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

User · Answer

I wrote the following code in one of my projects  It basically compares each individual element of the list  Feel free to use it  if it works for your requirement   def reachedGoal a b       if len a   len b            raise ValueError  Wrong lists provided        for val1 in range 0 len a            temp1 a val1          temp2 b val1          for val2 in range 0 len b                if temp1 val2   temp2 val2                    return False     return True

User · Answer

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    gt  0

User · Answer

You could change the lists to sets and then compare both sets using the  amp  function  eg   list1    1  2  3  4  5  list2    5  6  7  8  9   if set list1   amp  set list2       print  Number was found  else      print  Number not in list    The   amp   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

User · Answer

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

User · Answer

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

User · Answer

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

[python] Checking if any elements in one list are in another

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