Subscripted assignment dimension mismatch

Question

So, I'm trying to do the Gauss-Seidel method in Matlab and I found a code that does this but when I apply it to my matrices I get the Subscripted assignment dimension mismatch. error. I will show you my code in order to get a better idea.

%size of the matrix
n = 10;

%my matrices are empty in the beginning because my professor wants to run the algorithm for n = 100
and n = 1000. A's diagonal values are 3 and every other value is -1. b has the constants and the
first and last value will be 2,while every other value will be 1.
A = [];
b = [];

%assign the values to my matrices
for i=1:n
  for j=1:n
     if i == j
         A(i,j) = 3;
     else
         A(i,j) = -1;
   end
 end
end

for i=2:n-1
    b(i) = 1;
end

%here is the Gauss-Seidel algorithm
idx = 0;
while max(error) > 0.5 * 10^(-4)
    idx = idx + 1;
    Z = X;
    for i = 1:n
        j = 1:n; % define an array of the coefficients' elements
        j(i) = [];  % eliminate the unknow's coefficient from the remaining coefficients
        Xtemp = X;  % copy the unknows to a new variable
        Xtemp(i) = [];  % eliminate the unknown under question from the set of values
        X(i) = (b(i) - sum(A(i,j) * Xtemp)) / A(i,i);
    end
    Xsolution(:,idx) = X;
    error  = abs(X - Z);
end

GaussSeidelTable = [1:idx;Xsolution]'
MaTrIx = [A X b]

I get the error for the Xsolution(:,idx) = X; part. I don't know what else to do. The code posted online works though, and the only difference is that the matrices are hardcoded in the m-file and A is a 5x5 matrix while b is a 5x1 matrix.

Daniel Daniel · Accepted Answer · 2015-01-01T13:44:11

I am unable to run your code because some variables are not initialised, at least error and X. I assume the Problem is caused because Xsolution is already initialised from a previous run with a different size. Insert a Xsolution=[] to fix this.

Besides removing the error I have some suggestions to improve your code:

Use Functions, there are no "left over" variables from a previous run, causing errors like you got here.
Don't use the variable name error or i. error is a build-in function to throw errors and i is the imaginary unit. Both can cause hard to debug errors.
Initialise A with A=-1*ones(n,n);A(eye(size(A))==1)=3;, it's faster not to use a for loop in this case. To initialise b you can simply write b(1)=0;b(2:n-1)=1;
Use preallocation

Subscripted assignment dimension mismatch

2 Answers