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Re: Curve Tracking and fetching Locator coordinates

  • To: mathgroup at smc.vnet.net
  • Subject: [mg119477] Re: Curve Tracking and fetching Locator coordinates
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Sun, 5 Jun 2011 07:05:01 -0400 (EDT)

DynamicModule[{pt = {1, 0}},
 Column[{
   Graphics[{
     Circle[],
     Locator[Dynamic[pt, (pt = Normalize[#]) &]]},
    PlotRange -> 2],
   Dynamic[pt],
   Dynamic[pt[[1]]]}]]


Bob Hanlon

---- Just A Stranger <forpeopleidontknow at gmail.com> wrote: 

=============
Hello, I'm trying to figure out how to constrain a locator's movement along
a curve, but then fetch the coordinates of the locator to use in a
calculation.

The documentation has an example of a Locator moving along a circle, but
it's strange, because the way they do it using Normalize seems to make it
not clear how to access the coordinates of the locator.

This is the example code from the Locator documentation:

DynamicModule[{pt = {1, 0}},
 Graphics[{Circle[], Locator[Dynamic[pt, (pt = Normalize[#]) &]]},
  PlotRange -> 2]]

I'm guessing the normalize function is being applied to the locator
position, and turning it into a unit vector (not entirely clear on how that
works in the code though). That has the effect of tracking the locator on a
unit circe But there is no variable for the locator positon. pt is simply a
list of constants {1,0} (although, I don't understand that entirely either,
because it also appears to be set to simply being a Normalize function)

Anyway, if someone could give me a hint as to what is going on in that code
I would much appreciate it (documentation seems sparse on the locator)

Barring that could someone just give me a quick hack for fetching the
locator's coordinates when it is being tracked along a circle (or better yet
if it is being tracked along an arbitrary curve.)?

Admittedly I have a foggy grasp about how Dynamic modules work, I'm able to
do basic stuff, but it starts to get unwieldy if I branch out.

Thank you for any help :)



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