Dynamic changing values

REN
2011 3 8
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Hello, I have a question about dynamic changing values.
A list of values (or maybe vectors or maybe matrices): a, b, c, d, e, f, ... ...
There are equations between a, b, c, and d, e, f
---------------
*a* , b, c,
---------------
*d*, e, f ....
---------------
a = x1*b + x2 * c
d = x1*e + x2 * f
values *a* depends on value b and c
values *d* depends on value e and f
x1, x2 are two constant,
And b, c, e, f are variables, their values keep updating in a loop. How to update a, d's values dynamically once b, c, e, f are changed?
I see matlab has no pointer like C, C++. Can matlab do changing values dynamically?
Thanks in advance for your help!

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Walter Roberson
Walter Roberson 2011-3-8

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If you have access to the symbolic toolbox, then you can define your values symbolically, and then when you need a particular numeric value, subs() the current values of the independent variables and double() the result of subs() to get the double precision representation of the resulting symbolic number.

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REN
REN 2011-3-8
hello Walter, I have symbolic Math toolbox, unsure that's what you mean, I am not familiar with subs(),
read help subs
any documents, code example to read about this?
Walter Roberson
Walter Roberson 2011-3-8
syms B C E F
a = x1*B + x2 * C;
d = x1*E + x2 * F;
Then as you change the value of the variables,
current_a = double(subs(a, {B,C,E,F}, {b,c,e,f}));
current_d = double(subs(d, {B,C,E,F}, {b,c,e,f}));
REN
REN 2011-3-8
thank a lot Walter.
R = subs(S, old, new) is what I am looking for
Jiro Doke
Jiro Doke 2011-3-8
Just curious, in your comment to my answer, you mentioned there too many variables like a and d. This applies to the symbolic solution as well. You need
a_new = subs(a, old, new);
d_new = subs(d, old, new);
...
for each variable. It is just as "tedious".
Walter Roberson
Walter Roberson 2011-3-8
Jiro, they don't have to be done one at a time: as in the example I gave, you can do them simultaneously.
Jiro Doke
Jiro Doke 2011-3-8
Walter, I'm not seeing that in your example (I assume you're talking about your second comment to this answer). You have
current_a = double(subs(a, {B,C,E,F}, {b,c,e,f}));
current_d = double(subs(d, {B,C,E,F}, {b,c,e,f}));
One for "a" and one for "d". I was just stating that this is essentially the same amount of "tediousness" as doing it numerically. I don't think you can do this:
[a_new, d_new] = subs({a, d}, {B,C,E,F}, {b,c,e,f})
Walter Roberson
Walter Roberson 2011-3-8
True. Though if the symbolic equations were all in an array or vector,then subs() could operate over the array or vector simultaneously. If need be you could then mat2cell(), assign that to a variable, and then
[a_new,d_new,...] = deal(t{:});
Walter Roberson
Walter Roberson 2011-3-8
num2cell(), not mat2cell().
REN
REN 2011-3-9
Yes, thanks again Jiro and Walter for your good idea . I do use mat2cell()(because thess variables are from large matrix) then deal(t{:}) as you suggest.

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更多回答（2 个）

Jiro Doke
Jiro Doke 2011-3-8

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Just redefine a and d after you update the other values:
for id = 1:100
a = x1*b + x2 * c
d = x1*e + x2 * f
...
<do your stuff>
...
<update b, c, e, f>
end

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REN
REN 2011-3-8
This isn't going to work..I try to update a list of values, a lot
these values dynamically linked with other variables.
Jiro Doke
Jiro Doke 2011-3-8
Well, I guess I don't fully understand your question. In my above example, every time through the loop, a and d are updated with the new set of values for b, c, e, f. Can you explain why this doesn't work?
REN
REN 2011-3-8
In my case, there are too many variables like a, d.
Writing a long list of varibles in for loop is a bit tedious. Thanks your help anyway...

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Daniel Shub
Daniel Shub 2011-3-8

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Seems like this might be related to the polynomial class example included in the OOP guide
web([docroot '/techdoc/matlab_oop/f3-39071.html'])

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