Compressed sparse vectors. Let a be a vector stored in an array, and assume that the only non-zero elements of a are the following:
a[k1], a[k2], a[k3] . . . a[knz],
where nz is the total number of non-zero elements in a.
In Sparse BLAS, this vector can be represented in compressed form by two arrays, x (values) and indx (indices). Each array has nz elements:
x=a[k1], x=a[k2], . . . x[nz-1]= a[knz],
indx=k1, indx=k2, . . . indx[nz-1]= knz.
Thus, a sparse vector is fully determined by the triple (nz, x, indx). If you pass a negative or zero value of nz to Sparse BLAS, the subroutines do not modify any arrays or variables.
Full-storage vectors. Sparse BLAS routines can also use a vector argument fully stored in a single array (a full-storage vector). If y is a full-storage vector, its elements must be stored contiguously: the first element in y, the second in y, and so on. This corresponds to an increment incy = 1 in BLAS Level 1. No increment value for full-storage vectors is passed as an argument to Sparse BLAS routines or functions.