Reverse sort and argsort in python

Question

I'm trying to write a function in Python (still a noob!) which returns indices and scores of documents ordered by the inner products of their tfidf scores. The procedure is:

• Compute vector of inner products between doc idx and all other documents
• Sort in descending order
• Return the "scores" and indices from the second one to the end (i.e. not itself)

The code I have at the moment is:

import h5py
import numpy as np

def get_related(tfidf, idx) :
    ''' return the top documents '''

    # calculate inner product   
    v = np.inner(tfidf, tfidf[idx].transpose())

    # sort
    vs = np.sort(v.toarray(), axis=0)[::-1]
    scores = vs[1:,]

    # sort indices
    vi = np.argsort(v.toarray(), axis=0)[::-1]
    idxs = vi[1:,] 

    return (scores, idxs)

where tfidf is a sparse matrix of type '<type 'numpy.float64'>'.

This seems inefficient, as the sort is performed twice (sort() then argsort()), and the results have to then be reversed.

• Can this be done more efficiently?
• Can this be done without converting the sparse matrix using toarray()?

Answer 1

I don't think there's any real need to skip the toarray. The v array will be only n_docs long, which is dwarfed by the size of the n_docs × n_terms tf-idf matrix in practical situations. Also, it will be quite dense since any term shared by two documents will give them a non-zero similarity. Sparse matrix representations only pay off when the matrix you're storing is very sparse (I've seen >80% figures for Matlab and assume that Scipy will be similar, though I don't have an exact figure).

The double sort can be skipped by doing

v = v.toarray()
vi = np.argsort(v, axis=0)[::-1]
vs = v[vi]

Btw., your use of np.inner on sparse matrices is not going to work with the latest versions of NumPy; the safe way of taking an inner product of two sparse matrices is

v = (tfidf * tfidf[idx, :]).transpose()