Looping over the non-zero elements of a uBlas sparse matrix
I have the following sparse matrix that contains O(N)
elements
boost::numeric::ublas::compressed_matrix<int> adjacency (N, N);
I could write a brute force double loop to go over all the entries in O(N^2)
time like belo开发者_Go百科w, but this is going to be too slow.
for(int i=0; i<N; ++i)
for(int j=0; j<N; ++j)
std::cout << adjacency(i,j) std::endl;
How can I loop over only the non-zero entries in O(N)
time? For each non-zero element I would like to have access to its value, and the indexes i,j
.
You can find the answer in this FAQ: How to iterate over all non zero elements?
In your case it would be:
typedef boost::numeric::ublas::compressed_matrix<int>::iterator1 it1_t;
typedef boost::numeric::ublas::compressed_matrix<int>::iterator2 it2_t;
for (it1_t it1 = adjacency.begin1(); it1 != adjacency.end1(); it1++)
{
for (it2_t it2 = it1.begin(); it2 != it1.end(); it2++)
{
std::cout << "(" << it2.index1() << "," << it2.index2() << ") = ";
std::cout << *it2 << std::endl;
}
}
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