Re: Curve Tracking and fetching Locator coordinates

*To*: mathgroup at smc.vnet.net*Subject*: [mg119512] Re: Curve Tracking and fetching Locator coordinates*From*: Murray Eisenberg <murray at math.umass.edu>*Date*: Tue, 7 Jun 2011 06:48:01 -0400 (EDT)

With that code and, say, the example alongTheCurve[Sin, 0, 2 \[Pi]], as I move the locator, I get a huge list of messages generated, all of the form "Set::write : Tag Sin in Sin[xxxxx] is Protected." On 6/6/2011 6:24 AM, Alexei Boulbitch wrote: > This is straightforward. If your Locator coordinates are denoted as pt, and a function along which you want to move the Locator is f[x], then you simply need to use the construction: > > Locator[Dynamic[{pt[[1]], f[pt[[1]]] > } > ] > ] > > The function entitled "alongTheCurve" plots a curve f[x] from xMin to xMax and the locator slides along this curve. The value of locator can be seen in the upper right corner of the plot: > > alongTheCurve[f_, xMin_, xMax_] := > DynamicModule[{pt = {xMin, f[xMin]}}, > (* The Plot and the Locator are combined by Show *) > Show[{ > (* Here is the plot of your curve *) > Plot[f[x], {x, xMin, xMax}, > > (* This is the inset into the plot showing the locator value *) > Epilog -> > Inset[Style[Dynamic[{pt[[1]], f[pt[[1]]]}], Red, 14], > Scaled[{0.8, 0.8}]]], > > (* This part draws the locator *) > Graphics[Locator[Dynamic[{pt[[1]], f[pt[[1]]]}]]] > }] > ]; > > Try this: > alongTheCurve[Sin, 0, 2 \[Pi]] > alongTheCurve[Cos, 0, 2 \[Pi]] > > This function will not work in case you give it the combination of the functions like > alongTheCurve[Sin+Cos, 0, 2 \[Pi]] > > Anyway, my aim is not to give a general solution, but only to show one possible way to answer your question. > > Have fun, Alexei > > > 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 :) > > -- Murray Eisenberg murray at math.umass.edu Mathematics & Statistics Dept. Lederle Graduate Research Tower phone 413 549-1020 (H) University of Massachusetts 413 545-2859 (W) 710 North Pleasant Street fax 413 545-1801 Amherst, MA 01003-9305