Sparse Matrix Collapse
One of the problems of storing data in a matrix (a two-dimensional Cartesian structure) is that if not all of the elements are used, there might be quite a waste of space. In order to handle this, we can use a construct called a "sparse matrix", where only the active elements appear. Each such element is accompanied by its two indexes (the row and the column).
Discuss in what ways such a structure is similar to and/or different than a list.