vpa function did not convert sym results as intended

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Leonie Bule
Leonie Bule 2022-5-30
回答： SAI SRUJAN 2023-10-27
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
Torsten
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
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
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?

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回答（1 个）

SAI SRUJAN
SAI SRUJAN 2023-10-27
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

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