When i run the blow code, the loop gets stuck on the underline line and it says that the indices on the left are not compatible with the indices on the right.

clc
clear all
sigma1=0.002;
sigma2=-0.003;
tau_12=0.004;
E1=181;
E2=10.3;
mu_12=0.28;
G12=7.17;
tetha=1:90;
for i=1:90
    c(i)=cosd(tetha(i));
    s(i)=sind(tetha(i));
    Stress_12=[sigma1; sigma2; tau_12];
    %S matrix calculation
    S=[1/E1 -mu_12/E1 0;-mu_12/E1 1/E2 0;0 0 1/G12];
    %Q matrix calculation
    Q=inv(S);
    %Strain Vector calculation
    Strain_12=S*Stress_12;
    %Transformation matrix calculation
    T(i)=[c(i).^2 s(i).^2 2.*s(i).*c(i); s(i).^2 c(i).^2 -2.*s(i).*c(i); -s(i).*c(i) s(i).*c(i) (c(i).^2)-(s(i).^2)];
    %Inverse of the transformation matrix
    T_1(i)=inv(T(i));
end

Answer 1

[KSSV]

clc
clear all
sigma1=0.002;
sigma2=-0.003;
tau_12=0.004;
E1=181;
E2=10.3;
mu_12=0.28;
G12=7.17;
tetha=1:90;
c=cosd(tetha);
s=sind(tetha);

T = zeros(3,3,90) ; 
for i=1:90
    
    Stress_12=[sigma1; sigma2; tau_12];
    %S matrix calculation
    S=[1/E1 -mu_12/E1 0;-mu_12/E1 1/E2 0;0 0 1/G12];
    %Q matrix calculation
    Q=inv(S);
    %Strain Vector calculation
    Strain_12=S*Stress_12;
    %Transformation matrix calculation
    T(:,:,i)=[c(i).^2 s(i).^2 2.*s(i).*c(i); s(i).^2 c(i).^2 -2.*s(i).*c(i); -s(i).*c(i) s(i).*c(i) (c(i).^2)-(s(i).^2)];
end

Answer 2

[Mathieu NOE]

hello

try this (T is a 3 x 3 matrix not a scalar so it must be stored in a cell)

clc
clear all
sigma1=0.002;
sigma2=-0.003;
tau_12=0.004;
E1=181;
E2=10.3;
mu_12=0.28;
G12=7.17;
tetha=1:90;

% all this could be removed from the for loop (no need to repeat all the
% time the same code )
    c=cosd(tetha);
    s=sind(tetha);
    Stress_12=[sigma1; sigma2; tau_12];
    %S matrix calculation
    S=[1/E1 -mu_12/E1 0;-mu_12/E1 1/E2 0;0 0 1/G12];
    %Q matrix calculation
    Q=inv(S);
    %Strain Vector calculation
    Strain_12=S*Stress_12;
    
for i=1:numel(tetha)
    %Transformation matrix calculation
    T{i}=[c(i)^2 s(i)^2 2*s(i)*c(i);
          s(i)^2 c(i)^2 -2*s(i)*c(i);
          -s(i)*c(i) s(i)*c(i) (c(i)^2)-(s(i)^2)];
    %Inverse of the transformation matrix
    T_1{i}=inv(T{i});
end