Partial pivoting problem in matrix

Question

Netanel Malihi 2021-12-4

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1603090-partial-pivoting-problem-in-matrix

回答： Pranjal Kaura 2021-12-30

I'm getting a mistake in my matrix when trying to get the max number at the column and swiching rows , I'm pretty sure its got to do with an incorrect index.

Im using the partial pivoting method.

thank you for your help.

clear;
clc;
n=5;
v=repelem(4,n);
A=diag(v);
i=1;
j=1;
A(1,n)=-1
A(n,1)=-1
for i=1:n
    for j=1:n
        if i==j+1 ||j==i+1
        A(i,j)=-1;
        end
    end
end
b=1:n;
B=b'/n;
A=[A,B]
for k=1:n-1
    [max_A1,row_i]=max(abs(A(k:end,k)))
    if row_i~=k
        A1=A(k,:)
        A(k,:)=A(k,:)
        A(row_i,:)=A1
    else
    end
    for i=k+1:n
        L=A(i,k)/A(k,k)
        for j=k:n+1
            A(i,j)=A(i,j)-L*A(k,j)
        end
    end
end

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Answer 1

Pranjal Kaura 2021-12-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1603090-partial-pivoting-problem-in-matrix#answer_865035

Hey,

Please refer the code below

clear;
clc;
n=5;
v=repelem(4,n);
A=diag(v);
i=1;
j=1;
A(1,n)=-1;
A(n,1)=-1;
for i=1:n
    for j=1:n
        if i==j+1 ||j==i+1
        A(i,j)=-1;
        end
    end
end
b=1:n;
B=b'/n;
A=[A,B];
% test matrices
% A = [25 5 1 106.8;
%     64 8 1 177.2;
%     144 12 1 279.2];
% A = [1 2 -4 3;
%     2 5 -6 10;
%     -2 -7 3 -21;
%     2 8 -5 38];
% n = 4;
%partial pivoting code
for k=1:n-1
    [max_A1,row_i]=max(abs(A(k:end,k)));
    row_i = row_i - 1 + k;
    if row_i~=k
        A1=A(k,:);
        A(k,:)=A(row_i,:);
        A(row_i,:) = A1;
    end
    for i=k+1:n
        L=A(i,k)/A(k,k);
        A(i, k:end) = A(i, k:end) - A(k, k:end)*L;
%         for j=k:n+1
%             A(i,j)=A(i,j)-L*A(k,j)
%         end
    end
end