Matrix with angles that change with each iteration. For loop?

I trying to create a plot using a matrix that changes angles from 0 to 180. I have two issues that I can't figure out. 1 How can I make it so my T value changes every time with each angle, and 2.How do I extract specific values from the resultant matrix sigma? I only want all the values in the first row first column and the first row second column in separate values so I can plot them? Maybe I'm approaching this while thing wrong, below is the code I came up with. Any help is highly appreciated.

c

lc; close all; clear all;
sig = [45 30; 30 -60]

for i = 0:180
    T = [cosd(i) sind(i);-sind(i) cosd(i)]

      sigma=T*sig*(T)'

      primesig = sigma(1,1)
      primetau = sigma(1,2)
  end

plot(primesig,primetau)
xlabel('sigma');
ylabel('tau');
title('Stress VS Strain');

Answer 1

[Walter Roberson]

Your code already uses a different T for each iteration and already extracts sigma values as needed. Your code has a different problem: it overwrites the prime* variables on each iteration. Instead of your code

      primesig = sigma(1,1)
      primetau = sigma(1,2)

you should have

      primesig(i) = sigma(1,1)
      primetau(i) = sigma(1,2)

[Carlos So]

I did that and now I'm getting an error.

"Attempted to access primesig(0); index must be a positive integer or logical"

Any idea what it could be?

[Walter Roberson]

      primesig(i+1) = sigma(1,1)
      primetau(i+1) = sigma(1,2)

[Walter Roberson]

More generally when you have a list of values that you want to go through, then a useful technique is to assign the list to a variable and step through the variable:

ivals = 0:180;                %put values in variable
for K = 1 : length(ivals)     %iterate over indices of variable
  i = ivals(K);               %retrieve the value for this iteration

    T = [cosd(i) sind(i);-sind(i) cosd(i)];

    sigma=T*sig*(T)';

    primesig(K) = sigma(1,1);   %store relative to loop count
    primetau(K) = sigma(1,2);   %store relative to loop count
  end