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Re: What is the point of having Initializations in DynamicModule and Manipulate?

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  • Subject: [mg123068] Re: What is the point of having Initializations in DynamicModule and Manipulate?
  • From: Armand Tamzarian <mike.honeychurch at>
  • Date: Tue, 22 Nov 2011 05:36:01 -0500 (EST)
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  • References: <jad5u1$ehq$>


I do NOT expect plot or g to dynamically update. That is not what this
is about.

On Nov 21, 8:33 pm, John Fultz <jfu... at> wrote:
> On Mon, 21 Nov 2011 15:57:00 +1100, Mike H wrote:
> > So the output cell containing the list {plot, "Silder", g[0]} is not a
> > display of the DynamicModule?
> To refresh, the code you included was...
> DynamicModule[{c}, {plot, Slider[Dynamic[c]], g[Dynamic@c]},
> Initialization :> (g[x_] := Cos[x]; plot = Plot[g[x], {x, 1, 10}]),
> UntrackedVariables -> {g}]
> * plot is not in a Dynamic.  You get exactly what the Shift+Enter evaluation
> gave you.
> * g[] is not in a Dynamic (regardless of what argument you pass it).  You get
> exactly what the Shift+Enter evaluation gave you.
> The Shift+Enter evaluation has completed long before the DynamicModule is
> displayed and the Initialization is executed.
> And the fact that you expected g[Dynamic@c] and/or Cos[Dynamic@c] to return
> anything sensible (which I had overlooked in your example before) indicates that
> you are failing to understand some really fundamental stuff about Dynamic.

On the contrary I have already repeatedly explained what I expect:
that is that plot would appear as the rendered plot because its global
value was set during the initialization. As far as my lack fundamental
understanding of Dynamic goes I repeat again that the example was
provided to me by Wolfram Research Inc. It is also worth noting that
when given the example I was also advised by Wolfram Research that
"The evaluation of g here is triggered by making the argument to g

I actually disagree with that advice provided to me by *your company*
and in fact it is wrong -- making the argument does not trigger the
evaluation of g. So rather than cast aspersions about my understanding
of Dynamic you might want to consider the knowledge base within

> here:
> and especially the post I link to here:
> Simply put, what is the value of?
> Cos[Dynamic[Pi]]
> Answer...Cos[Dynamic[Pi]], but because of the way Dynamic typesets, it will look
> like Cos[Pi] (because Dynamic[Pi] looks like Pi).  That is because Dynamic[Pi],
> in the Shift+Enter evaluation, always evaluates to Dynamic[Pi].  Dynamic is a
> HoldFirst function which never evaluates its merely creates a
> front end construct which evaluates and displays its argument.

yes. I agree but repeat that it is Wolfram providing this example

My question is solely about why the global value is not rendered. I am
not suggesting or expecting these variables to be dynamic so lets get
back on point.

> > I understand the point you make about if a colon were placed and no
> > output were generated but what is your definition of "first displayed"?
> It actually shows up in the front end.
> > I
> > guess this must be another point of difference because I would consider
> > the first display of the dynamic module as being the first time I see the
> > list {plot, "slider", g[0]} rendered.
> Yes, that's correct.  But why would you expect 'plot' or 'g' to magically
> update, since they are not inside of Dynamic?

I don't expect them to "update." I expect them to have global values
at the time of the rendering. The values are set once only. Based on
the language which you say you are going to re-write, I expect an
initialization to set plot to the graphic (once) and then when the
dynamicmodule content is evaluated and the rendered output shows the

> > So when I first see {plot, "slider", g[0]} the initialized value of plot
> > and g are not rendered. Apologies if it seems I'm laboring this point but
> > it remains confusing.
> > So it seems that the key to understanding this now becomes what the
> > definition of "display" is. Am I correct is saying that if a
> > DynamicModule is displayed -- and I await what this means -- the
> > initialization will have taken place and therefore the initialized value
> > will be shown in the display?
> The initialization will have taken place, but initialized values will only show
> if the FE is kicking off further evaluations to actually display them...which is
> what Dynamic does.

We're going in circles here, I do not understand how, if the
initializations have taken place, prior to the evaluation of the the
list (contents of the DM) that plot is not rendered as a graphic. If
the initialization has taken place then Global`plot must surely have a
value at the time the next step is executed -- which is the evaluation
of the DM body.

If I had evaluated plot=Plot[x,{x,0,1}] in a separate cell prior to
evaluating the DM the global value of plot would be (and is) shown in
the display. It is not a question of whether plot is updated -- I
don't expect it to be and don't want it to be (otherwise I would use
Dynamic) it is a question of whether the global value of plot is set
in the initializations prior to the evaluation of the body. If you
have an evaluation order like this:

evaluate initialization plot = plot[....]


evaluate {plot, slider, g[0]}

then I am still struggling to see how the global value of plot would
not be used in the displayed list if the initialization occurs first.

I'll try to explain what i expect again using a more simplified

In[]= ClearAll[g];
g = "hello world";

Out[]= "hello world"

that was what I expected.

 g = "hello world";

Out[]="hello world"

that was what I expected. whereas

 Initialization :> {g = "hello world"}]

Out[]= g

this is not what I expected because I thought the initialization would
establish the global value of g and therefore that value would be
*displayed*. (I do not expect g to dynamically update. How can it?)


> > FWIW I had assumed a display to be the rendered output.
> That is correct.
> > thanks
> > Mike
> Sincerely,
> John Fultz
> jfu... at
> User Interface Group
> Wolfram Research, Inc.

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