Compressed sparse row format (CSR)

The compressed sparse row (CSR) format uses three arrays: data, indptr, and indices:

• The 1D array data stores all the nonzero values in order. It has as many elements as there are nonzero elements, often denoted by the variable nnz.

• The 1D array indptr contains integers such that indptr[i] is the index of the element in data, which is the first nonzero element of row i. If the entire row i is zero, then indptr[i]==indptr[i+1]. If the original matrix has m rows, then len(indptr)==m+1.
• The 1D array indices contains the column index information in such a way that indices[indptr[i]:indptr[i+1]] is an integer array with the column indexes of the nonzero elements in row i. Obviously, len(indices)==len(data)==nnz.

Let's see an example:

The CSR format of the matrix:

is given by the...