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MathGroup Archive 2004

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Re: PlotPoints in plot

  • To: mathgroup at smc.vnet.net
  • Subject: [mg48581] Re: [mg48520] PlotPoints in plot
  • From: DrBob <drbob at bigfoot.com>
  • Date: Sat, 5 Jun 2004 07:20:06 -0400 (EDT)
  • References: <200406040849.EAA23664@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On my machine, at least, the count is 41:

plot = Plot[Sqrt[1 - x^2], {x, -1, 1}, PlotPoints -> 3];
Length@Flatten[Cases[plot, Line[a__] :> a, Infinity], 1]

41

Help documentation on PlotPoints is obviously incorrect (again), or at least misleading (again). An adaptive procedure is used whether you want it or not, obviously. I suspect PlotPoints sets a MINIMUM number of points.

Here's an attempt with only 4 points:

plot = Plot[
     Sqrt[1 - x^2], {x, -1, 1}, PlotPoints -> 3, PlotDivision -> 1,
       MaxBend -> 90];
Length@Flatten[Cases[plot, Line[a__] :> a, Infinity], 1]

4

Here again, the documentation for MaxBend is incorrect; increasing it should decrease the number of plot points, but...

plot = Plot[
     Sqrt[1 - x^2], {x, -1, 1}, PlotPoints -> 3, PlotDivision -> 1,
       MaxBend -> 100];
Length@Flatten[Cases[plot, Line[a__] :> a, Infinity], 1]

5

I'm thinking you'll have to pick your points manually, something like this:

Show[Graphics[Line@Table[{x, Sqrt[1 - x^2]}, {x, -1, 1}]]]

DrBob

On Fri, 4 Jun 2004 04:49:37 -0400 (EDT), "G.L.Narasimham" <mathma18 at hotmail.com> wrote:

> For the plot command  Plot[Sqrt[1-x^2],{x,-1,1},PlotPoints->3]
> I expected to see a triangle joining 3 points on a circle, but a
> continuous line ( default PlotPoints->20? )appears. However,
> ParametricPlot3D[{Cos[t],Sin[t],0},{t,0,Pi},PlotPoints->3,ViewPoint->{0,0,5}]
> gives a triangle. How to get a polygon of straight sides in the
> unparametered Plot?
>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net/index.html


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