check membership of a set in python

Question

I had a statement in my python code something like

msk = (var1 == 49) | (var1 == 26) | (var1 == 28)

Now, instead of (49,26,28), I want to have a longer list, and want to make it variable,

I tried the solution at Python equivalent of MATLAB's "ismember" function but it is not exactly what I want.

Something like:

msk = (var1 == list1)

What is `|` in this context? For Python `set` operations, `|` stands for `set` union. — Vedang Mehta, May 22 '16 at 18:15
Closing as duplicate of the linked question in the post, as one of the answers to it has already mentioned use of `np.in1d`. — Divakar, May 22 '16 at 18:35
Python set membership can be checked with the `in` operator. For example with `my_set = set([1, 2, 3])` use `[i in my_set for i in [2, 4]]`. However from the context of the question, I'm not entirely sure what you're looking for is what the title says. — Taylor D. Edmiston, May 22 '16 at 18:37

Simon Fromme · Answer 1 · 2016-05-22T18:32:39.663

2

Using the set intersection operator:

import numpy as np

list1 = np.Array([1, 2, 3, 49])
list2 = np.Array([1, 2, 3, 4])

msk1 = any({49, 26, 28} & set(list1))
# => True

msk2 = any({49, 26, 28} & set(list2))
# => False

This works for both numpy arrays and regular lists.

edited May 22 '16 at 18:32

answered May 22 '16 at 18:26

Simon Fromme

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score 2 · Accepted Answer · answered May 22 '16 at 18:37

2

You can use the in operator:

msk = var1 in {49, 26, 28}

answered May 22 '16 at 18:37

Nagasaki45

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ayhan · Answer 3 · 2016-05-22T18:30:08.960

numpy.in1d should work for this case:

a
Out[153]: array([ 1, 10, 26, 28, 40, 49])

b
Out[154]: array([26, 28, 49, 50])

To check whether an element in array a is also in array b:

np.in1d(a, b)
Out[156]: array([False, False,  True,  True, False,  True], dtype=bool)

Of course you can use it for single elements as well:

np.in1d(28, b)
Out[158]: array([ True], dtype=bool)

np.in1d(43, b)
Out[159]: array([False], dtype=bool)

check membership of a set in python

3 Answers3