Subscript indices error when plotting

Running a relatively simple code to plot 2 variables against each other.

      clear;clc;
      syms A1 B2

      R=0.5;
      Cp=0.5;

      Cv=0.3;
      i = 0;
      while Cv<1.11
          i = i + 0.01;
          Cv= Cv + 0.01;

          R = Cv^2*((1/Cv-tan(A1))^2 - (tan(B2))^2)/2/(1-Cv*tan(A1) - Cv*tan(B2));

          Cp = 1 - (tan(B2)/(1/Cv - tan(A1)))^2;

          [A1,B1] = vpasolve([R, Cp], [A1,B2]);

          SWR = 1 - Cv*(tan(A1) + tan(B2));

          X(i,2) = Cv;
          Y(i,2) = SWR;

      end

      plot(X(:,2),Y(:,2),'g-');
      hold on

And the error I get is as following

Subscript indices must either be real positive integers or logicals.

Error in gasturbine_hw7 (line 22)
    x(i,2) = Cv;

I think the error is due to Cv not being an integer but how do I make a matrix and call individual values from it that aren't integers without receiving that error message?

My end goal here is to plot Cv on the X axis and SWR on the Y axis.

Answer 1

[Roger Stafford]

This is your problem:

   X(i,2) = Cv;
   Y(i,2) = SWR;

Matlab is objecting to the fact that 'i' is being used as an index, but its value is not a positive integer. You need to revise your code so that the index value in X and Y is a positive integer.

[Vidhan Malik]

ah it was having problems with the index not the value assigned to it, okay makes sense, thanks!

Answer 2

[John D'Errico]

Why should CV be a problem here? What variable is a subscript? i is a variable. What values does i take on?

i = 0;
...
  i = i + 0.01;

    X(i,2) = Cv;
    Y(i,2) = SWR;

Now read the error message again. You cannot use a non-integer number (or any zero or negative integer) as an index.

Think of it like this: Where would MATLAB store the 0.01 element of a vector? It might get a little tight between elements at times. :)

[Vidhan Malik]

thanks! Yeah for some reason I was associating the word indices with some else entirely.

[Vidhan Malik]

Just wondering if you could continue your help from before. How do you return a variable back to being a variable after you previously have defined it?

As in if A1=54 in the first loop and now for the second iteration of the loop, I want it being a variable again.

I think one way would be to move where I defined the equations to before the loop, but what if I had some reason not to? How would I go about un-defining a variable?