Python list have common elements

Python | Check if given two Lists have any Element in Common

In Python, if you have to check if the given two lists have any elements in common, we can do so by traversing the lists and comparing them or by using the Python Set data structure. In this tutorial, we will see different ways of looking for common elements in two given Python lists.

Check if the given two Lists have any Elements in Common in Python

1. By traversing over the Lists

The naive way of doing it is to iterate over both lists and check if the elements are equal.

def common_elements(list_1, list_2): result = False for x in list_1: for y in list_2: if x == y: result = True return result return result list_one = [1, 2, 5, 7, 58] list_two = [87, 58, 0,3,1] print("Are elements common between list_one and list_two?") print(common_elements(list_one, list_two)) list_three = [0,5,7,89,34,4] list_four = [45,78,90] print("Are elements common between list_three and list_four?") print(common_elements(list_three, list_four))

Are elements common between list_one and list_two?
True
Are elements common between list_three and list_four?
False

2. Using Python Sets

A better way is to use a Python set since the set data structure only holds unique data values.

def common_elements(list_1, list_2): a_set = set(list_1) b_set = set(list_2) if (a_set & b_set): return True else: return False list_one = [1, 2, 5, 7, 58] list_two = [87, 58, 0,3,1] print("Are elements common between list_one and list_two?") print(common_elements(list_one, list_two)) list_three = [0,5,7,89,34,4] list_four = [45,78,90] print("Are elements common between list_three and list_four?") print(common_elements(list_three, list_four))

Are elements common between list_one and list_two?
True
Are elements common between list_three and list_four?
False

2.1 Using set intersection

Another method is to use the property of the Python set, i.e. intersection. Both lists can be converted into a set data structure and intersected to get the common values amongst both sets.

def common_elements(list_1, list_2): a_set = set(list_1) b_set = set(list_2) if len(a_set.intersection(b_set)) > 0: return(True) return(False) list_one = [1, 2, 5, 7, 58] list_two = [87, 58, 0,3,1] print("Are elements common between list_one and list_two?") print(common_elements(list_one, list_two)) list_three = [0,5,7,89,34,4] list_four = [45,78,90] print("Are elements common between list_three and list_four?") print(common_elements(list_three, list_four))

Are elements common between list_one and list_two?
True
Are elements common between list_three and list_four?
False

Conclusion

Identifying common elements between two lists is a common operation in Python programming. By employing the techniques discussed in this article, developers can efficiently compare lists and determine their intersection, gaining valuable insights for data analysis, streamlining algorithms, or simply finding commonalities.

With the variety of methods available, such as set operations, list comprehensions, and the use of built-in functions, Python offers a powerful toolkit to tackle this task with elegance and efficiency. By understanding the underlying principles and considering performance implications, you can choose the approach that best suits your specific use case.

We hope this guide has shed light on the techniques available in Python for checking common elements between two lists. Remember to analyze the complexity of different approaches, consider the size of your data, and experiment with the provided examples to solidify your understanding.

Continue to explore the vast array of Python’s capabilities, and leverage its flexibility to solve problems, process data, and build robust applications. Happy coding!

Frequently Asked Questions(FAQs)

1. How can I check if two lists have any common elements in Python?

One common approach is converting the lists into sets and then using the intersection operator «&» to find the common elements. Another method involves iterating over one list and checking for element membership in the other.

2. Can I use the «in» keyword to check for common elements between two lists?

Yes, you can iterate over one list and use the «in» keyword to check if each element exists in the other list. However, this approach may have slower performance compared to using sets or other techniques for larger lists.

3. What is the advantage of using sets to check for common elements?

Sets offer a fast lookup time for element membership, making them an efficient choice when checking for common elements. Converting lists to sets eliminates duplicates and allows the use of set operations, such as intersection and union.

4. How can I handle duplicate elements when checking for common elements?

If duplicate elements are significant and need to be considered, sets may not be suitable, as they eliminate duplicates. In such cases, iterating over the lists and keeping track of encountered elements would be more appropriate.

5. Are there any specialized libraries in Python for list comparison and finding common elements?

Python provides built-in functions such as intersection() and issubset() for set operations, which can be used to find common elements. Additionally, the NumPy library offers advanced array operations and comparisons that can be utilized for comparing and finding common elements efficiently.

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Python list have common elements

Last updated: Feb 22, 2023
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# Table of Contents

# Find common values in multiple lists in Python

To find the common values in multiple lists:

1. Convert the first list to a set object.
2. Use the intersection() method on the set .
3. The intersection method will return the common elements in the lists.

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list1 = ['a', 'b', 'c'] list2 = ['a', 'z', 'c'] list3 = ['a', 'x', 'c'] common_elements = list( set(list1).intersection(list2, list3) ) print(common_elements) # 👉️ ['a', 'c'] 

If you need to find the common elements in a list of lists, click on the following subheading:

The example finds the common elements in three lists, but you can use this approach with as many lists as necessary.

We used the set() class to convert the first list to a set .

The set() class takes an iterable optional argument and returns a new set object with elements taken from the iterable.

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list1 = ['a', 'b', 'c'] print(set(list1)) # 👉️ 

The intersection method returns a new set with elements common to the provided objects.

