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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:

def intersection_by(a, b, fn):
  _b = set(map(fn, b))
  return [item for item in a if fn(item) in _b]
from math import floor
print(intersection_by([2.1, 1.2], [2.3, 3.4], floor))

Sample Output:

[2.1]

Flowchart:

Flowchart: Get a list of elements that exist in both lists.

Visualize Python code execution:

The following tool visualize what the computer is doing step-by-step as it executes the said program:


Sample Solution-2:

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

Python Code:

def similarity(x, y):
  return [item for item in x if item in y]
nums1 = [1, 2, 3, 4]
nums2 = [1, 2, 4, 5]
print("Original lists:")
print(nums1)    
print(nums2)
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:

Flowchart: Get a list of elements that exist in both lists.

Visualize Python code execution:

The following tool visualize what the computer is doing step-by-step as it executes the said program:


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:

def common_elements(l1, l2):
  _l1, _l2 = set(l1), set(l2)
  return list(_l1 & _l2)
nums1 = [1, 2, 3, 4]
nums2 = [1, 2, 4, 5]
print("Original lists:")
print(nums1)    
print(nums2)
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:

Flowchart: Get a list of elements that exist in both lists.

Visualize Python code execution:

The following tool visualize what the computer is doing step-by-step as it executes the said program:


Python Code Editor:

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Previous: Write a Python program to retrieve the value of the nested key indicated by the given selector list from a dictionary or list.
Next: Write a Python program to get the symmetric difference between two lists, after applying the provided function to each list element of both.

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Python: Tips of the Day

Floor Division:

When we speak of division we normally mean (/) float division operator, this will give a precise result in float format with decimals.

For a rounded integer result there is (//) floor division operator in Python. Floor division will only give integer results that are round numbers.

print(1000 // 300)
print(1000 / 300)

Output:

3
3.3333333333333335