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PlanarAngle issues?

Werner Geiger
Posted 5 years ago

I am observing a very strange behavior from PlanarAngle:

  1. The result of PlanarAngle depends on the previous history.
  2. PlanerAngle assumes for the first few calls that its arguments are complex. After that, it doesn't.

Fortunately, however, all results are correct.

Attachments:
210717 PlanarAng...nb
POSTED BY: Werner Geiger
13 Replies
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Werner Geiger
Werner Geiger
Posted 5 years ago

Hans, you're right. I didn't really realize what "generic" means.

However, that makes the symbolic result of PlanarAngle pretty meaningless.
POSTED BY: Werner Geiger
Hans Milton
Hans Milton
Posted 5 years ago

Your findings are touched upon in the PlanarAngle documentation

enter image description here
POSTED BY: Hans Milton
Werner Geiger
Werner Geiger
Posted 5 years ago

Meanwhile I found out that the symbolic result of PlanarAngle is wrong. The correct symbolic result of PlanarAngle would have to be a Piecewise expression that depends on the relative positions of the points p, q1, q2.

See the following and attached Notebook "210717b PlanarAngle Issues 2.nb".

Attachments:
210717b PlanarAn...nb
POSTED BY: Werner Geiger
Werner Geiger
Werner Geiger
Posted 5 years ago
POSTED BY: Werner Geiger
Hans Milton
Hans Milton
Posted 5 years ago
POSTED BY: Hans Milton
Werner Geiger
Werner Geiger
Posted 5 years ago

@Jason: This makes results at least reproducible, just like Remove, ClearAll do (see above). But it is a very expensive operation since it clears all cached intermediate results so far.

We now have: enter image description here

Of course it doesn't prevent PlanarAngle from changing its behavior.

I reported this as a bug to Wolfram Support.

POSTED BY: Werner Geiger

This is clearly a bug and should be reported here. To work around the issue until it is fixed, I recommend calling ClearSystemCache[] before PlanarAngle.
POSTED BY: Jason Biggs
Werner Geiger
Werner Geiger
Posted 5 years ago

Very very strange. Does anybody have any idea what happens here?

I think repeatability and reproducibility shoul be one of the most basic features of programming languages. PlanarAngle seems to show a kind of randomness instead.
POSTED BY: Werner Geiger
Werner Geiger
Werner Geiger
Posted 5 years ago

I did not try that within the cloud but on the desktop only. I use a Wolfram|One license.
POSTED BY: Werner Geiger
Hans Milton
Hans Milton
Posted 5 years ago
POSTED BY: Hans Milton
Hans Milton
Hans Milton
Posted 5 years ago
POSTED BY: Hans Milton
Werner Geiger
Werner Geiger
Posted 5 years ago

Hmm, this is another miracle.

For me, your code executed in a freshly loaded kernel looks like: enter image description here
POSTED BY: Werner Geiger
Hans Milton
Hans Milton
Posted 5 years ago

I am also using Windows 10 and Mathematica 12.3.1. And I can not reproduce the issue. No matter how many times PlanarAngle[{0,1} -> {{-Sin ... is evaluated the output always contains Conjugate

Table[PlanarAngle[{0, 1} -> {{-Sin[\[Phi]], Cos[\[Phi]]}, {Cos[\[Phi]], Sin[\[Phi]]}}], 50]
POSTED BY: Hans Milton
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