I want to create a matrix combining 2 matrices. The new one must have from the diagonal up de elements of matrix A and from the diagonal down the elements of matrix B. Help

I want to create a matrix combining 2 matrices. The new one (matrix C) must have from the diagonal up de elements of matrix A and from the diagonal down the elements of matrix B. Both matrices A and B have 0 in the diagonal, they have the same size and are simetric. Can anyone help with the code for that? Thank you

Answer 1

[Voss]

You can use triu() and tril() to get upper- and lower-triangular matrices, respectively, and logical indexing to combine them:

% Create some symmetric matrices:
A = [0 1 2; 1 0 3; 2 3 0];
B = 10*A;
display(A); display(B);

A = 3×3

0 1 2 1 0 3 2 3 0

B = 3×3

0 10 20 10 0 30 20 30 0

% get upper-triangular matrix of A:
A_tri = triu(A);
% get lower-triangular matrix of B:
B_tri = tril(B);
display(A_tri); display(B_tri);

A_tri = 3×3

0 1 2 0 0 3 0 0 0

B_tri = 3×3

0 0 0 10 0 0 20 30 0

% get logical matrix where A_tri is non-zero:
A_idx = logical(A_tri);
% get logical matrix where B_tri is non-zero:
B_idx = logical(B_tri);
display(A_idx); display(B_idx);

A_idx = 3×3 logical array
   0   1   1
   0   0   1
   0   0   0

B_idx = 3×3 logical array
   0   0   0
   1   0   0
   1   1   0

% make a new matrix C and fill in the upper and lower triangular parts using the logical matrices:
C = zeros(size(A));
C(A_idx) = A(A_idx);
C(B_idx) = B(B_idx);
display(C)

C = 3×3

0 1 2 10 0 3 20 30 0

[Margaret Mora]

Thank you so much Benjamin! It worked perfectly, I would not have figure it out on my on. Thanks for your kind, smart and quick response. Happy 2022

[Voss]

Excellent! Please mark my answer as 'Accepted', if you don't mind.

[Margaret Mora]

Done! I am new on the forum.. Thanks again

[Voss]

Welcome, and thank you! Any other questions, feel free to post them