Python | Ways to invert mapping of dictionary

Dictionary is a collection which is unordered, changeable and indexed. In Python, dictionaries are written with curly brackets, and they have keys and values. It is widely used in day to day programming, web development, and machine learning. Let’s discuss a few ways to invert mapping of a dictionary. 

Method #1: Using Dictionary Comprehension. 

Python3

# Python code to demonstrate
# how to invert mapping
# using dict comprehension

# initialising dictionary
ini_dict = {101: "akshat", 201 : "ball"}

# print initial dictionary
print("initial dictionary : ", str(ini_dict))

# inverse mapping using dict comprehension
inv_dict = {v: k for k, v in ini_dict.items()}

# print final dictionary
print("inverse mapped dictionary : ", str(inv_dict))

Output:

initial dictionary :  {201: 'ball', 101: 'akshat'}
inverse mapped dictionary :  {'ball': 201, 'akshat': 101}

Time complexity: O(n), where n is the number of key-value pairs in the dictionary.
Auxiliary space: O(n), to store the keys and values in dictionary.

Method #2: Using dict.keys() and dict.values() 

Python3

# Python code to demonstrate
# how to invert mapping
# using zip and dict functions

# initialising dictionary
ini_dict = {101: "akshat", 201 : "ball"}

# print initial dictionary
print("initial dictionary : ", str(ini_dict))

# inverse mapping using zip and dict functions
inv_dict = dict(zip(ini_dict.values(), ini_dict.keys()))

# print final dictionary
print("inverse mapped dictionary : ", str(inv_dict))

Output:

initial dictionary :  {201: 'ball', 101: 'akshat'}
inverse mapped dictionary :  {'ball': 201, 'akshat': 101}

Method #3: Using map() and reversed 

Python3

# Python code to demonstrate
# how to invert mapping
# using map and reversed

# initialising dictionary
ini_dict = {101: "akshat", 201 : "ball"}

# print initial dictionary
print("initial dictionary : ", str(ini_dict))

# inverse mapping using map and reversed
inv_dict = dict(map(reversed, ini_dict.items()))

# print final dictionary
print("inverse mapped dictionary : ", str(inv_dict))

Output:

initial dictionary :  {201: 'ball', 101: 'akshat'}
inverse mapped dictionary :  {'akshat': 101, 'ball': 201}

Method #4: Using lambda 

Python3

# Python code to demonstrate
# how to invert mapping
# using lambda

# initialising dictionary
ini_dict = {101 : "akshat", 201 : "ball"}

# print initial dictionary
print("initial dictionary : ", str(ini_dict))

# inverse mapping using lambda
lambda ini_dict: {v:k for k, v in ini_dict.items()}

# print final dictionary
print("inverse mapped dictionary : ", str(ini_dict))

Output:

initial dictionary :  {201: 'ball', 101: 'akshat'}
inverse mapped dictionary :  {201: 'ball', 101: 'akshat'}

The time complexity of this code is O(n), where n is the number of key-value pairs in the dictionary ini_dict. dictionary.

The auxiliary space complexity of this code is O(n), where n is the number of key-value pairs in the dictionary ini_dict. 

Method #5: Using the zip function and dictionary constructor:

Approach:

• Use the zip function to create a list of tuples with the original dictionary’s values and keys
• Pass the list of tuples to the dictionary constructor to create the inverted dictionary
• Return the inverted dictionary

Python3

original_dict = {"a": 1, "b": 2, "c": 3}
inverted_dict = dict(zip(original_dict.values(), original_dict.keys()))
print(inverted_dict)

Output

{1: 'a', 2: 'b', 3: 'c'}

Time complexity: O(n), where n is the number of key-value pairs in the dictionary
Space complexity: O(n), where n is the number of key-value pairs in the dictionary