Columnwise maximum of scipy.sparse.csc_matrix, in Cython

csc_columnwise_max.pyx

cimport numpy as np


def csc_columnwise_max(np.ndarray[np.float64_t, ndim=1] data,
                       np.ndarray[int, ndim=1] indices,
                       np.ndarray[int, ndim=1] indptr,
                       np.ndarray[np.float64_t, ndim=1] out):
    cdef double mx
    cdef int n_features = indptr.shape[0] - 1
    cdef int i, j

    for i in range(n_features):
        mx = 0
        for j in range(indptr[i], indptr[i + 1]):
            if data[j] < 0:
                raise ValueError("negative value %r in column %d"
                                 % (data[j], i))
            mx = max(data[j], mx)
        out[i] = mx