Google found this:

I don't think you'd need a hash table for calculating a norm. But then, was that a math or a programming question?One measures the distance between vectors by the norm of their difference. In particular, the size of length of a vector x in this sense is its norm, x , i.e., its distance from the zero vector. One also measures the distance between two matrices by the norm of their difference, with the norm of a matrix A its corresponding distance A from the zero matrix.