# [scikit-learn] Adding BM25 to scikit-learn.feature_extraction.text

Basil Beirouti basilbeirouti at gmail.com
Fri Jul 1 17:17:43 EDT 2016

```Hi everyone,

to put it succinctly, here's the BM25 equation:

f(w,D) * (k+1) / (k*B + f(w,D))

where w is the word, and D is the document (corresponding to rows and
columns, respectively). f is a sparse matrix because only a fraction of the
whole vocabulary of words appears in any given single document.

B is a function of only the document, but it doesn't matter, you can think
of it as a constant if you want.

The problem is since f(w,D) is almost always zero, I only need to do the
calculation (ie. multiply by (k+1) then divide by (k*B + f(w,D))) when
f(w,D) is not zero. Is there a clever way to do this with masks?

You can refactor the above equation to get this:

(k+1)/(k*B/f(w,D) + 1) but alas we still have f(w,D) appearing in a
denominator, which is bad (because of dividing by zero).

So anyway, currently I am converting to a coo_matrix and iterator through
the non-zero values like this:

cx = x.tocoo()
for i,j,v in itertools.izip(cx.row, cx.col, cx.data):
(i,j,v)

That iterator is incredibly fast, but unfortunately coo_matrix does
not support assignment. So I create a new copy of either a dok sparse
matrix or a regular numpy array and assign to that.

I could also deal directly with the .data, .indptr, and indices
attributes of csr_matrix, and see if it's possible to create a copy of
.data attribute and update the values accordingly. I was hoping
somebody had encountered this type of issue before.

Sincerely,

Basil Beirouti
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