What is the best way to create a sparse array in C++?

Question

I am working on a project that requires the manipulation of enormous matrices, specifically pyramidal summation for a copula calculation.

In short, I need to keep

Answer 1

I would suggest doing something like:

typedef std::tuple coord_t;
typedef boost::hash coord_hash_t;
typedef std::unordered_map sparse_array_t;

sparse_array_t the_data;
the_data[ { x, y, z } ] = 1; /* list-initialization is cool */

for( const auto& element : the_data ) {
    int xx, yy, zz, val;
    std::tie( std::tie( xx, yy, zz ), val ) = element;
    /* ... */
}

To help keep your data sparse, you might want to write a subclass of unorderd_map, whose iterators automatically skip over (and erase) any items with a value of 0.