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MathGroup Archive 2000

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Re: newbie PolarPlot question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg25076] Re: newbie PolarPlot question
  • From: leko at ix.netcom.com (J. Leko)
  • Date: Thu, 7 Sep 2000 22:28:22 -0400 (EDT)
  • References: <8ov0jn$9p8@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Doug,

You entered:

PolarPlot[r, {t, 0, 2 Pi}]

when actually, what you meant to say was:

PolarPlot[r[t], {t, 0, 2 Pi}].

Also, your function definition should read:

r[t_] := 16/(5 + 3 Cos[t])

Note the underscore in the brackets on the left hand side! This is
important (see below).

I know that's a subtle difference, but what you are asking Mathematica to
do is to create a polar plot of the function r(t). It would be the same
difference as if in calculus you wrote "f" instead of "f(t)." One is a
variable, and the other is a function.

In this case, Mathematica sees "r" as an assignment to a symbol which
remains static (i.e., constant) throughout the Kernel's lifetime. "r[t_]"
on the other hand, says that we have a function of t (the underscore means
pattern, or "replace any occurrance of t on the right hand side with the
value specified when the function is called), and that the value of this
function can change depending on what the value of t is when it is called.

I can't help you with your editing. I'm new to this newsgroup myself.

J. Leko
University of Alabama in Huntsville

Please reply to: leko*j at cspar.uah.edu  and remove the *.

In article <8ov0jn$9p8 at smc.vnet.net>, Doug Magnoli <dmagnoli at home.com> wrote:

> I've defined:
> 
> r[t] := 16/(5 + 3 Cos[t])
> 
> Then I load Graphics, with:
> 
> <<Graphics`
> 
> and ask for:
> 
> PolarPlot[r, {t, 0, 2 Pi}], which tells me:
> 
> ParametricPlot::pptr:
> {r Cos[t], r Sin[t]} does not evaluate to a pair
> of real numbers at t = 2.617993877991494`*^-7
> 
> ParametricPlot::pptr:
> {r Cos[t], r Sin[t]} does not evaluate to a pair
> of real numbers at t = 0.25488992540742256`.
> 
> ParametricPlot::pptr:
> {r Cos[t], r Sin[t]} does not evaluate to a pair
> of real numbers at t = 0.5328694051959509`.
> 
> General::stop:
> Further output of ParametricPlot ::pptr will be
> suppressed during this calculation.
> 
> Near as I can make out, x and y should both evaluate to real numbers for
> any value of theta between 0 and 2 Pi, so I don't know what I'm doing
> wrong.
> 
> ....and, BTW, how do you get all those quotes from mathematica pages into
> this newsgroup without having to edit?  I tried to copy, e.g.,
> 
> In[32]:= PolarPlot[4, {t, 0, 2 Pi}]
> 
> and I don't get the In[32]:= part.  Also, when copying those error
> messages, I had to edit extensively, or else they look like this when
> copied / pasted:
> 
> ParametricPlot::"pptr": "\!\({\(\(r\\ \(\(Cos[t]\)\)\)\), \(\(r\\ \
> \(\(Sin[t]\)\)\)\)}\) does not evaluate to a pair of real numbers at
> \!\(t\) \
> = \!\(2.617993877991494`*^-7\)."
> 
> tia,
> 
> -Doug Magnoli


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