Re: How to Enable Automatic Recalculation

• To: mathgroup at smc.vnet.net
• Subject: [mg109883] Re: How to Enable Automatic Recalculation
• From: John Fultz <jfultz at wolfram.com>
• Date: Fri, 21 May 2010 06:44:49 -0400 (EDT)

```I think Daniel's analysis is pretty good, and I was a bit surprised that Dynamic
was raised as a possibility to solve this problem.  It would be possible, in
principle, to create an automated computational interface that keeps everything
up to date, but it would have to be in the context of a sequestered interface.

I do occasionally hear requests for Dynamic to behave differently...to
essentially bind calculations together invisibly for an extended duration of
time without the requirement of being displayed.  Such a construct wouldn't even
require a front end.  Maybe such an idea has merit, although I'm not convinced.
But it's well outside the mission of Dynamic, which is to efficiently bind state
for display and interaction.  If we ever did do such a thing, I would strongly
oppose calling it Dynamic.  It shares the notion of dependency tracking, but is
otherwise just a completely different concept.

Sincerely,

John Fultz
jfultz at wolfram.com
User Interface Group
Wolfram Research, Inc.

On Thu, 20 May 2010 06:39:57 -0400 (EDT), dh wrote:
> Hi,
> there seems to be two pitfalls in method 2 and 3.
> Dynamic is thought to update screen outputs, not to recalculate
> variables. If nothing is printed, Dynamic is not even evaluated.
> Let's look at method2:
> (* method 2: use Dynamic *)
> Clear[a,b]
> Dynamic[a]
> a
> a == 3;  (* preceding output changes to 3 *)
> b == 3 + a
> 6
> a == Pi;
> b
> 3 + Pi
> The last answer is WRONG. b is definitly set to 6 and is NEVER
> recalculated when a is changed. This is so because we use a un-delayed
> assign.
>
> Method 3 is even more dangerous:
> (* method 3: another way to use Dynamic *)
> Clear[a,b]
> a == 3;
> Dynamic[b == 3 + a]
> 6
> a == Pi;  (* preceding ouptput changes to 3 + Pi *)
> b
> 3 + Pi
>
> The last answer is again wrong in a devious way. The correct value is 6.
> However, if you reevaluate b again, it is suddenly 3+Pi. The reason is,
> that the Dynamic statement is evaluated AFTER b has been printed.
> Therefore, b obtains its new value too late.
>
> What can we learn from this is that Dynamic should be used for display
> jobs and not anything else. The right thing to do here is to use an
> delayed assign as in method 1.
> cheers, Daniel
>
> Am 20.05.2010 02:15, schrieb Murray Eisenberg:
>> There IS a "simple" answer. Actually, two answers. it used to be that
>> the only way to do this was to use SetDelayed ( :== ) for the symbol you
>> wanted to be recalculated whenever you changed another variable whose
>> values was specified by Set ( == ).  Since Mathematica 6, you also have
>> the more sophisticated method of using Dynamic quantities.
>>
>> (* method 1: use SetDelayed instead of Set for b *)
>> a == 5;
>> b :== 3 + a
>> b
>> 8
>> a == Pi;
>> b
>> 3 + Pi
>>
>>
>> (* method 2: use Dynamic *)
>> Clear[a,b]
>> Dynamic[a]
>> a
>> a == 3;  (* preceding output changes to 3 *)
>> b == 3 + a
>> 6
>> a == Pi;
>> b
>> 3 + Pi
>>
>> (* method 3: another way to use Dynamic *)
>> Clear[a,b]
>> a == 3;
>> Dynamic[b == 3 + a]
>> 6
>> a == Pi;  (* preceding ouptput changes to 3 + Pi *)
>> b
>> 3 + Pi
>>
>>
>> On 5/19/2010 7:01 AM, Victor Roberts wrote:
>>> I'm new to Mathematica, so perhaps there is a simple answer
>>> to this question.
>>>
>>> How to I set Mathematica so that when I change the value
>>> assigned to a variable it will automatically change all
>>> later calculations that use that variable.
>>>
>>> For example, if I set
>>>
>>> a == 5
>>>
>>> and
>>>
>>> b == 3 + a
>>>
>>> I would like the value of b updated each time I change the
>>> value of a.
>>>
>>> Right how, I need to recalculate each and every expression
>>> that uses the variable a if I change its value.  There must
>>> be a better way.
>>>
>>>
>>> --
>>> Vic Roberts
>>> Replace xxx with vdr in e-mail address.

```

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