Python: Get a list of elements that exist in both lists

Last update on April 19 2025 12:58:00 (UTC/GMT +8 hours)

Common Elements with Function

Write a Python program to get a list of elements that exist in both lists, after applying the provided function to each list element of both.

Create a set, using map() to apply fn to each element in b.
Use a list comprehension in combination with fn on a to only keep values contained in both lists.

Sample Solution-1:

Applying a given function:

Python Code:

# Import the necessary modules and functions.
from math import floor

# Define a function 'intersection_by' that takes two lists 'a' and 'b' and a function 'fn' as input.
def intersection_by(a, b, fn):
    # Create a set '_b' containing the results of applying 'fn' to each element in list 'b'.
    _b = set(map(fn, b))
    # Return a list containing elements from list 'a' where 'fn(item)' is in the set '_b'.
    return [item for item in a if fn(item) in _b]
	
# Provide an example of using 'intersection_by' function to find the intersection of two lists using 'floor' function as the key.
print(intersection_by([2.1, 1.2], [2.3, 3.4], floor))

Sample Output:

[2.1]

Flowchart:

Sample Solution-2:

Without applying any function:
Use a list comprehension on x to only keep values contained in both lists.

Python Code:

# Define a function 'similarity' that takes two lists 'x' and 'y' as input.
def similarity(x, y):
    # Return a list containing elements from list 'x' that also exist in list 'y'.
    return [item for item in x if item in y]
	
# Define two lists 'nums1' and 'nums2'.
nums1 = [1, 2, 3, 4]
nums2 = [1, 2, 4, 5]

# Display a message and the original lists.
print("Original lists:")
print(nums1)
print(nums2)

# Display a message and the list of elements that exist in both lists using the 'similarity' function.
print("\nList of elements that exist in both lists:")
print(similarity(nums1, nums2))

Sample Output:

Original lists:
[1, 2, 3, 4]
[1, 2, 4, 5]

List of elements that exist in both lists:
[1, 2, 4]

Flowchart:

Sample Solution-3:

Without applying any function:

Create a set from a and b.
Use the built-in set operator & to only keep values contained in both sets, then transform the set back into a list.

Python Code:

# Define a function 'common_elements' that takes two lists 'l1' and 'l2' as input.
def common_elements(l1, l2):
    # Convert 'l1' and 'l2' into sets to eliminate duplicate values.
    _l1, _l2 = set(l1), set(l2)
    # Find the intersection (common elements) between the sets and convert it back to a list.
    return list(_l1 & _l2)

# Define two lists 'nums1' and 'nums2'.
nums1 = [1, 2, 3, 4]
nums2 = [1, 2, 4, 5]

# Display a message and the original lists.
print("Original lists:")
print(nums1)
print(nums2)

# Display a message and the list of elements that exist in both lists using the 'common_elements' function.
print("\nList of elements that exist in both lists:")
print(common_elements(nums1, nums2))

Sample Output:

Original lists:
[1, 2, 3, 4]
[1, 2, 4, 5]

List of elements that exist in both lists:
[1, 2, 4]

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to get the common elements between two lists after converting all string elements to uppercase.
Write a Python program to find common elements between two lists using a custom function that extracts a numeric field from dictionary elements.
Write a Python program to compute the intersection of two lists after applying a function that rounds float values to one decimal place.
Write a Python program to filter two lists with a provided function and then return the common elements that satisfy the condition.

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