How does the inertia cython implementation in scikit-learn for kmeans work?
&amp; is the &quot;address-of&quot; operator in c, and that appears to be how it's being used here.  See these two SO posts.
Note the signature of _euclidean_dense_dense:
cdef floating _euclidean_dense_dense(
        floating* a, # IN
        floating* b, # IN
        int n_features,
        bint squared) nogil:

The first two inputs are pointers. So you need to pass the address, not a copy of the data.
Notice too that only the address of the element in the first column gets passed. If you look at the definition for _euclidean_dense_dense that becomes clearer: that function actually loops over the addresses of the rest of the columns in its computation.