# How to change values of elements in a sparse matrix

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Ismaeel on 9 Aug 2018
Edited: James Tursa on 9 Aug 2018
I have the following sparse matrix:
(1,1) 1.1000
(2,2) 2.1000
(3,3) 0.1000
(4,4) 5.1000
(5,5) 0.6000
Created from a full matrix. I want to modify the sparse matrix by changing some values in the second column above. I do not want to change them in the original matrix. The reason is the full matrix is very large and the majority of elements are zeros. It takes a big portion of my computer memory (greater than 8 GB of my RAM capacity). The above matrix is only a small sample of my problem. Note that for this problem, the nonzero elements are located in the diagonal of the matrix.
Any idea?
Thanks
Rik on 9 Aug 2018
Variable in Matlab in general don't affect each other, so you can just change the elements. What errors are you encountering?

James Tursa on 9 Aug 2018
Edited: James Tursa on 9 Aug 2018
Not sure what your problem is. Sparse matrix elements can be changed directly just like full matrices. E.g.,
S = sparse(F); % your sparse matrix
S(3,2) = something; % change a value in S, does not affect F
F(5,2) = something else; % change a value in F, does not affect S
How many elements will you be changing?
James Tursa on 9 Aug 2018
Edited: James Tursa on 9 Aug 2018
If there are already non-zero elements in the diagonal and you are changing them to other non-zero values, then a loop probably won't hurt you since in theory it would not require large amounts of memory to be copied. But if that is not the case, then doing it all at once will probably be a better method. E.g.,
v = a vector of 82000 values for the diagonal
S(1:82001:end) = v; % change all the diagonals at once using linear indexing

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