Re: Parametric Plot

• To: mathgroup at smc.vnet.net
• Subject: [mg27098] Re: Parametric Plot
• From: Tom Burton <tburton at cts.com>
• Date: Sun, 4 Feb 2001 02:58:24 -0500 (EST)
• Organization: Brahea Consulting
• References: <95gma2\$8ir@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Hello Winston,

Your asking the wrong question. The right question is, why does Plot work?

Looking carefully as functions xd and xr, you'll notice that each returns a list containing one number, because the solution pend has this structure. This is NDSolve's way of reporting that the set of solutions contains one element. If you take the First element somewhere, either to solution itself (pend) or to xd and xr, then ParametricPlot will work (and so will Plot).

Why does Plot work? Experimenting a bit, I find that Plot is tolerant of extra braces in its first argument, the list of functions to plotted. Perhaps Plot, taking advantage of the fact that a higher-level structure is not meaninful, flattens this argument. Note that ParametricPlot cannot do this safely, because a list of lists is a meaninful structure as input to ParametricPlot.

Tom

On 3 Feb 2001 05:25:06 -0500, in comp.soft-sys.math.mathematica you wrote:

>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...

Tom

```

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