To simplify my actual problem, let's define the following symbolic expressions.
a=sym('a',[1,3]);
b=a(1);
If I somehow assign a new thing to a, I cannot get b to update. What is the proper way of doing this?
Example 1:
a=sym('a',[1,3]);
b=a(1);
a=sym([1,2,3]);
b
subs(b)
subs(b,a(1),100)
Example 1 Output:
b =
a1
ans =
a1
ans =
a1
Example 2:
a=sym('a',[1,3]);
b=a(1);
a=subs(a,a(1),3);
b
subs(b)
subs(b,a(1),100)
Example 2 Output:
b =
a1
ans =
a1
ans =
a1
Example 3:
a=sym('a',[1,3]);
b=a(1);
a(1)=sym(3);
b
subs(b)
subs(b,a(1),100)
Example 3 Output:
b =
a1
ans =
a1
ans =
a1
My actual problem is the following. I have N unknowns
a=sym('a',[1,N]);
and N equations.
EQN=sym('EQN',[1,N]);
N is defined according to user input and I don't know what it will be beforehand. I also have other variables that are dependent on a.
% some code here such as b = [2*a(1),3*a(2),..] etc.
Then I solve the equations for the unknowns
% some code here to derive EQN(1), EQN(2), ... EQN(N).
SOL=solve(EQN,a);
and substitute the solutions to a
SOL = struct2cell(SOL);
SOL = [SOL{:}];
a=subs('a',a,SOL);
Now I want to get rid of unknowns in b but I cannot due to the reason I tried to explain in the beginning.
The problem is that you are overwriting a instead of assigning a new value to a1, a2 and a3. So, you should change the underlying variable instead of a itself.
a=sym('a',[1,3]);
b = a(1)
a1 = 1;
subs(b) % returns 1
subs(b, 'a1', a1) % returns 1
subs(a) % [ 1, a2, a3]
If you want to use it to substitute the result of a solve operation, the best solution is to use subs with the output structure of solve as input:
sol = solve(a == [1 2 3], a);
subs(b, sol) % returns 1
Note that you can also manually construct this structure as follows:
sol.(char(a(1))) = 1 % sol.a1 = 1;