﻿ Python: Get a list of elements that exist in both lists - w3resource

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

## Python List: Exercise - 226 with Solution

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:

Python Code Editor:

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