Check for common element in three lists: it checks for identical lists instead

Question

I need to input three lists and make a Boolean check whether there is a common element to all lists. So far it the main part of the code looks like this:

def in_all_three(listA, listB, listC):
    for x in listA:
        if x in listB and x in listC:
            return True
        else: 
            return False

I don't know why, but it only returns True if the lists are identical. What is wrong with my code?

Answer 1

You can use intersection of sets for this purpose.

def check_common_element(listA, listB, listC):
    common_elements = set.intersection(set(listA), set(listB), set(listC))
    if common_elements:
        return True
    else:
        return False

# Test case 1: Testing when there is a common value among lists
list_a = [1,2,3]
list_b = [1,5,6]
list_c = [1,8,9]

print(check_common_element(list_a,list_b,list_c))
# output: True

# Test case 2: Testing when there is no common value among lists
list_a = [1,2,3]
list_b = [1,5,6]
list_c = [7,8,9]

print(check_common_element(list_a,list_b,list_c))
# output: False

As @Reti43 suggested, you can do return bool(common_elements) instead of if-else as its a better option for the reasons described in the comments below. In that case, the modified function would look like this:

def check_common_element(listA, listB, listC):
    common_elements = set.intersection(set(listA), set(listB), set(listC))
    return bool(common_elements)

Answer 2

Logically you need to return False only after all items were checked and no match was found. In order to do that you need to RETURN FALSE; after the loop done.

def in_all_three(listA, listB, listC):
for x in listA:
    if x in listB and x in listC:
        return True
return False