How do I make a lower triangular matrix from a column vector?
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I am trying to make a lower triangular matrix from a column vector to then produce a fully populated 24x24 symmetric matrix.
The column vector I'm using (A as a simple example) has 300 elements and I need to populate the triangular matrix (B as a simple example) as follows:
A = [i1 i2 i3 i4 i5 i6 i7 i8 i9 i10 ...]
B = [i1; i2 i3; i4 i5 i6; i7 i8 i9 i10; ....]
Any help would be appreciated. It may be that it's easier to make the symmetric matrix straight away, if so I'm open to suggestions. Thanks in advance.
采纳的回答
Jan
2021-3-9
Create the symmetric matrix directly:
n = 24;
Ind = zeros(n, n);
Ind((1:n) >= (1:n).') = 1:300;
Ind = Ind + Ind.' - diag(diag(Ind));
B = A(Ind);
3 个评论
Leo Cordery Smith
2021-3-9
Leo Cordery Smith
2021-3-28
Jan
2021-3-29
Sorry, I have basic programming skills in Python only. Ask this in a python forum.
更多回答（1 个）
The functions you should be looking for are tril and triu. You should be using triu, because you seem to want to fill your array in row-major order, while the Matlab default is column-major.
A=1:21;%replace with your data
%sz will only be a positive integer if numel(A) is a triangle number
sz=-0.5+sqrt(0.25-4*0.5*-numel(A));
B=triu(ones(sz));
B(logical(B))=A;
B=B.';
disp(B)
2 个评论
Leo Cordery Smith
2021-3-29
Rik
2021-3-29
Similar to Jan, my Python skill are only rudimentary. If you are an actual Python user, your success in googling documentation will probably be higher.
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