I'm still a bit confused as to why I would use c_face_loop as this loops over the faces of the cells, whereas I'm just interested in the cell centroids and the boundary face centroids. Suppose I wanted to print the distance between the centroid of every cell to the centroid of every boundary face. How would you achieve this with c_face_loop? It seems that I would run into the same issue since I would not be able to pass information into the begin_f_loop once I start looping over the boundary faces.