Python | Combine the values of two dictionaries having same key

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. Combining dictionaries is very common task in operations of dictionary. Let’s see how to combine the values of two dictionaries having same key. 

Method #1: Using Counter 

Counter is a special subclass of dictionary that performs acts same as dictionary in most cases. 

Step-by-step approach:

• Import the Counter class from the collections module.
• Define two dictionaries called “ini_dictionary1” and “ini_dictionary2” with some key-value pairs.
• Print the initial dictionaries using the print() function.
• Combine the two dictionaries using the “+” operator and the Counter() function.
• Store the resulting dictionary in a variable called “final_dictionary”.
• Print the final dictionary using the print() function.

Below is the implementation of the above approach:

initial 1st dictionary Counter({'akshat': 15, 'manjeet': 10, 'akash': 5, 'nikhil': 1})
initial 2nd dictionary Counter({'m': 15, 'akash': 7, 'akshat': 5})
final dictionary Counter({'akshat': 20, 'm': 15, 'akash': 12, 'manjeet': 10, 'nikhil': 1})

Time Complexity: O(n)
Auxiliary Space: O(1)

Method #2: Using dict() and items This method is for Python version 2. 

Step-by-step approach:

• Two dictionaries are initialized with key-value pairs, stored in the variables ini_dictionary1 and ini_dictionary2 respectively.
• The initial dictionaries are printed using the print() function, to show their content before combining.
• The dictionaries are combined into a final dictionary, using the dict() constructor and the items() method.
• This creates a new dictionary that includes all the key-value pairs from both initial dictionaries.
• To handle cases where the same key exists in both initial dictionaries, a list comprehension is used to iterate over the set intersection of the keys in both dictionaries (i.e., the keys that exist in both dictionaries). For each of these keys, a new tuple is created with the key and the sum of the values for that key in both dictionaries. This tuple is then added to the list of key-value pairs being passed to the dict() constructor, so that it gets included in the final dictionary.
• The final dictionary is printed using the print() function. It includes all the key-value pairs from both initial dictionaries, as well as any key-value pairs where the same key exists in both initial dictionaries (with the values added together).

Below is the implementation of the above approach:

('initial 1st dictionary', "{'manjeet': 10, 'nikhil': 1, 'akshat': 15, 'akash': 5}")
('initial 2nd dictionary', "{'m': 15, 'akshat': 5, 'akash': 7}")
('final dictionary', "{'nikhil': 1, 'm': 15, 'manjeet': 10, 'akshat': 20, 'akash': 12}")

Time complexity: O(n), where n is the number of elements in both dictionaries.
Auxiliary space: O(n), where n is the size of the final dictionary created by combining both dictionaries.

Method #3: Using dict comprehension and set 

initial 1st dictionary {'nikhil': 1, 'akash': 5, 'manjeet': 10, 'akshat': 15}
initial 2nd dictionary {'akash': 7, 'akshat': 5, 'm': 15}
final dictionary {'m': 15, 'manjeet': 10, 'akshat': 20, 'nikhil': 1, 'akash': 12}

Time complexity: O(n), where n is the total number of key-value pairs in both dictionaries.
Auxiliary space: O(n), where n is the total number of key-value pairs in both dictionaries

Method #4: Using dict() and for loop

Combine two dictionaries with the same keys using a for loop and the dict() constructor to create a new dictionary. 

final dictionary {'nikhil': 1, 'akash': 12, 'manjeet': 10, 'akshat': 20, 'm': 15}

Time complexity: O(n), where n is the total number of key-value pairs in both dictionaries.
Auxiliary space: O(n), where n is the total number of key-value pairs in both dictionaries

Method #5:  use the update() method. 

Step-by-step approach:

• Define the initial dictionaries:
• Create a new dictionary and update it with the first dictionary.
• Use the update() method to add or update the key-value pairs from the second dictionary:
• If there are keys in the second dictionary that are not in the first one, they will be added to the final dictionary automatically.
• Print the final result.

Below is the implementation of the above approach:

final dictionary {'nikhil': 1, 'akash': 7, 'manjeet': 10, 'akshat': 5, 'm': 15}

Time complexity: O(n+m), where n and m are the sizes of the two dictionaries. 
Auxiliary space: O(n), where n is the size of the first dictionary.

Method #6: Using the ** operator

Step-by-step approach:

• Define two dictionaries called ini_dictionary1 and ini_dictionary2 with some key-value pairs.
• Create a new dictionary called final_dictionary by using the double asterisk operator (**) to unpack the two dictionaries into a single dictionary.
• Print the final result using the print() function and passing the string representation of final_dictionary.

Below is the implementation of the above approach:

final dictionary {'nikhil': 1, 'akash': 7, 'manjeet': 10, 'akshat': 5, 'm': 15}

Time complexity: O(n), where n is the total number of elements in both dictionaries.
Auxiliary space: O(n), where n is the total number of elements in both dictionaries. 

Method #7: Using Merge Operator

Step-by-Step Approach:

• Initialize an empty dictionary to store the combined result.
• Use the merge operator (**), which combines two dictionaries into a single dictionary, to merge the two dictionaries ini_dictionary1 and ini_dictionary2.
• For each key in the merged dictionary, sum up the values for the same keys.
• Store the result in the final dictionary.
• Print the final dictionary.

initial 1st dictionary {'nikhil': 1, 'akash': 5, 'manjeet': 10, 'akshat': 15}
initial 2nd dictionary {'akash': 7, 'akshat': 5, 'm': 15}
final dictionary {'nikhil': 1, 'akash': 7, 'manjeet': 10, 'akshat': 5, 'm': 15}

Time Complexity: O(m+n), where m and n are the number of items in the two dictionaries.
Auxiliary Space: O(m+n), where m and n are the number of items in the two dictionaries.