A list is one of Python’s fundamental in-built data types and clusters several items in a single variable.<\/p>\n
In this article, we will demonstrate the different ways available to retrieve the list intersection in Python.<\/p>\n
The intersection of two lists returns the elements that are present in both lists. There are several methods to find these elements.<\/p>\n
for<\/code> loop to achieve list intersection in Python<\/h3>\nInstead of using any built-in functions available for this purpose, we can always create a simple user-defined function using the for<\/code> loop.<\/p>\nFor example,<\/p>\n
\nX = [1,3,5,7,9]\nY = [1,5,10,15]\ndef intsec(A, B):\nintsec = []\nfor val in A:\nif val in B:\nintsec.append(val)\nprint(intsec)\nintsec(X,Y)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nExplanation<\/h5>\n\n- We first define two lists named
X<\/code> and Y<\/code> in the above code.<\/li>\n- We create a user-defined function that uses the for loop to traverse the list and find the common elements.<\/li>\n
- We store these elements in a new list
intsec<\/code> and display them.<\/li>\n<\/ul>\nList comprehension can simplify the code and make the syntax compact in this case. <\/p>\n
For example,<\/p>\n
\nX = [1,3,5,7,9]\nY = [1,5,10,15]\nZ = [val for val in X if val in Y]\nprint(Z)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nThis method increases the program’s efficiency, dropping its complexity to O(n)<\/code>.<\/p>\nUsing the set()<\/code> function to find list intersection in Python<\/h3>\nWe first convert the given lists into sets individually using the set()<\/code> method. Then we apply the intersection on the two sets using the intersection &<\/code> operator, which returns the intersection between the two sets.<\/p>\nSee the code below.<\/p>\n
\nX = [1,3,5,7,9]\nY = [1,5,10,15]\nZ = list(set(X) & set(Y))\nprint(Z)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nExplanation<\/h5>\n\n- The two lists
X<\/code> and Y<\/code> are defined.<\/li>\n- Both the lists are converted into sets using the
set()<\/code> method. <\/li>\n- We carry out the intersection operation using the
&<\/code> operator.<\/li>\n- The result is stored in
Z<\/code> and is displayed.<\/li>\n<\/ul>\nUsing the intersection()<\/code> function to find list intersection in Python<\/h3>\nSets have an in-built method, intersection()<\/code>, which can quickly get the common items between sets. The intersection()<\/code> function will return the common values from both sets in a freshly generated set. <\/p>\nThe lists need to be explicitly converted into sets first, as done in the previous method to use this function.<\/p>\n
See the code below.<\/p>\n
\nX = [1,3,5,7,9]\nY = [1,5,10,15]\nZ = list(set(X).intersection(set(Y)))\nprint(Z)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nUsing the filter()<\/code> and lambda<\/code> functions to find list intersection in Python<\/h3>\nPython’s filter()<\/code> function can filter out elements of a given list based on some condition. Only the elements that satisfy this condition will be returned by this function.<\/p>\nHere, we will also take help from the lambda<\/code> function. A lambda function is a single-expression, one-line function that can take any number of arguments.<\/p>\nWe use both these functions in the following example.<\/p>\n
\nX = [1,3,5,7,9]\nY = [1,5,10,15]\nZ = list(filter(lambda x:x in X,Y))\nprint(Z)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nExplanation<\/h5>\n\n- We create the two lists.<\/li>\n
- The
filter()<\/code> function takes each item of the first list and compares it with the items in the second list to check whether it is present in both lists.<\/li>\n- If this is
True<\/code>, then we store the element in the list Z<\/code>.<\/li>\n<\/ul>\nThe advantage of this method over its peers mentioned in this article is that we can also use it over sub-lists contained within the other lists.<\/p>\n
Using the Counter<\/code> class to find list intersection in Python<\/h3>\nThe Counter<\/code> is a class from the collections<\/code> module that keeps track of all the items’ frequency of occurrence in a given iterable. We can use it to return the list intersection of two lists. <\/p>\nFor example,<\/p>\n
\nfrom collections import Counter\nX = Counter([1,3,5,7,9])\nY = Counter([1,5,10,15])\nZ = X & Y\nZ = list(Z.elements())\nprint(Z)\n<\/pre>\nThe above code provides the following output:<\/p>\n
\n [1, 5]\n<\/div>\nExplanation<\/h5>\n\n- We apply the
Counter()<\/code> constructor to both lists.<\/li>\n- We perform the intersection operation on both
X<\/code> and Y<\/code> using the &<\/code> operator.<\/li>\n- We store the elements of this class in
Z<\/code> using the elements()<\/code> function.<\/li>\n- Finally, we display the common elements.<\/li>\n<\/ul>\n
Using the numpy.intersect1d()<\/code> function to find list intersection in Python<\/h3>\nThe numpy.intersect1d<\/code> function returns an array containing the intersection of two given arrays. We can convert this final result to a list using the list()<\/code> function.<\/p>\nFor example,<\/p>\n
\n import numpy as np\n X = [1,3,5,7,9]\n Y = [1,5,10,15]\n Z = list(np.intersect1d(X, Y))\n print(Z)\n<\/pre>\nOutput:<\/p>\n