C++ armadillo sparse matrix batch insertion

Question

I am looking at batch insertion for sparse matrices in armadillo in the docs "http://arma.sourceforge.net/docs.html#batch_constructors_sp_mat".

It defines form1 as:

form 1: sp_mat(rowind, colptr, values, n_rows, n_cols)

What does colptr hold? If I understand correctly, it should have the actual address to whatever columns we want to insert at ?

Seems strange to me rowind are not pointers but colptr are pointers. Any reason for this?

Answer 1

Armadillo uses the standard Compressed Sparse Column (CSC) format for storing sparse matrix data. The format is also known as Compressed Column Storage (CCS) and Harwell-Boeing. The row indices and column pointers are explained on several sites:

• Wikipedia: http://en.wikipedia.org/wiki/Sparse_matrix#Compressed_sparse_column_.28CSC_or_CCS.29
• Netlib: http://netlib.org/linalg/html_templates/node92.html
• http://www.cs.colostate.edu/~mroberts/toolbox/c++/sparseMatrix/sparse_matrix_compression.html

The CSC format is used for compatibility with existing sparse solvers, etc.