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Manipulate and ParametricPlot Control?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg114328] Manipulate and ParametricPlot Control?
  • From: AES <siegman at stanford.edu>
  • Date: Wed, 1 Dec 2010 02:11:32 -0500 (EST)

This is a "what did I do wrong" post?  It's related to the 
"re-virginating" thread I started recently, but quite separate (and I 
think genuinely puzzling).

As a simple test file, I created a notebook (which I don't have any 
more) containing the one cell

   Manipulate[ Plot[x^a, {x,0,1}], {a,0,5}]

(May have had a PlotRange option also; don't recall for sure.)

Executed it; worked as expected.  

Decided to set an initial value for a; as best I recall (details are 
fuzzy), I edited the cell to change {a,0,5} to {{a,1},0,5}  (that's what 
I intended to do, anyway).

When I re-executed the cell, I got a live Manipulate that displayed a 
***two-dimensional*** (2D) Control -- something I had never seen before, 
but which looked exactly like the one I more recently found in the 
ParametricPlot example in the Manipulate Help.  

Exercising this Manipulate displayed **two** x^a curves, with values 
corresponding to the H and V axes of the 2D Control.  Functioned fine; 
no error msgs at any time.

I made a (presumably erroneous) guess that the edited cell must have 
somehow been influenced by the earlier execution of the initial, 
un-edited version of the same cell (I had two implementations of the "a" 
variable floating around, maybe??).  So, I tried adding Removes before 
the cell; tried Saving it, Quiting Mathematica, and re-opening it.  The 2D 
Control obstinately remained (and worked).  This then led to the 
re-virginating query.

I may have made a typo in the editing I noted above, but I sure as hell 
did NOT change Plot to ParametricPlot.  And when I try some tests now 
using Plot[ {f1, f2}, ...] inside a Manipulate, I get two curves -- but 
never a 2D Control.

Any ideas what went wrong here?


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