Programming

The CSR (compressed sparse row) format is a very simple but efficient way of storing sparse matrices, meaning matrices with a large amount of zero entries, which should not all occupy memory. It has three arrays: one holds all non-zero entries in order, read row by row, the next array contains the column indices of each non-zero element (and therefore has the same length as the first array), the third array indices into the first array for the first element of each row, so we can tell where a new row starts.

On sparse matrices it has optimal memory efficiency and fast lookups, the main downside is that adding or removing elements from the matrix requires shifting all three arrays, so it is mostly useful for immutable data.