okay it will produce this
[[1], [0], [0, 1, 1, 1]]

deciding how many elements are in row 0.

IA[1] - IA[0] gives 1
column is [0], from JA

so the matrix is 
[1]

Now, number of elements in row 1,
IA[2] - IA[1] gives 0

So, no elements.....