Dynamic changing values

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!

Answer 1

[Walter Roberson]

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.

[REN]

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]

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]

thank a lot Walter.

R = subs(S, old, new) is what I am looking for

[Jiro Doke]

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]

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]

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]

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]

num2cell(), not mat2cell().

[REN]

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.

Answer 2

[Jiro Doke]

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

[REN]

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]

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]

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...

Answer 3

[Daniel Shub]

Seems like this might be related to the polynomial class example included in the OOP guide

web([docroot '/techdoc/matlab_oop/f3-39071.html'])

[REN]

thanks for your link too