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Re: Scoping constructs Block, Module, ModuleBlock violate principle

  • To: mathgroup at smc.vnet.net
  • Subject: [mg111111] Re: Scoping constructs Block, Module, ModuleBlock violate principle
  • From: István Zachar <zac at freemail.hu>
  • Date: Tue, 20 Jul 2010 07:55:20 -0400 (EDT)
  • References: <i23k4v$euh$1@smc.vnet.net>

I think the answer for your DynamicModule-related question is:
DynamicModule *does not* wrap its local variables in Dynamic. It is
never stated in the Documentation.
Morover, it is somewhat explicitely stated, that you *should use*
Dynamic wrappers when needed:

"DynamicModule[{x,y,...},expr] represents an object which maintains
the same local instance of the symbols x, y, ... in the course of all
evaluations of Dynamic objects in expr. ... "

The following produces the behaviour you're looking for:

DynamicModule[{n = 7}, Row[{Dynamic@n, Button["X", --n; Print[n]]}]]

Cheers
Istv=E1n



On Jul 20, 9:44 am, Michael <michael2... at gmail.com> wrote:
> This statement yields unexpected behavior:
>
> DynamicModule[{n = 7},
>   Row[{n, Button["X", --n; Print[n]]}]
>   ]
>
> Why doesn't the "n" that appears as the first element in Row[] change
> when I click on the button?  The Print[] statement clearly shows it is
> being decremented.  I can look at "n" outside of the DynamicModule[] an=
d
> see that it is not defined, so Button[] can't be working with a
> different "n" than Row is trying to display, can it?
>
> n2=7;
> Dynamic[
>   Row[{n2, Button["X", --n2; Print[n2]]}]
>   ]
>
> This behaves as expected, but I need localization and this doesn't
> provide it.  Ok, I will use a Block[].  But wait...
>
> Block[{n3 = 7},
>   Dynamic[
>    Row[{n3, Button["X", --n3; Print[n3]]}]
>    ]
>   ]
>
> Mathematica highlights n3 in Red, and n3 appears to lose its value
> inside the Dynamic.  I have looked, but I can't find anything in the
> documentation for Block[] or Dynamic[] suggesting this sort of behavior.
>
> Ok, how about a Module[] instead of a Block[]?
>
> Module[{n3 = 7},
>   Dynamic[
>    Row[{n3, Button["X", --n3; Print[n3]]}]
>    ]
>   ]
>
> This works fine.  So why doesn't DynamicModule[] work???  There is al=
so
> another problem with this approach.  I might want to define a more
> complex interface than just a button:
>
> f[x_] := Button[x, --n3; Print[n3]]
> Module[{n3 = 7},
>   Dynamic[
>    Row[{n3, f["X"]}]
>    ]
>   ]
>
> This is why I wanted to use Block[] or even DynamicModule[], because
> then f[] would see n3 in the context of the Block.  But since Module[]
> creates internally some name like n3$1, f[] can't see it.
>
> I'm using Version 7.
>
> What's going on here?  Any suggestions on how I can accomplish what
> seemed like any easy task but wound up eating up my entire evening with
> nothing to show for it?
>
> Thanks,
>
> Michael



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