CODEWITHSUNDEEP
pythonarraysnumpy
farnold
farnold

Reputation: 339

Elementwise comparison of numpy arrays with different lengths

I want to compare the elements of two 3D numpy arrays of different lengths. The goal is, to find overlapping elements in the two arrays.

All functions I found so far, rely on the two arrays being of the same lengths.

Is there an efficient way to do compare the 2D-elements (for loops will be very inefficient, since each array has tens of thousands of elements)?

Upvotes: 0

Views: 2547

Answers (2)

hpaulj
hpaulj

Reputation: 231325

Here a few ways of comparing 2 1d arrays:

In [325]: n=np.arange(0,10)
In [326]: m=np.arange(3,9)

In [327]: np.in1d(n,m)
Out[327]: array([False, False, False,  True,  True,  True,  True,  True,  True, False], dtype=bool)

In [328]: np.in1d(m,n)
Out[328]: array([ True,  True,  True,  True,  True,  True], dtype=bool)

In [329]: n[:,None]==m[None,:]
Out[329]: 
array([[False, False, False, False, False, False],
       [False, False, False, False, False, False],
       [False, False, False, False, False, False],
       [ True, False, False, False, False, False],
       [False,  True, False, False, False, False],
       [False, False,  True, False, False, False],
       [False, False, False,  True, False, False],
       [False, False, False, False,  True, False],
       [False, False, False, False, False,  True],
       [False, False, False, False, False, False]], dtype=bool)

and farenorths suggestion 

In [330]: np.intersect1d(n,m)
Out[330]: array([3, 4, 5, 6, 7, 8])

In [331]: np.where(np.in1d(n,m))
Out[331]: (array([3, 4, 5, 6, 7, 8], dtype=int64),)

Upvotes: 1

farenorth
farenorth

Reputation: 10771

Is intersect1d what you want? For example, if your arrays are a and b, you could simply do:

duplicates = np.intersect1d(a, b)

Upvotes: 0

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