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Re: Strange Manipulate+ContourPlot behavior

  • To: mathgroup at smc.vnet.net
  • Subject: [mg82172] Re: Strange Manipulate+ContourPlot behavior
  • From: Yaroslav Bulatov <yaroslavvb at gmail.com>
  • Date: Sat, 13 Oct 2007 04:05:49 -0400 (EDT)
  • References: <fei3de$qn1$1@smc.vnet.net><200710110416.AAA21323@smc.vnet.net>

I suspect the discrepancy between "Manipulate[ContourPlot[e" and
"Manipulate[ContourPlot[Evaluate[e" has to do with automatic
simplification rules. The expression being plotted evaluates to 1, 0,
1/2 or Indeterminate depending on the order in which replacement rules
are applied.

e = (b (a - a p + b p))/(2 a b + (a - b)^2 p);
rules = {a -> 0, b -> 0, p -> 0};
Fold[ReplaceAll[#1, #2] &, e, #] & /@ Permutations[rules]

Is it possible to disable automatic simplification rules to prevent
expressions like a / (2 a) simplifying to 1/2?


On Oct 12, 12:11 am, Syd Geraghty <sydgerag... at mac.com> wrote:
> Not so weird...
>
> In[]:= a = 0; b = 0; p = 0; (b (a - a p + b p))/(-2 a b (-1 + p) +
> a^2 p + b^2 p)
>
> Out[]:= Indeterminate
>
> Syd
>
> Syd Geraghty B.Sc., M.Sc.
> sydgerag... at mac.com
> San Jose, CA
>
> On Oct 10, 2007, at 9:16 PM, Will Robertson wrote:
>
> > On Oct 10, 5:44 pm, Yaroslav Bulatov <yarosla... at gmail.com> wrote:
> >> Manipulate[
> >>  ContourPlot[(
> >>   b (a - a p + b p))/(-2 a b (-1 + p) + a^2 p + b^2 p), {a, 0, 1},
> >> {b,
> >>     0, 1}], {p, 0, 1}]
>
> > That is weird. Sticking in an Evaluate[] around the expression fixes
> > the problem -- until the slider is first moved. I guess I'd just
> > recommend avoiding p = 0 ... :)
>
> > Will




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