How to check if a value in one list is in another list with a one-liner for an if statement in Python if I'm not using sets?

Question

I\'m trying to construct a one liner that would check if any of the values in one list are present in another list and return True or False if it does or does not.

T

Answer 1

You can us an any(..) builtin function with a generator expression:

any(e in list2 for e in list1)

So this will check if there is at least one element that occurs in both lists.

Note however that this will result in a worst-case O(n²) algorithm. If the elements are hashable for instance, and you can use a set, we can make it an O(n) average-case algorithm.

Answer 2

You could also do

set(list1).intersection(list2)

to get the set of elements that occur in both; the length of the set is 0 if there's no intersection, otherwise positive. You can treat this set as a boolean, since Python evaluates empty sets to False and nonempty sets to True.

if set(list1).intersection(list2):
    print ('Lists have elements in common')
else: 
    print ('No elements in common')

Runtime is O(n) average-case, O(n^2) worst-case: https://wiki.python.org/moin/TimeComplexity

Answer 3

Supose we have this lists:

list1 = ['bar', 'foo', 'qwerty', 9, 1]
list2 = [1, 2, 3, 'foo']

If we want to see the repeated values:

[i for i in list1 if i in list2]

The input:

['foo', 1]

If we want True false:

(map(lambda each: each in list1, list2))

The input:

[True, False, False, True]

The input is checking list2, the first value '1' exists in list1 and the last 'foo' too.