How can I do dynamic calculation?

How can I update the following variable?

a = 1; b = a + 2;

then I changed a = 2, but b is still 3.

How can I change or update b without executing b = a + 2?

If Matlab supports pointer in C, I guess the above problem is solved easily.

[John D'Errico]

But you are using MATLAB, not C. No pointers available.

Answer 1

[Matt J]

Even in C, I think a re-calculation would have to occur somewhere in the code to get a revised value. However, the kind of auto-updating I believe you're looking for is something a dependent class property can do:

 classdef myclass

     properties 
      a   
     end

     properties (Dependent)
      b   
     end

     methods  

         function val=get.b(obj)

             val=obj.a+2;

         end
     end
 end

Now you can do things like this,

    >> obj=myclass;
    >> obj.a=1; obj.b

    ans =

         3

    >> obj.a=3; obj.b

    ans =

         5

Answer 2

[Walter Roberson]

syms a
b = a + 2
a = 1
subs(b)
a = 2
subs(b)