Plotting Results of a Matrix from a For Loop Help

Hey guys, this is a snippet of my code I can't seem to get to graph. S is a 3x3 matrix and I need to plot points (1,1), (2,2) and (3,3) of the transformed 3x3 Sxy matrix for values of theta from 0:90. How to I get the figures to work and not just be blank? I would very much appreciate help with this. So far I've tried using the 'hold on' and moving the plot functions inside the for loop.

I would do this without the for loop but the matrix S does not change with each iteration so without the for loop, the inner matrix indices do not match.

for theta=0:90
    % Angles 
    c=cosd(theta);
    s=sind(theta);
 
    % Transformation Matrix
    T=[c.^2 s.^2 2.*c.*s; s.^2 c.^2 -2.*c.*s; -c.*s c.*s c.^2-s.^2];
    Tinv=[c.^2 s.^2 -2.*c.*s; s.^2 c.^2 2.*c.*s; c.*s -c.*s c.^2-s.^2];
 
    % Coordinate Transformations
    Sxy=Tinv*S*T;
 
    Exx=1/Sxy(1,1);
    Eyy=1/Sxy(2,2);
    Gxy=(1/Sxy(3,3))/2; 
end
 
figure(1)
plot(theta,Exx) 
xlabel('Theta in Degrees')
ylabel('Exx')
 
figure(2)
plot(theta,Eyy)
xlabel('Theta in Degrees')
ylabel('Eyy')
 
figure(3)
plot(theta,Gxy)
xlabel('Theta in Degrees')
ylabel('Gxy')

Answer 1

[Luke Horstman]

Your Exx Eyy and Gxy variables are only saving the last value of the loop. So, you are only plotting a single point. You need to index your variables in the loop so those are arrays instead of single values.

%initialize variables
theta = 0:1:90;
Exx = zeros(1,length(theta));
Eyy = zeros(1,length(theta));
Gxy = zeros(1,length(theta));
%Loop
for 1=1:length(theta)
    % Angles 
    c=cosd(theta(i));
    s=sind(theta(i));
 
    % Transformation Matrix
    T=[c.^2 s.^2 2.*c.*s; s.^2 c.^2 -2.*c.*s; -c.*s c.*s c.^2-s.^2];
    Tinv=[c.^2 s.^2 -2.*c.*s; s.^2 c.^2 2.*c.*s; c.*s -c.*s c.^2-s.^2];
 
    % Coordinate Transformations
    Sxy=Tinv*S*T;
 
    Exx(i)=1/Sxy(1,1);
    Eyy(i)=1/Sxy(2,2);
    Gxy(i)=(1/Sxy(3,3))/2; 
end

I haven't tested the code, but I'm pretty sure it should work.

[Briana Staheli]

Yes this worked! thank you so much!