vpa function did not convert sym results as intended

I have a function that applied gaussian elimination to a matrix and is suppose to eliminate values from a few rows and columns. A few days ago, the code was working fine and I got a bunch of complex elements as the output of the user defined function below. Today, the output is just a sym format and not double complex. I cannot figure out what the problem is.

I have attached the csv file of the ybus matrix.

function yreduced=gauss_elimination(ybus,yb,Nb)
indexyb=ybus(1,:);
MM=ybus(2:end,2:end);   % Augmented Matrix
MM=sym(MM);
%%%%%%%%%%%%%%%%  Gauss elimination method %%%%%%%%%%%%%%
disp('Gauss elimination method:');
[m]=size(MM);
y1=size(Nb);
y2=sort(Nb,'descend')


for j=1:y1
    for k=1:m
        if k==y2(j)
            for z=k-3:m
                if MM(k,k)==0
                    disp("replace row"+k + "with row" +z)
                    tmp=MM(k,:); MM(k,:)=MM(z,:);
                    MM(z,:)=tmp;
                end
            end
            %         end
            i=k-3;
            ss=1;
            while ss<=3
                disp("Eliminate ybus("+i+","+k+")")
                MM(i,:)=MM(i,:)-MM(k,:)*(MM(i,k)/MM(k,k))             
                
                %             i=i-3;
                ss=ss+3;
            end
            
        end
    end
end
% yreduced=M;
yreduced=vpa(MM,6);

[Torsten]

Why do you define MM as sym ?

MM is a numerical matrix from the .csv file. You will not gain higher precision this way.

[Leonie Bule]

I have a live script so I declared MM as sym for algebraic visualization. I have noticed a problem with the for loop

for z=k-3:m

if MM(k,k)==0

disp("replace row"+k + "with row" +z)

tmp=MM(k,:); MM(k,:)=MM(z,:);

MM(z,:)=tmp;

end

end

It doesn't replace the MM(k,k) k rows with non-zero rows. Maybe the sym and vpa function fail when the elements are not double precision?

Answer 1

[SAI SRUJAN]

Hi Leonie Bule,

I understand that you are facing an issue regarding the implementation of gauss elimination method.

You can follow the given example to proceed further.

a = [3 4 -2 2 
    4 9 -3 5 
    -2 -3 7 6 
    1 4 6 7 ];

a=sym(a);

[m,n]=size(a);
for j=1:m-1
    for z=2:m
        if a(j,j)==0
            t=a(j,:);
            a(j,:)=a(z,:);
            a(z,:)=t;
        end
    end
    for i=j+1:m
        a(i,:)=a(i,:)-a(j,:)*(a(i,j)/a(j,j));
    end
end
vpa(a,6);

You can refer to the following documentation to understand more about "vpa" MATLAB function.

https://www.mathworks.com/help/symbolic/vpa.html