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 :

Step 1: Import the Counter class from the collections module.

Step 2: Define two dictionaries called “ini_dictionary1” and “ini_dictionary2” with some key-value pairs.

Step 3: Print the initial dictionaries using the print() function.

Step 4: Combine the two dictionaries using the “+” operator and the Counter() function.

Step 5: Store the resulting dictionary in a variable called “final_dictionary”.

Step 6: Print the final dictionary using the print() function.

Python3

# Python code to demonstrate combining
# two dictionaries having same key

from collections import Counter

# initialising dictionaries
ini_dictionary1 = Counter({'nikhil': 1, 'akash' : 5,
                    'manjeet' : 10, 'akshat' : 15})
ini_dictionary2 = Counter({'akash' : 7, 'akshat' : 5,
                                        'm' : 15})

# printing initial dictionaries
print ("initial 1st dictionary", str(ini_dictionary1))
print ("initial 2nd dictionary", str(ini_dictionary2))

# combining dictionaries
# using Counter
final_dictionary = ini_dictionary1 + ini_dictionary2

# printing final result
print ("final dictionary", str(final_dictionary))

Output

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. 

Python

# Python code to demonstrate combining
# two dictionaries having same key


# initialising dictionaries
ini_dictionary1 = {'nikhil': 1, 'akash' : 5,
            'manjeet' : 10, 'akshat' : 15}
ini_dictionary2 = {'akash' : 7, 'akshat' : 5,
                                    'm' : 15}

# printing initial dictionaries
print ("initial 1st dictionary", str(ini_dictionary1))
print ("initial 2nd dictionary", str(ini_dictionary2))

# combining dictionaries
# using dict() and items()
final_dictionary = dict(ini_dictionary1.items() + ini_dictionary2.items() +
                    [(k, ini_dictionary1[k] + ini_dictionary2[k])
                    for k in set(ini_dictionary2)
                    & set(ini_dictionary1)])

# printing final result
print ("final dictionary", str(final_dictionary))

Output

('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

Python3

# Python code to demonstrate combining
# two dictionaries having same key

# initialising dictionaries
ini_dictionary1 = {'nikhil': 1, 'akash' : 5,
            'manjeet' : 10, 'akshat' : 15}
ini_dictionary2 = {'akash' : 7, 'akshat' : 5,
                                    'm' : 15}

# printing initial dictionaries
print ("initial 1st dictionary", str(ini_dictionary1))
print ("initial 2nd dictionary", str(ini_dictionary2))

# combining dictionaries
# using dict comprehension and set
final_dictionary = {x: ini_dictionary1.get(x, 0) + ini_dictionary2.get(x, 0)
                    for x in set(ini_dictionary1).union(ini_dictionary2)}

# printing final result
print ("final dictionary", str(final_dictionary))

Output

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

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

Python3

# initialising dictionaries
ini_dictionary1 = {'nikhil': 1, 'akash' : 5, 'manjeet' : 10, 'akshat' : 15}
ini_dictionary2 = {'akash' : 7, 'akshat' : 5, 'm' : 15}

# combining dictionaries using a for loop and dict() constructor
final_dictionary = {}
for key in ini_dictionary1:
    final_dictionary[key] = ini_dictionary1[key] + ini_dictionary2.get(key, 0)
for key in ini_dictionary2:
    if key not in final_dictionary:
        final_dictionary[key] = ini_dictionary2[key]

# printing final result
print("final dictionary", str(final_dictionary))

Output

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. 

Here is the step-by-step approach:

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

Python3

# initialising dictionaries
ini_dictionary1 = {'nikhil': 1, 'akash': 5, 'manjeet': 10, 'akshat': 15}
ini_dictionary2 = {'akash': 7, 'akshat': 5, 'm': 15}

# combining dictionaries using update() method
final_dictionary = ini_dictionary1.copy()
final_dictionary.update(ini_dictionary2)

# printing final result
print("final dictionary", str(final_dictionary))

Output

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

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