Convert from sparse external format.
S = SPCONVERT(D) converts an nnz or nnz+1 by 3 or 4 array
with rows containing [i,j,s] or [i,j,real(s(i,j)),imag(s(i,j))]
to the corresponding sparse matrix. A row of the form [m n 0]
or [m n 0 0] anywhere in D may be used to specify size(S).
If D is already sparse no conversion is done, so SPCONVERT can
be used after D is loaded from either a MAT or an ASCII file.
See also SPARSE1V5MDWP, FULL .