Sparse matrices

Sparse matrices are special in that zeros aren't stored; this is done in an effort to save space in memory. This creates an issue for the binarizer, so to combat it, a special condition for the binarizer for sparse matrices is that the threshold cannot be less than zero:

from scipy.sparse import coo
spar = coo.coo_matrix(np.random.binomial(1, .25, 100))
preprocessing.binarize(spar, threshold=-1)

ValueError: Cannot binarize a sparse matrix with threshold < 0