[Numpy-discussion] Lookup array

[Olivier Delalleau]

2011/10/10 Andrey N. Sobolev <inconnu at list.ru>

> В Mon, 10 Oct 2011 10:03:48 +0100
> Bob Dowling <rjd4+numpy at cam.ac.uk> пишет:
>
> >
> > On 10/10/11 09:53, Andrey N. Sobolev wrote:
> >
> > > I have 2 arrays - A with the dimensions of 1000x4 and B with the
> > > dimensions of 5000x4. B doesn't (hopefully) contain any rows that
> > > are not in A. I need to create a lookup array C, the i-th value of
> > > which will be the index of B[i] in A. In the (very rare) case when
> > > B[i] is not in A C[i] should be equal to -1.
> >
> > May we assume that there are no repeats in A?  (i.e. no cases where
> > two different indices are both valid?)
> >
>
> Yes, rows in A are unique and sorted.
> One more typo found - instead of np.which in the previous e-mail it has
> to be np.where, I don't know what I thought about :)
>
> Thanks in advance!
> Andrey
>

The following doesn't use numpy but seems to be about 20x faster:

    A_rows = {}
    for i, row in enumerate(A):
        A_rows[tuple(row)] = i
    for i, row in enumerate(B):
        C[i] = A_rows.get(tuple(row), -1)

-=- Olivier
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