cant mutplaye matrix by vector

Question

tomer polsky 2018-1-4

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编辑： Guillaume 2018-1-4

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 ?

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Jan 2018-1-4

编辑：Jan 2018-1-4

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 2018-1-4

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.

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Answer 1

Jan 2018-1-4

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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.

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tomer polsky 2018-1-4

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 2018-1-4

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.

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Answer 2

Star Strider 2018-1-4

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在 MATLAB Online 中打开

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.)