Prev: Find value of unknown const that causes integral to equal some value
Next: Number to string headed by zeros
From: Antonio De Juárez on 12 Jun 2010 05:31
Sometimes, I would find useful a method to update variables
automatically. For example, given the delayed rule

b:=3 a+1;

it would be useful to have b updated whenever a is assigned a new
value, like a=37. Is there any method to do this in Mathematica?

Thanks,

Antonio.


From: Sjoerd C. de Vries on 13 Jun 2010 04:09
Are you kidding us? Using SetDelayed (:=) in the definition of b means
the most current value of a are used anytime b is used.

Check this:

b := a
a = 2;
b
a = 4;
b

yields 2, then 4.

Or try
b := a
Dynamic[b]

and the assign various values to a and see the dynamically displayed
value of b changing immediately.

By the way: b:=3 a +1 is not a delayed rule. That would be b:>3 a +1

Cheers -- Sjoerd

On Jun 12, 11:31 am, Antonio De Ju=E1rez <adejua...(a)gmail.com> wrote:
> Sometimes, I would find useful a method to update variables
> automatically. For example, given the delayed rule
>
> b:=3 a+1;
>
> it would be useful to have b updated whenever a is assigned a new
> value, like a=37. Is there any method to do this in Mathematica?
>
> Thanks,
>
> Antonio.


From: Peter Pein on 13 Jun 2010 04:11
Am Sat, 12 Jun 2010 09:31:06 +0000 (UTC)
schrieb Antonio De Ju=E1rez <adejuarez(a)gmail.com>:

> Sometimes, I would find useful a method to update variables
> automatically. For example, given the delayed rule
>
> b:=3 a+1;
>
> it would be useful to have b updated whenever a is assigned a new
> value, like a=37. Is there any method to do this in Mathematica?
>
> Thanks,
>
> Antonio.
>
>

Hi Antonio,

I'm afraid I do not understand your problem:

In[1]:= b:=3 a+1;
In[2]:= b
Out[2]= 1+3 a
In[3]:= a:=123
In[4]:= b
Out[4]= 370

As you see, b changes its value as a does.



From: Erik Max Francis on 13 Jun 2010 04:12
Antonio De Ju=E1rez wrote:
> Sometimes, I would find useful a method to update variables
> automatically. For example, given the delayed rule
>
> b:=3 a+1;
>
> it would be useful to have b updated whenever a is assigned a new
> value, like a=37. Is there any method to do this in Mathematica?

I don't really follow the goal of what you're doing here. Using `:=`
for assignment explicitly means that you do _not_ want the right-hand
side of the expression evaluated. If `b` depends on `a` then the
sensible thing to do here is to make `b` a function:

b[a_] := 3 a + 1;

and then do the appropriate substitute `b[somethingOrOther]` when the
time is right.

The only other thing I can think of that you might be trying to get at
would be `Dynamic`, which you can look up.

Perhaps if you said more about the problem you're trying to solve more
generally, people could suggest a more appropriate solution.

--
Erik Max Francis && max(a)alcyone.com && http://www.alcyone.com/max/
San Jose, CA, USA && 37 18 N 121 57 W && AIM/Y!M/Skype erikmaxfrancis
War is like love, it always finds a way.
-- Bertolt Brecht

From: telefunkenvf14 on 13 Jun 2010 04:12
On Jun 12, 4:31 am, Antonio De Ju=E1rez <adejua...(a)gmail.com> wrote:
> Sometimes, I would find useful a method to update variables
> automatically. For example, given the delayed rule
>
> b:=3 a+1;
>
> it would be useful to have b updated whenever a is assigned a new
> value, like a=37. Is there any method to do this in Mathematica?
>
> Thanks,
>
> Antonio.

You could tell Mathematica to treat b as Dynamic:

Dynamic[b]

b := 3 a + 1;

Each time you change a, b now reflects the change.

a = 2

a = 3

-RG

 |  Next  |  Last
Pages: 1 2
Prev: Find value of unknown const that causes integral to equal some value
Next: Number to string headed by zeros