Re: Eternal Trouble with Dynamic: fishing for tips on my

*To*: mathgroup at smc.vnet.net*Subject*: [mg129520] Re: Eternal Trouble with Dynamic: fishing for tips on my*From*: Brentt <brenttnewman at gmail.com>*Date*: Sat, 19 Jan 2013 01:18:10 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-newout@smc.vnet.net*Delivered-to*: mathgroup-newsend@smc.vnet.net*References*: <20130118055133.1B044687E@smc.vnet.net>

That is exactly what I needed. I'd blindly fudged around the code and got it working but didn't really understand what the issue was until reading this. I think had I been thinking of "interpol" as a function I probably wouldn't have had the problem. So I learned a couple things here, thank you. On Fri, Jan 18, 2013 at 6:38 AM, John Fultz <jfultz at wolfram.com> wrote: > The Big Trouble from your code comes down to your doing: > > Show[{Graphics[=85], Dynamic[=85]}] > > This is trouble for the same reason that something like this would be > trouble: > > f[x] + Dynamic[g[x]] > > Show, like Plus, is a kernel function. It takes two Graphics objects and > combines them into a single Graphics object. It doesn't know what to do > with an argument of Dynamic. So, at the simplest level, you'll need to > remove Dynamic from the 'interpol' definition (since Show doesn't > understand Dynamic) and wrap it around the Show (to force Show to > re-evaluate every time its contents update). > > So why is this different from Graphics? Graphics[{Dynamic[=85]}] works > because Graphics doesn't need to evaluate its content. All Graphics does= , > when viewed as an output, is to display the content on-screen. I.e., if > one were to evaluate=85 > > Graphics[{obj1, obj2, obj3}] > > The result of that evaluation would be Graphics[{obj1, obj2, obj3}]. Wha= t > turns it into an onscreen graphic isn't the evaluation, but the act of > displaying the result=85or in Mathematica parlance, "typesetting" the > graphic. Compared with Show where the expected output of > > Show[{obj1, obj2, obj3}] > > is Graphics[{some stuff pulled from obj1/2/3}, some options pulled form > obj1/2/3] > > Does that make sense? > > Now that's enough to make your example work, but I also recommend that yo= u > turn your 'interpol' definition into a function definition. E.g., > something like: > > DynamicModule[{r = {{0, 0}}, interpol}, > interpol[x_] := > If[Length[x] >= 3, Graphics[{Circle[Last[x], 1/Length[x]]}], {}]; > EventHandler[ > Dynamic@ > Show[{Graphics[{Line[Dynamic[r]]}], interpol[r]}, > PlotRange -> ({{-1, 1}, {-1, 1}})], {"MouseDragged" :> (r = > DeleteCases[AppendTo[r, MousePosition["Graphics"]], None])}]] > > John Fultz > jfultz at wolfram.com > User Interface Group > Wolfram Research, Inc. > > > > On Jan 17, 2013, at 11:51 PM, Brentt <brenttnewman at gmail.com> wrote: > > > > > Hi, everytime I think I have Dynamic down it seemingly inexplicably > breaks. > > I'd much appreciate if I could step through the process that leaves me > with > > code that, with one small change, just stops working. (Maybe some > designers > > might get something out of seeing the coding process of an idiot?) > > > > So I want to draw a set of points on a graphic and then show a > dynamically > > updated interpolating function for those points. This is my interpolati= ng > > function, having this dynamically update upon drawing a set of points i= n > > the graphic is the goal: > > > > > > parametricInterpolation[param_, pointList_] := > >> Function[{t}, > >> Function[{f}, f[t]] /@ > >> Quiet[(ListInterpolation /@ Transpose[pointList])]][param]; > >> > > > > The function works if I take a random set of points. So I set it aside = to > > get the dynamic interface working using a more simple function in its > place > > (just so I know if any problems arise, which they have, it has nothing > > within the above slightly complicated function. ) > > > > Ok, so here is where I start. The point set are to be drawn when the > mouse > > is dragged on the graphic. To keep the code as simple as possible I sta= rt > > with a point at the origin: > > > > DynamicModule[ > >> {r = {{0, 0}}, interpol = {}}, > >> > >> interpol = > >> Dynamic@If[Length[r] >= 3, Circle[Last[r], 1/Length[r]], {}]; > >> EventHandler[ > >> Show[ > >> { > >> Graphics[{Line[Dynamic[r]]}], > >> Graphics[{interpol}] > >> }, > >> PlotRange -> ( { > >> {-1, 1}, > >> {-1, 1} > >> } ) > >> ], > >> {"MouseDragged" :> (r = > >> DeleteCases[AppendTo[r, MousePosition["Graphics"]], None])}] > >> ] > >> > > > > > > The variable interpol is going to eventually hold my interpolating > > function. Since it needs at least 3 points to work properly, I have the > > conditional so it need not evaluate until at least 3 points are drawn. > The > > Circle[Last[r], 1/Length[r]] I'm using as a test function in the > > interpolating function's place. (it simply draws a circle which shrink= s > as > > function of the the number of points). > > > > Code works so far. But the problem is I need the interpolating function > to > > be plotted using ParametricPlot. I need to replace Graphics[{Interpol}] > > with a ParametricPlot. > > > > Now this seems like it should be a rather simple step. But alas, no suc= h > > luck. My apparently naive approach is to have interpol hold a graphics > > object, and then use that as an element in Show's list argument. This > way, > > if it would work for the simpler function which draws the shrinking > circle, > > it would just be a matter of replacing this with a ParametricPlot which > > plots my interpolation function. But the code breaks before I even get > > there. Here is the seemingly small step that breaks the code > > > > DynamicModule[ > >> {r = {{0, 0}}, interpol = {}}, > >> > >> interpol = > >> Dynamic@If[Length[r] >= 3, Graphics[{Circle[Last[r], 1/Length[r]]}]= , > >> Graphics[{}]]; > >> EventHandler[ > >> Show[ > >> { > >> Graphics[{Line[Dynamic[r]]}], > >> interpol > >> }, > >> PlotRange -> ( { > >> {-1, 1}, > >> {-1, 1} > >> } ) > >> ], > >> {"MouseDragged" :> (r = > >> DeleteCases[AppendTo[r, MousePosition["Graphics"]], None])}] > >> ] > >> > > > > > > With the result > > > > Show::gcomb: Could not combine the graphics objects in Show[{\!\(\* > > GraphicsBox[LineBox[Dynamic[r$4494]]]\),\!\(\* > > GraphicsBox[{}]\)},PlotRange->{{-1,1},{-1,1}}]. >> > > > > > > I'm not sure what to make of the error message. It seems like what I di= d > > should work. I'm just passing a graphics object instead of an argument > for > > Graphics and I'm not sure why should that be a problem? Is there > something > > about Graphics that behaves differently that other functions I suppose, > but > > I haven't been able to discern what that is. Any tips would be greatly > > appreciated (whether it is about this code in particular, or anything > about > > the process. And please forgive me if I seem thick, I'm an undergraduat= e > > mathematics major, but not a terribly good one. This isn't for school > work, > > I'm just trying to figure out Mathematica to explore ideas I've learned > > about. I feel like I have a good sense of how it works except when it > comes > > to this Dynamic functionality.) > > > > > >

**References**:**Eternal Trouble with Dynamic: fishing for tips on my coding process?***From:*Brentt <brenttnewman@gmail.com>