Question

Problem in MATLAB Code for solving desired ‘n’ number of simultaneous equations of the type Ax = b provided that the solving involves the method of upper triangular matrix and the values of A and b are evolved into Aprime and bprime along with the x values.

The problem is to write a code that can solve “n” number of simultaneous equation of the type Ax = b using upper triangulation matrix. The values of A and b are given as matrix in the command window. The code should return Aprime, bprime and x values as the answer when the program is run successfully. The code should also give the output as “Error, Matrix dimensions doesn’t match” (or whatever !) for certain equations ! The code works fine except it shows an error along with the above given Error message.

The code I used is as follows,

function [x, Aprime, bprime]=solved(A,b)
    n = size(A);
    % Assign the size of A to n.
    if (n(1)~= n(2)) || (det(A) == 0)
        % Checking through the determinant method for dimension error.
        disp('ERROR!! Matrix dimensions should agree.')
    else
        for j=1 %:n-1
            % Fix the first value of j to 1.
            if A(j,j)==0
                u=A(j,:);
                A(j,:)=A(j+1,:);
                A(j+1,:)=u;
                %using u as a temperary value "u", to save the row,to swap the positions of two rows.
                v=b(j);
                b(j)=b(j+1);
                b(j+1)=v;
                %using u as a temperary variable "v", to save the row,to interchange the positions of two rows in b matrix.
            end

            for i=j+1:n
                if A(i,j)~=0
                    %If the first number of the particular row  be zero.
                    b(i)=b(j)+(b(i)*(-A(j,j)/A(i,j)));
                    A(i,:) = A(j,:)+(A(i,:)*(-A(j,j)/A(i,j)));
                end
                %After this 'for'loop, the matrix becomes a upper triangle matrix.
            end

            Aprime=A;
            bprime=b;
            x=A\b;
            % Using this command the values of x,y,z can be found.
        end
    end
end

Please provide the suitable correction….

Results as obtained on the command window,

A = [1 1 0;2 1 1;1 2 3]

A =

 1     1     0
 2     1     1
 1     2     3

b= [3;7;14]

b =

 3
 7
14

[x, Aprime, bprime] = solved(A, b)

x =

 1
 2
 3

Aprime =

1.0000    1.0000         0
     0    0.5000   -0.5000
     0   -1.0000   -3.0000

bprime =

3.0000

-0.5000
-11.0000

The second type is,

A = [1 2 3; 4 5 6]

A =

 1     2     3
 4     5     6

b = [7;8;9;10]

b =

 7
 8
 9
10

[x, Aprime, bprime] = solved(A, b)
ERROR!! Matrix dimensions should agree.
Error in solved (line 2)
n = size(A);
Output argument “x” (and maybe others) not assigned during call to
“C:\Users\Hari\Documents\solved.m>solved”.

Answer 1

Instead of using disp, use the function error to communicate an error. That will tell MATLAB not to try to continue with execution after the error. Plus, the style will match builtin MATLAB errors.