Find arrays of ndarray that are in another ndarray - Python

Question

I was wondering if there's any built-in method to find arrays of a ndarray that are contained into another ndarray. i.e.

arr1 = [[1,2], [2,9], [3,7], [5,11], [32,5], [11,4], [89,3], [37,5]]

and

arr2 = [[12,44], [2,9], [125,3], [37,5]]

I would like to know if there's any built-in method to identify that [2,9] and [37,5] are the only arrays of arr2 contained into arr1. Or to identify their indexes.

Why don't you try `[x for x in arr1 if x in arr2]` This will not give the indexes, but the elements themselves. — Sruthi, Mar 24 '18 at 15:08

score 0 · Accepted Answer · answered Mar 24 '18 at 15:11

You could use sets and tuples, to do this, using:

arr1 = [[1,2], [2,9], [3,7], [5,11], [32,5], [11,4], [89,3], [37,5]]
arr2 = [[12,44], [2,9], [125,3], [37,5]]

set1 = {tuple(i) for i in arr1}
set2 = {tuple(i) for i in arr2}

arr3 = [list(i) for i in set1 & set2]

But it would be much simpler just to use a list comprehension

arr3 = [i for i in arr1 if i in arr2]

score 0 · Answer 2 · answered Mar 24 '18 at 16:55

0

If you use sets:

a = [1,2,3,4,"B"]
b = [2, "B", 5]
sa = set(a)
sb = set(b)
c = list(sa.intersection(sb))

c will be:

[2, "B"]

answered Mar 24 '18 at 16:55

Artemis

2,553
7
21
36

it will work whatever the types in the original arrays – Artemis Mar 24 '18 at 16:55

score 0 · Answer 3 · answered Mar 24 '18 at 17:05

One way is to use intersection of rows of the 2 lists, after mapping each row to tuple.

The reason why conversion to tuple is necessary is because tuples are immutable and hashable. Therefore, they can be included in a set, while lists cannot.

arr1 = [[1,2], [2,9], [3,7], [5,11], [32,5], [11,4], [89,3], [37,5]]
arr2 = [[12,44], [2,9], [125,3], [37,5]]

res = set(map(tuple, arr1)) & set(map(tuple, arr2))

# {(2, 9), (37, 5)}

If required, convert the result back to lists:

res2 = list(map(list, res))

# [[37, 5], [2, 9]]

Alternative, possibly more efficient, solutions if numpy arrays are viable:

Find arrays of ndarray that are in another ndarray - Python

3 Answers3