Allocate space for sparse matrix
S = spalloc(m,n,nzmax)
S = spalloc(m,n,nzmax) creates an all zero sparse matrix S of size m-by-n with room to hold nzmax nonzeros. The matrix can then be generated column by column without requiring repeated storage allocation as the number of nonzeros grows.
spalloc(m,n,nzmax) is shorthand for