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Re: Eternal Trouble with Dynamic: fishing for tips on my coding process?

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  • Subject: [mg129518] Re: Eternal Trouble with Dynamic: fishing for tips on my coding process?
  • From: John Fultz <jfultz at wolfram.com>
  • Date: Sat, 19 Jan 2013 01:17:30 -0500 (EST)
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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}].  What 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 you 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 interpolating
> function, having this dynamically update upon drawing a set of points in
> 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 start
> 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 shrinks 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 such
> 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 did
> 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 undergraduate
> 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.)
>
>




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