> Is there a concise, Numpythonic way to copy the values of x[:,1] over to > y[:,1] where x[:,0] = y[:,0]? Resulting in, z: First use mask = (x[:,0] == y[:,0]) # integers or mask = np.abs(x[:,0] - y[:,0]) < eps # floats and then y[mask,1] = x[mask,1] Sturla