Imagine that you want to calculate the centroid of some triangles given in matrix form. First the coordinates of the vertices of the triangles are in a matrix called coords where the line number represents the vertex number and the column number is the dimension, in this case it is equal to 2 (x, y). The way the nodes connect are found in the lnods matrix where the line number is the triangle number and the column number is the vertex number that makes up each triangle (v_i, v_j, v_k).