cant mutplaye matrix by vector

heloo i have this code

clc;
clear all;
R=5;
C=100e-6;
L=10e-3;
U=5;
A=[0 1/C;-1/L -R/L];
x=[0;0];
t=linspace(0,0.5,4);
t_delta=t(2)-t(1);
x_0=[0;0];
for i=1:length(t)
    x(:,i+1)=t(i)*A
end

i have this code and i want to multipay A by t,but i get this eror: Assignment has more non-singleton rhs dimensions than non-singleton subscripts

how do i fix it ?

[Jan]

There is no "b" in your code. Which vector do you want to multiply with which matrix? What is the meaning of x1 and x_0?

[Guillaume]

how do i fix it ?

We don't know since we have no idea what you're trying to do.

t(i)*A results in a 2x2 matrix that you're trying to put in a 2x1 section of a matrix. That's never going to work.

In addition, as commented in your previous question, growing the x matrix is a bad idea. In this particular case, it's not even clear that the loop is needed.

Answer 1

[Jan]

Your A is 2x2 matrix and x is a 2x1 vector. Then:

x(:,i+1)=t(i)*A

has a 2x2 matrix on the right and a 2x1 vector on the left side.

It is not clear what you want to calculate. Start with writing it down with pencil and paper. Then create a clear an meaningful comment in the code, which explains, what you want to achieve. Afterwards writing the code (or suggesting an improvement) is much easier.

[tomer polsky]

clc;
clear all;

R=5;
C=100e-6;
L=10e-3;
U=5;
A=[0 1/C;-1/L -R/L];

x=[0;0];
t=linspace(0,0.5,4);
t_delta=t(2)-t(1);
x_0=[0;0];
for i=1:length(t)

    x(:,i+1)=t(i)*A

end

i want to multpliey each x(i) by A

[Guillaume]

Please take the time to

• proofread your posts
• understand the comments you're given and answer them clearly

The code you posted multiply t(i) by A. Then, you state you want to multiply x(i). Which is it?

You've been told by two different persons that the result for each step is 2x2 matrix, which you can't stuff into a 2x1 vector. Until we know what you want to do with these 2x2 matrices, we can't help you.

edit: could have done with proofreading my own post!

[tomer polsky]

ok i am sorry for the mess, so as i said i want to mulyipliey t(i) by A, i need to get vecotr of 2x1 ; becouse I need to get vecotr in the size of 2x1 for each i , and then multiplay this vector by vecotr A that his size is 2x2

[Jan]

t(i) is a scalar, A is a 2x2 matrix. Multiplying them creates a 2x2 matrix. If you state, that you want to get a 2x1 vector, a multiplication is not the right operation.

Answer 2

[Star Strider]

If you are doing a state-space simulation, you are doing it incorrectly. You need to use a matrix exponential (the expm function), with the correct matrix multiplications.

See if this does what you want:

for i=1:length(t)
    x(:,i) = expm(t(i)*A) * [1; 1] + x_0
end

You need to include the ‘A’, ‘B’, and ‘C’ matrices. (I created ‘B’ as [1;1] to produce a column vector output for ‘x’. Use your own matrices in the calculation.)