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MathGroup Archive 2003

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Re: Baffling Failure when plotting multiple curves.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg44171] Re: Baffling Failure when plotting multiple curves.
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Sat, 25 Oct 2003 06:26:20 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <bnao48$4ei$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

and

Plot[Evaluate[voteCurves[{p1, p2}]], {\[Theta], -90, 180}, 
  AxesOrigin -> {0, 0}, PlotRange -> {{-90, 180}, {0, 255 Sqrt[2]}}, 
  AspectRatio -> 1, Frame -> True]

works also. 

Regards
  Jens

Bob Harris wrote:
> 
> Howdy,
> 
> Am having trouble getting collections of 2D function plots to work.  I have
> created a function that returns a list of "equations" in one variable,
> theta.  I.e. an equation is an expression like 17 Cos[theta] + 10
> Sin[theta].  When I try to plot this list of functions AS THE RETURN VALUE
> OF MY FUNCTION, Mathematica complains (details below).  However, if I
> evaluate the function first, then cut and paste the result into the call to
> Plot, it works ok (but this is not very useful to me, since I want to
> incorporate this into a larger function).  Further, if my function returns a
> list with only one equation in it, then handing the function call to Plot
> works.  I'm clueless as to what he difference is.
> 
> Here are the details.
> 
> This function returns a list of eqations, given a list of 2D points (cos[]
> and sin[] are degree-based versions of Cos[] and Sin[] that I've defined)
> 
> voteCurves[pnts_List] := Module[{vc, xx, yy},
>     vc = {};
>     For[ix = 1, ix ¾ Length[pnts], ix++,
>       xx = pnts[[ix]][[1]];
>       yy = pnts[[ix]][[2]];
>       vc = Append[vc, xx cos[\[Theta]] + yy sin[\[Theta]]];
>       ];
> 
>     vc
>     ]
> 
> Here are two typical points:
> 
> p1 = {158, 116};
> p2 = {74 , 135};
> 
> and here's a typical list of equations result:
> 
> voteCurves[{p1, p2}]
> 
> { 158 Cos[\[Pi] \[Theta] /180] + 116 Sin[\[Pi] \[Theta] /180],
>    74 Cos[\[Pi] \[Theta] /180] + 135 Sin[\[Pi] \[Theta] /180]}
> 
> This works (two equations, by cut and paste):
> 
> Plot[
> { 158 Cos[\[Pi] \[Theta] /180] + 116 Sin[\[Pi] \[Theta] /180],
>    74 Cos[\[Pi] \[Theta] /180] + 135 Sin[\[Pi] \[Theta] /180]},
>    {\[Theta], -90, 180},
>    AxesOrigin -> {0,0},
>    PlotRange -> {{-90, 180}, {0, 255 Sqrt[2]}},
>    AspectRatio -> 1,
>    Frame -> True])
> 
> And this works (one equation, as function return value):
> 
> Plot[voteCurves[{p1}],{\[Theta], -90, 180},
>    AxesOrigin -> {0,0},
>    PlotRange -> {{-90, 180}, {0, 255 Sqrt[2]}},
>    AspectRatio -> 1,
>    Frame -> True])
> 
> But this fails (two equations, as function return value):
> 
> Plot[voteCurves[{p1,p2}],{\[Theta], -90, 180},
>    AxesOrigin -> {0,0},
>    PlotRange -> {{-90, 180}, {0, 255 Sqrt[2]}},
>    AspectRatio -> 1,
>    Frame -> True])
> 
> The errors I get are
>     Plot::"plnr": "\!\(voteCurves[\(\({p1, p2}\)\)]\) is not
>     a machine-size real number at \!\(\[Theta]\) = \!\(-89.99998875`\)."
> 
> My best guess is that the theta I return from my function call is considered
> to be a different theta than the one I use in the plot statement.  But if
> that is true, why does the function call usage work ok when the function
> just returns one value?  I'm baffled trying to figure out the difference,
> and in trying to come up with a solution.
> 
> Thanks for any help,
> Bob Harris
> 
> P.S. I'm using Mathematica 4.0.1.0 on Mac OS 9.04 at home, *AND* Mathematica
> 5.something on Mac OSX at school.  Had the same problem on both.


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