Creating (and Accessing) a Sparse Matrix with NA default entries

Question

After learning about the options for working with sparse matrices in R, I want to use the Matrix package to create a sparse matrix from the following data frame and have all

Answer 1

Yes, Thierry's answer is definitely true I can say as co-author of the 'Matrix' package...

To your other question: Why is accessing "M" slower than "Y"? The main answer is that "M" is much much sparser than "Y" hence much smaller and -- depending on the sizes envolved and the RAM of your platform -- the access time is faster for much smaller objects, notably for indexing into them.

Answer 2

Why do you want default NA values? As far as I know matrices are only sparse if they have zero-cells. As NA is a non-zero value, you loose all the benefits from the sparse matrix. A classic matrix is even more efficient if the matrix has hardly any zeros. A classic matrix is like a vector that will be cut according to the dimensions. So it only has to store the data vector and the dimensions. The sparse matrix stores only the non-zero values, but also stores there location. This is an advantage if and only if you have enough zero values.