vpa function did not convert sym results as intended

Question

Leonie Bule 2022-5-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1729905-vpa-function-did-not-convert-sym-results-as-intended

回答： SAI SRUJAN 2023-10-27

New_Y_Bus.csv

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);

2 个评论
显示无隐藏无

Torsten 2022-5-30

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 2022-5-30

编辑：Leonie Bule 2022-5-30

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

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?

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

SAI SRUJAN 2023-10-27

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1729905-vpa-function-did-not-convert-sym-results-as-intended#answer_1342191

在 MATLAB Online 中打开

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);