CODEWITHSUNDEEP
pythonlistmatching
tylcole
tylcole

Reputation: 179

How to return list of booleans to see if elements of one list in another list

I have two lists:

A = [1,2,3,4,5,6,7,8]
B = [2,3,4]

and want to get a boolean list of length(A) where the element at each index indicates whether the element at the same index in A is in anywhere in the list B. The return value would be:

[False, True, True, True, False, False, False, False]

It would be easy to write a function, but want to know if there is a paradigmatic way of doing it in Python.

In R, the counterpart would be 

which(A %in% b)

Upvotes: 5

Views: 6665

Answers (3)

Katriel
Katriel

Reputation: 123762

In [1]: A = [1,2,3,4,5,6,7,8]

In [2]: B = [2,3,4]

In [3]: map(B.__contains__, A)
Out[3]: [False, True, True, True, False, False, False, False]

Upvotes: 4

rburny
rburny

Reputation: 1569

Another way to write it (but slower due to lambda usage):

>>> A = [1,2,3,4,5,6,7,8]
>>> B = [2,3,4]
>>> S = set(B)
>>> map(lambda x: x in S, A)
[False, True, True, True, False, False, False, False]

Upvotes: 3

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 251106

use a list comprehension:

In [164]: A = [1,2,3,4,5,6,7,8]

In [165]: B = [2,3,4]

In [166]: [x in B for x in A]
Out[166]: [False, True, True, True, False, False, False, False]

If B is huge then better convert it to a set first. As, membership test for sets is O(1) compared to O(n) in lists.

In [167]: b=set(B)

In [168]: [x in b for x in A]
Out[168]: [False, True, True, True, False, False, False, False]

Upvotes: 12

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