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Manipulate subtleties?

  • To: mathgroup at
  • Subject: [mg101318] Manipulate subtleties?
  • From: AES <siegman at>
  • Date: Wed, 1 Jul 2009 06:34:07 -0400 (EDT)
  • Organization: Stanford University

If I type in (as three separate cells)

   makeFig := Graphics[{ Circle[{0,0}, 1], Circle[{0,0}, r]}];

   g1 = makeFig; 
   Show[g1, ImageSize->1*72],
   {{r, 0.5}, 0,2}]

   r = rc;  (* rc short for "control variable" *)
   g1 = makeFig;
   Show[g1, ImageSize->1*72],
   {{rc, 0.5}, 0,2}]

the second and third cells both produce Manipulate-type
output cells that appear to be OK.

The slider in the second cell output isn't active: Moving it 
doesn't cause anything to happen

The slider in the third cell animates *both* cells.

Reversing the order of the second and third cells gives 
the same behavior.


If I change g1 to g2 in the third line, the second cell gives
a Manipulate output cell with an error msg: "Radius r should be a 
positive number or pair of positive numbers, or a Scaled or Offset form."

If I remove the initial values for the slider from both lines, the 
second line still gives an error but the third doesn't, apparently using 
0 as the default value for rc (or does it use the 2nd value in {rc,0,1}?)

Seems like the second line might have done the same for r?

I guess my working rule from now on will be:

   "The control variable(s) of Manipulate slider(s) MUST 
    appear explicitly in the body of a Manipulate definition."

[But note that this is not logically necessary:  Any intelligent human, 
presented with either the second or the third cell would have all the 
information needed to do what's desired.]

Meta-question:  _Where_ in the user documentation would the mythical 
"ordinary user" have been expected to learn that the second line 
wouldn't work?

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