how to make sparse, group association

I have a 129202x2 double and this represents different people's associations with certain "groups". On a row we have a group number (column1) och a person id (column2). I wish to create a sparse matrix when G(i,j)=1 means that person j is connected to group i. I would prefer the matrix to be symmetric. Don't send me the link to matlab sparse page, please. I don't get it.

Answer 1

[Guillaume]

" I would prefer the matrix to be symmetric." Why should it be? If person 1 and 2 are associated with group 4, it does not mean that person 4 is associated with group 1 and 2. There's not even a guarantee your matrix will be square unless you have the same number of groups as persons.

"Don't send me the link to matlab sparse page, please. I don't get it." Maybe you should try getting it, otherwise you won't get very far.

demodata = [1 5
            1 3
            2 4
            1 4
            2 1
            3 2];
connections = sparse(demodata(:, 1), demodata(:, 2), 1)

Also note that your original storage is actually more efficient in term of memory than the sparse matrix since you don't have to store the '1' that carries no information.

[Michael Sohl]

Thank you. The fact is that I have tried to read the description for S = sparse(i,j,v) loads of times. Can't digest the mathematical way of describing it though. Thanks again!