# 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

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Margaret Mora on 22 Jan 2022
Commented: _ on 22 Jan 2022
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

_ on 22 Jan 2022
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
_ on 22 Jan 2022
Welcome, and thank you! Any other questions, feel free to post them