Compare a numpy array to each element of another one

Question

A = np.array([5,1,5,8])
B = np.array([2,5])

I want to compare the A array to each element of B. In other words I'm lookin for a function which do the following computations :

A>2
A>5
(array([ True, False,  True,  True]), array([False, False, False,  True]))

Yes lol. I was looking for something more numpyic but that's fine also. — hans glick
– hans glick, Commented Aug 13, 2018 at 12:41
Wonderful@Brenlla, thks. Could you esplain why does it work? — hans glick
– hans glick, Commented Aug 13, 2018 at 12:44
By adding a an extra dimension to B, it sort off "expands" the last dimension of B to match the dimension of A. I know it doesn't make a lot of sense, but it is posibly the most important feature of numpy. For more details see here — Brenlla
– Brenlla, Commented Aug 13, 2018 at 12:52

andrew_reece · Accepted Answer · 2018-08-13 13:10:11Z

7

Not particularly fancy but a list comprehension will work:

[A > b for b in B]

[array([ True, False,  True,  True], dtype=bool),
 array([False, False, False,  True], dtype=bool)]

You can also use np.greater(), which requires the dimension-adding trick that Brenlla uses in the comments:

np.greater(A, B[:,np.newaxis])

array([[ True, False,  True,  True],
       [False, False, False,  True]], dtype=bool)

edited Aug 13, 2018 at 13:10

answered Aug 13, 2018 at 12:47

andrew_reece

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4 Comments

hans glick Over a year ago

Yeah, right but I prefer the Brenlla version ... A>B[:,None] as it give me a numpy array

andrew_reece Over a year ago

Sure, I like that one too! (But you can always just wrap this answer in np.array().)

andrew_reece Over a year ago

@hans glick there is an explicit Numpy function for pairwise comparison of two vectors, see updated answer.

sil Over a year ago

I'm confused, is the greater function what you are referring to with your last comment, because that still uses explicit broadcasting. Is there any built in numpy function which produces an n*m output of comparisons without explicit broadcasting (even if such a function just performs broadcasting behind the scenes). I'm fine with doing the broadcasting myself, but if a method exists that wraps it, I'd rather use that for readability

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