[petsc-users] nonzero structure setting and remembering

I have a couple of questions about how to deal with the nonzero structure. 0) Is the "nonzero structure" the same as "number of non zeros per row"? Or are they different? 1) How much of a speedup do you get for copying/creating/loading matrices when you set the number of non zeros per row beforehand? (if the nonzero structure is different from the number of non zeros per row, what about the nonzero structure) 2) If I have a matrix, how do I add to the nonzero structure, without actually adding nonzero values, do I just add "0.0"? (the matrix will have values there, but doesn't at the moment) As usual, thanks for all the help -Andrew