How do I make a lower triangular matrix from a column vector?

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Leo Cordery Smith
评论： Rik 2021-3-29
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.

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Jan
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);
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Leo Cordery Smith
Leo Cordery Smith 2021-3-28
Hi Jan, do you know how to write the above code in Python? I've had to change my program from MATLAB to Python. Thanks.
Jan
Jan 2021-3-29
Sorry, I have basic programming skills in Python only. Ask this in a python forum.

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Rik
Rik 2021-3-9
Ran in:
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)
1 0 0 0 0 0 2 3 0 0 0 0 4 5 6 0 0 0 7 8 9 10 0 0 11 12 13 14 15 0 16 17 18 19 20 21
  2 个评论
Leo Cordery Smith
Leo Cordery Smith 2021-3-29
Hi Rik, do you know how to write the above code in Python? I've had to change my program from MATLAB to Python. Thanks.
Rik
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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