Re: Curve Tracking and fetching Locator coordinates

*To*: mathgroup at smc.vnet.net*Subject*: [mg119480] Re: Curve Tracking and fetching Locator coordinates*From*: John Fultz <jfultz at wolfram.com>*Date*: Sun, 5 Jun 2011 07:05:34 -0400 (EDT)

pt is only accessible from within the DynamicModule, and it will only be updated if you wrap it in Dynamic. Here's a very simple extension of your example which shows the value of pt at any given time: DynamicModule[{pt = {1, 0}}, Graphics[{Circle[], Locator[Dynamic[pt, (pt = Normalize[#]) &]]}, PlotRange -> 2, PlotLabel -> Dynamic[pt]]] I strongly encourage you to read the Introduction to Dynamic tutorial in the documentation (type tutorial/IntroductionToDynamic in the search field) and work through all of the examples. Sincerely, John Fultz jfultz at wolfram.com User Interface Group Wolfram Research, Inc. On Sat, 4 Jun 2011 06:19:42 -0400 (EDT), Just A Stranger 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 :)