Trying to run Gauss Siedel method, I have no idea where I'm going wrong in this code. I get an error of "Index exceeds matrix dimensions" Not sure how? Just need this to run Thanks

% Solve system from HW using Gauss Seidel
% System of Equations Simplified Below
% 2x1-3x2=-160 ---> x1=(3x2-160)/2
% x1+2x2-3x3 -----> x2=(3x3-x1+40)/2
% 2x3-x2=80 ------> x3=(x2+80)/2

clear;clc;
i=1;
x2(i)=0;x3(i)=0;
error_x1(i)=9999;

while error_x1(i) >= .01;
        x1(i+1)=(3*x2(i)-160+0*x3(i))/2;
        x2(i+1)=(3*x3(i)-x1(i+1)+40)/2;
        x3(i+1)=(x2(i+1)+80+0*x1(i+1))/2;

          errorx1(i+1)=abs((x1(i+1)-x1(i))/x1(i+1))*100;
          errorx2(i+1)=abs((x2(i+1)-x2(i))/x2(i+1))*100;
          errorx3(i+1)=abs((x3(i+1)-x3(i))/x3(i+1))*100;

          i=i+1;
  end

disp('                  x1               error(%)');
disp([x1',error_x1']);
disp('                  x2               error(%)');
disp([x2',error_x2']);
disp('                  x3               error(%)');
disp([x3',error_x3']);

Answer 1

[Marc Jakobi]

You have to pre-initialize x1, x2, x3, errorx1, errorx2 and errorx3. The index exceeds the matrix dimensions, because you index x1(i+1), which is x1(2) in the first iteration of your loop. Since x1 is initialized with a length of 1, the index 2 exceeds the dimension 1.

One solution would be

clear;clc;
i=1;
errorx1=9999;
errorx2 = 9999;
errorx3 = 9999;
x1 = 0;
x2 = 0;
x3 = 0;
while errorx1(i) >= .01
        x1 = [x1; (3*x2(i)-160+0*x3(i))/2];
        x2 = [x2; (3*x3(i)-x1(i+1)+40)/2];
        x3 = [x3; (x2(i+1)+80+0*x1(i+1))/2];
          errorx1 = [errorx1; abs((x1(i+1)-x1(i))/x1(i+1))*100];
          errorx2 = [errorx2; abs((x2(i+1)-x2(i))/x2(i+1))*100];
          errorx3 = [errorx3; abs((x3(i+1)-x3(i))/x3(i+1))*100];
          i=i+1;
end
disp('                  x1               error(%)');
disp([x1',errorx1']);
disp('                  x2               error(%)');
disp([x2',errorx2']);
disp('                  x3               error(%)');
disp([x3',errorx3']);

[Walter Roberson]

No, the code is careful not to use a value until it exists.

Answer 2

[Walter Roberson]

You initialized

error_x1(i)=9999;

and you have

while error_x1(i) >= .01;

which is consistent. But then you have

          errorx1(i+1)=abs((x1(i+1)-x1(i))/x1(i+1))*100;

which is assigning into a different variable, one that does not have the underscore.