but ParametricPlot
assumes that {{x1[t]},{x2[t]}} is an error for something like
{{x1[t]`y1[t]},{x2[t], y2[t]}} .

One way out is to replace

xd[t_] := Evaluate[x1[t] /. pend];
 etc

with

    xd[t_] = x1[t] /. pend[[1]];

etc

(the Evaluate is unnecessary if we use = instead of :=)

--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565

"Winston Garira" <uceswga at ucl.ac.uk> wrote in message
news:95gma2$8ir at smc.vnet.net...
> I am trying to solve a system of two coupled pendulums using the NDSolve.
> command. If I replace the Plot[{xd[t],xr[t]},{t,0,100}] with
> ParametricPlot[{xd[t],xr[t]},{t,0,100}] it does not work. Can someone tell
> me why the ParametricPlot command does not work in this case. I need to
> make parametric plots for this system.
>
> Thanking you in annticipation
>
> Winston
>
>
>  Pends[init1_, init2_, time_, k_, {c_, w_, p_}]:=
>  Module[{},
> pend=NDSolve[{x1''[t]+ c x1'[t]+ p Sin[x1[t]]==k(x2[t]-x1[t]),
>       x2''[t]+ c x2'[t]+ p Sin[x2[t]]==k(x1[t]-x2[t]),
> x1[0]==init1[[1]], x1'[0]==init1[[2]],
> x2[0]==init2[[1]], x2'[0]==init2[[2]]},
> {x1, x2},
> {t,0,time}, MaxSteps->200000];
> xd[t_] := Evaluate[x1[t] /. pend];
> xdd[t_]:= Evaluate[x1'[t] /. pend];
> xr[t_] := Evaluate[x2[t] /. pend];
> xrd[t_]:= Evaluate[x2'[t] /. pend];
> ];
>
>
> c=0.1; w=0.5; p=1.9;
>
> Pends[{1.57,0}, {-1.57,0}, 5000,0.6,  {c,w,p}];
> Plot[{xd[t],xr[t]},{t,0,100},
> PlotStyle\[Rule]{RGBColor[1,0,0.3],RGBColor[0,0.5,1]}];
>
>
>
> W.GARIRA
> E-mail: w.garira at ucl.ac.uk
> phone : +44-(0)20-7679-2521
> ___________________________________________________
>
>
>
>




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