What are the sparse and dense vector ? I cant undestand ,can you explain to me?please.Why do we use for?
A sparse matrix 

A sparse matrix is a matrix that is comprised of mostly zero values.
e.g

     1, 0, 0, 1, 0, 0
A =  0, 0, 2, 0, 0, 1 
     0, 0, 0, 2, 0, 0


A Dense matrix

Dense matrices with mostly non-zero values, which are referred to as dense matrices.

e.g.

     1, 1, 3, 1, 4, 5
A =  0, 1, 2, 0, 1, 1 
     1, 0, 0, 2, 1, 1