Hi again, I have a problem with the nonzero() function for matrix. The following test program: import numpy, scipy.sparse Z = numpy.zeros((10,10)) Z[0,0] = Z[1,1] = 1 i = Z.nonzero() print i Zc = scipy.sparse.coo_matrix((Z[i],i)) Z = numpy.matrix(Z) i = Z.nonzero() print i Zc = scipy.sparse.coo_matrix((Z[i],i)) gives me: (array([0, 1]), array([0, 1])) (matrix([[0, 1]]), matrix([[0, 1]])) Traceback (most recent call last): File "test.py", line 13, in <module> Zc = scipy.sparse.coo_matrix((Z[i],i)) File "/Volumes/Data/Local/lib/python2.6/site-packages/scipy/sparse/ coo.py", line 179, in __init__ self._check() File "/Volumes/Data/Local/lib/python2.6/site-packages/scipy/sparse/ coo.py", line 194, in _check nnz = self.nnz File "/Volumes/Data/Local/lib/python2.6/site-packages/scipy/sparse/ coo.py", line 187, in getnnz raise ValueError('row, column, and data arrays must have rank 1') ValueError: row, column, and data arrays must have rank 1 Is that the intended behavior ? How can I use nonzero with matrix to build the coo one ? Nicolas