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list1 = ['a', 'b', 'c'] list2 = ['a', 'z', 'c'] list3 = ['a', 'x', 'c'] common_elements = list( set(list1).intersection(list2, list3) ) print(common_elements) # 👉️ ['a', 'c'] print(len(common_elements)) # 👉️ 2 

Alternatively, you can use a list comprehension.

# Find common values in multiple lists using a list comprehension

This is a three-step process:

1. Use a list comprehension to iterate over the first list.
2. Check if each element is present in the other lists and return the result.
3. The new list will only contain the common elements between the lists.

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list1 = ['a', 'b', 'c'] list2 = ['a', 'z', 'c'] list3 = ['a', 'x', 'c'] common_elements = [ element for element in list1 if element in list2 and element in list3 ] print(common_elements) # 👉️ ['a', 'c'] 

We used a list comprehension to iterate over the first list.

List comprehensions are used to perform some operation for every element or select a subset of elements that meet a condition.

On each iteration, we check if the current item is present in the two other lists and return the result.

The new list only contains the elements that are present in all three lists.

The in operator tests for membership. For example, x in l evaluates to True if x is a member of l , otherwise, it evaluates to False .

Alternatively, you can use a simple for loop.

# Find common values in multiple lists using a for loop

This is a three-step process:

1. Use a for loop to iterate over the first list.
2. Check if each item is contained in the other lists.
3. Append the matching items to a new list.

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list1 = ['a', 'b', 'c'] list2 = ['a', 'z', 'c'] list3 = ['a', 'x', 'c'] common_elements = [] for element in list1: if element in list2 and element in list3: common_elements.append(element) print(common_elements) # 👉️ ['a', 'c'] 

We used a for loop to iterate over the first list.

On each iteration, we use the in operator to check if the current item is contained in the other two lists.

If the condition is met, we append the value to a new list.

The list.append() method adds an item to the end of the list.

Copied! 
my_list = ['bobby', 'hadz'] my_list.append('com') print(my_list) # 👉️ ['bobby', 'hadz', 'com'] 

The method returns None as it mutates the original list.

# Find the common elements in a List of Lists in Python

To find the common elements in a list of lists:

1. Convert the first element in the list to a set .
2. Call the intersection() method on the set .
3. The method will return the common elements in the list of lists.

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list_of_lists = [ [4, 7, 9], [4, 9, 9], [4, 1, 9] ] common_elements = set( list_of_lists[0] ).intersection(*list_of_lists) print(common_elements) # 👉️ print(list(common_elements)) # 👉️ [9, 4] 

We used the set() class to convert the first nested list to a set object.

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list_of_lists = [ [4, 7, 9], [4, 9, 9], [4, 1, 9] ] print(set(list_of_lists[0])) # 👉️ 

The intersection method returns a new set with elements common to both set objects.

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list_of_lists = [ [4, 7, 9], [4, 9, 9], [4, 1, 9] ] common_elements = set( list_of_lists[0] ).intersection(*list_of_lists) print(common_elements) # 👉️ print(list(common_elements)) # 👉️ [9, 4] 

We called the intersection() method on the set and used the iterable unpacking operator to unpack the list of lists in the call to the method.

The * iterable unpacking operator enables us to unpack an iterable in function calls, in comprehensions and in generator expressions.

You can imagine that the nested lists got passed as multiple, comma-separated arguments to the intersection method.

The intersection method then returned a set object containing the common elements in the lists.

Use the list() class if you need to convert the set to a list.

The list class takes an iterable and returns a list object.

Alternatively, you can use the intersection_update() method.

# Find the common elements in a List of Lists using intersection_update()

This is a three-step process:

1. Convert the first element in the list to a set .
2. Use a for loop to iterate over the rest of the list.
3. Use the intersection_update method to find the common elements.

Copied! 
list_of_lists = [ [4, 7, 9], [4, 9, 9], [4, 1, 9] ] common_elements = set(list_of_lists[0]) for l in list_of_lists[1:]: common_elements.intersection_update(l) print(common_elements) # 

We converted the first nested list to a set and stored the result in a variable.

We then used a for loop to iterate over the rest of the list.

On each iteration, we use the intersection_update method to update the set with the intersection of itself and the list.

The intersection_update method updates the set in place.

After the last iteration, the variable stores the common elements in the list of lists.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

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Find common elements in two lists in python

💡 Outline
You can first convert first list into set using set() and call intersection() by passing second list as parameter to find common element in two lists in Python.

In this tutorial, we will see different methods to find common elements in two lists in python.

Using the intersection() Function

This is the easiest method to find common elements in two lists in Python. As the name suggests, the intersection() function is a built-in python function that is used to return a set that contains the elements which are common in two sets. The sets can be of any form i.e a list or a dictionary.

Note that, __name__ variable is used in the above code. It is a built-in variable that is used to see whether the current code is being run on its own or an external module is being imported from somewhere. Thus, the __name__ variable checks the name of the current module in the ongoing code. It is usually used with an if statement and where it is assigned to ‘__main__’ when there is no module in the program. Otherwise, __name__ is assigned to the name of the module used in the program.

Using List Comprehension

List Comprehension is a very concise way which helps in creating a list based on values stored from existing list or lists. This is also one way of finding common elements from two lists.

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