I am creating a matrix of symbolic variables (A) and then creating an expression using the variables in this matrix (X). I intend to set the value of the symbol in a particular index in A (for example, in my code I do A(1,1) = 11), and then I want that to be reflected in the expression. However, when I do subs(X), I find that the symbol is not replaced. Is there any way I can achieve this?
Below is what I am trying:
>> A = sym('X', [2 2])
A =
[ X1_1, X1_2]
[ X2_1, X2_2]
>> X = A(1,1)*10 + A(2,2)*11
X =
10*X1_1 + 11*X2_2
>> A(1,1)=11
A =
[ 11, X1_2]
[ X2_1, X2_2]
>> subs(X)
ans =
10*X1_1 + 11*X2_2
I could of course do X1_1 = 2. My problem is that this is not amenable to looping. I'd like to set the values in a loop. Obviously A(*,*)=* is amenable to looping. Is there any way to set the value of X1_1 indirectly?
Edit: To achieve this, I can redefine X after setting the value of A(*,*). However this is not an option for me. Defining X is a very costly operation. Doing it multiple times is out of the question for my needs.
Instead of updating an index in A with a value, you can use the symbolic variable in the index of A to substitute that value in X:
>> A = sym('X', [2 2]);
>> X = A(1,1)*10 + A(2,2)*11;
>> X = subs(X, A(1,1), 11)
X =
11*X2_2 + 110
And if you want to do this for all the symbolic variables in A, you don't even have to use a loop. Just one call to subs will work:
>> Avalues = [11 0; 1 10]; % The values corresponding to symbolic variables in A
>> X = subs(X, A, Avalues)
X =
220