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Re: Dynamic: feature?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg88112] Re: Dynamic: feature?
  • From: Chris Osborn <chrisosb at gmail.com>
  • Date: Fri, 25 Apr 2008 05:27:59 -0400 (EDT)
  • References: <fupm3c$s4f$1@smc.vnet.net>

On Apr 24, 5:06 am, John Fultz <jfu... at wolfram.com> wrote:
> On Wed, 23 Apr 2008 04:10:12 -0400 (EDT), Jean-Marc Gulliet wrote:
> > Chris Osborn wrote:
>
> >> I have a cell containing this:
>
> >> Dynamic[f[a, b]]
>
> >> Then I do this:
>
> >> a /: f[a, b] = 12
>
> >> and the dynamic cell updates to 12, as I would expect.
>
> >> When I do this:
>
> >> b /: f[a, b] = 45
>
> >> The dynamic cell does *not* update for some reason.
> >> It seems as though it does not realize an update has occurred when it
> >> is associated with b.
>
> >> Does anyone know why this is?
>
> >> Thanks in advance,
> >> Chris
>
> > Weird. I witness the same behavior if the cells are evaluated in the
> > given order. However, ff one change the order of the assignments (i.e. b
> > first, then a) Dynamic[] works as expected. Evaluating the following
> > expressions in that order,
>
> > Dynamic[f[a, b]]
> > (* The first output cell is: f[a, b] *)
>
> > b /: f[a, b] = 45
> > (* The first output cell is now: 45 *)
>
> > a /: f[a, b] = 12
> > (* The first output cell is now: 12 *)
>
> > $Version => "6.0 for Mac OS X x86 (64-bit) (February 7, 2008)"
>
> > I have also recorded the sessions in a PDF file. See
>
> >http://homepages.nyu.edu/~jmg336/mathematica/weirddynamic.pdf
>
> > Regards,
> > -- Jean-Marc
>
> A principle rule of debugging Dynamics...consider what Shift-Enter evaluations
> do.  In this case, exactly the same thing.  In fact, Dynamic correctly reflects
> what a Shift-Enter evaluation on f[a,b] would do at that very moment.
>
> Sincerely,
>
> John Fultz
> jfu... at wolfram.com
> User Interface Group
> Wolfram Research, Inc.

Thanks to everyone for their comments; they have been quite helpful.
It seems that I was creating two separate rules, one associated with a
and one associated with b (I had expected the 2nd one to overwrite the
1st).

Because a is the first argument, its rule takes precedence.

Thanks also John for the tip on debugging Dynamic -- see what shift-
enter does.

Chris


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