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MathGroup Archive 1996

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Re: --->What is the problem with 'ParametricPlot'?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4675] Re: --->What is the problem with 'ParametricPlot'?
  • From: ags at seaman.cc.purdue.edu (Dave Seaman)
  • Date: Thu, 22 Aug 1996 03:55:25 -0400
  • Organization: Purdue University
  • Sender: owner-wri-mathgroup at wolfram.com

In article <4ve8rf$iro at dragonfly.wolfram.com>,
Seagull <whoami at hknet.com> wrote:
>I input the followings:
>x={Sin[t]}^3
>y={Cos[t]}^3
>ParametricPlot[{x, y}, {t, 0, Pi/2}]
>
>And got the following outputs:

 [ ... messages deleted ... ]

>ParametricPlot::pptr: 
>   x does not evaluate to a pair of real numbers at t = 0..

 [ ... messages deleted ... ]

>Can anyone tell me what is wrong?
>Thanks in advance!!

Your x and y should evaluate to one of two things for this example to work:

	1.  A real number,
	2.  A list consisting of a pair of real numbers.

In case (2) you would be plotting two separate curves in parametric
form on the same axis.  However, your x and y each evaluate to
something that matches neither (1) nor (2), namesly:

	3.  A list consisting of a single real number.

That's because the Power function (represented here by "^") is listable
and therefore x={Sin[t]}^3 sets x to {Sin[t]^3}, which is a one-element
list.  It should work much better if you remove the curly braces.

Mathematica notices that the value of x is not a real number (case (1))
and therefore assumes it must match case (2) -- but then the length of
the list turns out to be 1 instead of 2, hence the error message that x
"does not evaluate to a pair of real numbers...."

-- 
Dave Seaman			dseaman at purdue.edu
      ++++ stop the execution of Mumia Abu-Jamal ++++
    ++++ if you agree copy these lines to your sig ++++
++++ see http://www.xs4all.nl/~tank/spg-l/sigaction.htm ++++

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