CODEWITHSUNDEEP
matlabmatrixsparse-matrix
Umut Tabak
Umut Tabak

Reputation: 1942

create the complete symmetric matrix by copying the lower triangular of a sparse matrix in triplet format

As the subject line suggests, what would be the most efficient way to copy the lower triangular part of a sparse matrix to the upper triangular part and complete the matrix entries to create the symmetric sparse matrix?

Assume that I have the triplets I, J, X for the lower triangle including the diagonal. I am reading these arrays from a commercial program and for storage space reasons, I believe, they only store the lower triangular part.

Well I will start testing different options soon, but wanted to see if someone else has experienced this before or not.

Upvotes: 1

Views: 452

Answers (2)

Ahmed Mahfouz
Ahmed Mahfouz

Reputation: 11

Another way to appraoch this problem, which is what I usually do, is to use the following code:

L = sparse(I, J, X, n, n);
S = L + L' - diag(L);

Where L is the lower-triangular matrix, S is the required symmetric matrix, and n is the number of rows/columns of the full square matrix.

Upvotes: 1

rahnema1
rahnema1

Reputation: 15837

You can use sparse:

idx = I ~= J; %index of nondiagonals
result = sparse([I;J(idx)], [J;I(idx)], [X;X(idx)]);

Because sparse adds together elements in X that have duplicate subscripts in I and J we exclude diagonal elements when concatenating vectors.

Upvotes: 1

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