Thanks for the input on k-means clustering, but the main questionw as actully this... If I have the following: for i in xrange(k): w[i] = average(compress(C == i, V, 0)) ... can that be expressed without the Python for loop? (I.e. without using compress etc.) I want w[i] to be the average of the vectors in V[x] for which C[x] == i... -- Magnus Lie Hetland The Anygui Project http://hetland.org http://anygui.org