[Numpy-discussion] extract elements of an array that are contained in another array?

Alan G Isaac aisaac at american.edu
Thu Jun 4 10:13:19 EDT 2009

> On Thu, Jun 4, 2009 at 8:23 AM, Alan G Isaac <aisaac at american.edu> wrote:
>> a[(a==b[:,None]).sum(axis=0,dtype=bool)]

On 6/4/2009 8:35 AM josef.pktd at gmail.com apparently wrote:
> If b is large this creates a huge intermediate array

True enough, but one could then use fromiter:
setb = set(b)
itr = (ai for ai in a if ai in setb)
out = np.fromiter(itr, dtype=a.dtype)

I suspect (?) that b would have to be pretty
big relative to a for the repeated testing
to be more costly than sorting a.

Or if a stable order is not important (I don't
recall if the OP specified), one could just
np.intersect1d(a, np.unique(b))

On a different note, I think a name change
is needed for your function. (Compare
intersect1d_nu to see the potential
confusion. And btw, what is the use case
for intersect1d, which gives neither a
set intersection nor a multiset intersection?)

Alan Isaac

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