Checking if any elements in one list are in another

Question

I'm trying to compare two lists and simply print a message if any value from the first list is in the second list.

def listCompare():
  list1 = [1, 2, 3, 4, 5]
  list2 = [5, 6, 7, 8, 9]
  if list1 in list2:
    print("Number was found")
  else:
    print("Number not in list")

In this example, I want the if to evaluate to True because 5 is in both lists. This doesn't work, and I'm not sure of the simplest way to compare the two lists.

Answer 1

You could solve this many ways. One that is pretty simple to understand is to just use a loop.

def comp(list1, list2):     for val in list1:         if val in list2:             return True     return False 

A more compact way you can do it is to use map and reduce:

reduce(lambda v1,v2: v1 or v2, map(lambda v: v in list2, list1)) 

Even better, the reduce can be replaced with any:

any(map(lambda v: v in list2, list1)) 

You could also use sets:

len(set(list1).intersection(list2)) > 0 

Answer 2

There are different ways. If you just want to check if one list contains any element from the other list, you can do this..

not set(list1).isdisjoint(list2) 

I believe using isdisjoint is better than intersection for Python 2.6 and above.

Answer 3

Your original approach can work with a list comprehension:

def listCompare():
  list1 = [1, 2, 3, 4, 5]
  list2 = [5, 6, 7, 8, 9]
  if [item for item in list1 if item in list2]:
    print("Number was found")
  else:
    print("Number not in list")

Answer 4

There is a built in function to compare lists:

Following is the syntax for cmp() method −

cmp(list1, list2)

#!/usr/bin/python

list1, list2 = [123, 'xyz'], [123, 'xyz']

print cmp(list1,list2)

When we run above program, it produces following result −

0

If the result is a tie, meaning that 0 is returned

Answer 5

You could change the lists to sets and then compare both sets using the & function. eg:

list1 = [1, 2, 3, 4, 5]
list2 = [5, 6, 7, 8, 9]

if set(list1) & set(list2):
    print "Number was found"
else:
    print "Number not in list"

The "&" operator gives the intersection point between the two sets. If there is an intersection, a set with the intersecting points will be returned. If there is no intersecting points then an empty set will be returned.

When you evaluate an empty set/list/dict/tuple with the "if" operator in Python the boolean False is returned.