Comparing two lists in python if elements exist

Question

I have two lists where I want to check if elements from a exists in b

a=[1,2,3,4]
b=[4,5,6,7,8,1]

This is what I tried ( didn't work though!)

a=[1,2,3,4]
b=[4,5,6,7,3,1]

def detect(list_a, list_b):
    for item in list_a:
        if item in list_b:
            return True
    return False  # not found

detect(a,b)

I want to check if elements from a exists in b and should set a flag accordingly. Any thoughts?

Problem is that you return True as soon as you find the first element that is in both a and b — tobias_k
– tobias_k, Commented Feb 25, 2014 at 10:49
Do you need a list of flags to check one-by-one if an element of a exists in b, or only one bool if ALL elements of a exist in b? — leeladam
– leeladam, Commented Feb 25, 2014 at 10:49
Flag should be raised only when all elements in a exist in b. — user741592
– user741592, Commented Feb 25, 2014 at 10:51
Use the correct data structure, use docs.python.org/2/library/sets.html — shevski
– shevski, Commented Feb 25, 2014 at 10:56

BartoszKP · Accepted Answer · 2014-02-25 10:54:20Z

8

Your code returns as soon as first element exists in both lists. To check all elements you could try this for example:

def detect(list_a, list_b):
    return set(list_a).issubset(list_b)

Other possibility, without creating a set:

def detect(list_a, list_b):
    return all(x in list_b for x in list_a)

And in case you were curious what exactly in your code is wrong, this is the fix in its current form (but it's not very pythonic):

def detect(list_a, list_b):
    for item in list_a:
        if item not in list_b:
            return False       # at least one doesn't exist in list_b

    return True   # no element found that doesn't exist in list_b
                  # therefore all exist in list_b

And finally, your function name isn't very readable. detect is too ambiguous. Consider other more verbose names, like isSubset etc.

edited Feb 25, 2014 at 10:54

answered Feb 25, 2014 at 10:49

BartoszKP

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