Specifically, what do the
& symbol stand for? and why is the column index always 0?
cpdef floating _inertia_dense( np.ndarray[floating, ndim=2, mode='c'] X, # IN floating[::1] sample_weight, # IN floating[:, ::1] centers, # IN int[::1] labels): # IN """Compute inertia for dense input data Sum of squared distance between each sample and its assigned center. """ cdef: int n_samples = X.shape int n_features = X.shape int i, j floating sq_dist = 0.0 floating inertia = 0.0 for i in range(n_samples): j = labels[i] sq_dist = _euclidean_dense_dense(&X[i, 0], ¢ers[j, 0], n_features, True) inertia += sq_dist * sample_weight[i] return inertia