I am trying to make a new matrix for each iteration of a for loop

I am not sure how to make a new matrix for each iteration, I would like to have Q_bar1, Q_bar2, Q_bar3, Q_bar4 each having differnt values. I know I need to initalize if I am using Q_bar(i) but I am not sure how to do that.

%% Making Qbar matrices for all thetas

for i = 1:length(theta)

m = cosd(theta(i));

n = sind(theta(i));

T1 = [m^2, n^2, 2*n*m

n^2, m^2, -2*n*m

-n*m, n*m, (m^2)-(n^2)];

T2 = [m^2, n^2, n*m

n^2, m^2, -n*m

-2*n*m, 2*n*m, (m^2)-(n^2)];

Q_bar(i)= inv(T1)*Q*T2;

end

ERROR MESSAGE:

Unable to perform assignment because the indices on the left side are not compatible with the

size of the right side.

Error in project2 (line 85)

Q_bar(i)= inv(T1)*Q*T2;

[Stephen23]

You should replace this

inv(T1) * Q * T2

with the more efficient and numerically more robust

T1 \ Q * T2

See: https://www.mathworks.com/help/matlab/ref/inv.html#bu8tv8c-6

Answer 1

[Jan]

Q_bar = cell(1, length(theta));
for i = 1:length(theta)
    m = cosd(theta(i));
    n = sind(theta(i));
    T1 = [m^2, n^2, 2*n*m; ...
          n^2, m^2, -2*n*m; ...
         -n*m, n*m, m^2 - n^2];
    T2 = [m^2, n^2, n*m; ...
          n^2, m^2, -n*m: ...
         -2*n*m, 2*n*m, m^2 - n^2];
    Q_bar{i} = inv(T1) * Q * T2;
end

Or alterntively:

Q_bar = zeros(3, 3, length(theta));
for i = 1:length(theta)
    m = cosd(theta(i));
    n = sind(theta(i));
    T1 = [m^2, n^2, 2*n*m; ...
          n^2, m^2, -2*n*m; ...
         -n*m, n*m, m^2 - n^2];
    T2 = [m^2, n^2, n*m; ...
          n^2, m^2, -n*m: ...
         -2*n*m, 2*n*m, m^2 - n^2];
    Q_bar(:, :, i) = inv(T1) * Q * T2;
end

Note, that T1 \ Q * T2 is numerically more stable than calculating the inverse explicitely.

[Kiernan O'Boyle]

Thank you! This worked perfectly.