HB
Posts:
42
Registered:
2/26/11


Re: Matrix dimensions must agree
Posted:
Oct 20, 2011 11:41 AM


On 20 Okt., 17:11, "Halley " <halleykhol...@gmail.com> wrote: > I am trying to write a program that utilizes the inverse iterative method. I calculate an initial x value and rho value given the matrices A, B, and b. I calculate a value of x normalized to the matrix B (xnorm) as well as a new value for rho for each iteration. Given that the relative error of rho is less than my preset tolerance (TOL) the loop ends, otherwise xnorm becomes the new value for x and the rho(i+1) becomes rho(i). The problem is in calculating rho(i), I am told my matrix dimensions do not agree when I have checked the sizes multiple times. Any suggestions? Thanks. > > %Declare initial variables > A = [2 1;1 4]; > b = [1;1]; > B = [1 0;0 1]; > Nmax = 20; > TOL = 1*10^5; > k = 1; > x = zeros(2,21); > x_a = zeros(21,2); > rho = zeros(21,1); > xnorm = zeros(2,21); > x(:,1) = A\b; > x_a(1,:) = x(:,1)' > > for i =1:Nmax > > %Calculate old values > rho(i) = (x_a(i,:).*A.*x(:,i))./(x_a(i,:).*B.*x(:,i)) > > %Calculate new values > xnorm(:,i) = (1/sqrt(x_a(i,:).*B.*x(:,i))).*x(:,i) > rho(i+1) = (x_a(i,:).*A.*x(:,i))./(x_a(i,:).*B.*x(:,i)) > > %Check for convergence > relative_error = abs((rho(i+1)rho(i))/rho(i+1)) > > if ( relative_error ~= 0 && relative_error <= TOL ) > x(i+1) = xnorm(i) > rho(i+1) = rho(i+1) > k = k > return > elseif (relative_error ~= 0 && relative > TOL) > x(i+1) = xnorm(i) > rho(i) = rho(i+1) > k = k+1 > > end > end
Hi, if you run your code line by line, or set a breakpoint somewhere and go into debugmode, you will see that you have this error already in the first line of the loop. The problem is that you're trying to multiply (elementwise) the 2x2 matrices A and B with x_a(i,:), a 1x2 vector and also with x(:,i), a 2x1 vector.
For elementwise multiplication, all factors have to have the same size.
Depending on how your needs, you may have to "enlarge" your vectors. For example with repmat, see help repmat.

