Python | Subtraction of dictionaries

Sometimes, while working with dictionaries, we might have a utility problem in which we need to perform elementary operations among the common keys of dictionaries in Python. This can be extended to any operation to be performed. Let’s discuss the subtraction of like key values and ways to solve it in this article.

Example

Input: The original dictionary 1 : {'gfg': 6, 'is': 4, 'best': 7}
            The original dictionary 2 : {'gfg': 10, 'is': 6, 'best': 10}
Output: The difference dictionary is : {'gfg': 4, 'is': 2, 'best': 3}
Explanantion: In this, we are subtacting 2 dictionary and printing the difference dictionary using Python.

How to Subtract Two Dictionaries

In Python, we have different methods to subtract dictionaries. Here we will see different methods:

• Using dictionary comprehension
• Using Counter() + “-” operator 
• Using a for loop and dict.get()
• Using items() method

Subtract Values from two Dictionaries using dictionary comprehension + keys()

The combination of the above two can be used to perform this particular task. This is just a shorthand for the longer method of loops and can be used to perform this task in one line. 

Python3

# Initialize dictionaries
test_dict1 = {'gfg' : 6, 'is' : 4, 'best' : 7}
test_dict2 = {'gfg' : 10, 'is' : 6, 'best' : 10}

# printing original dictionaries
print("The original dictionary 1 : " +  str(test_dict1))
print("The original dictionary 2 : " +  str(test_dict2))

# Using dictionary comprehension + keys()
# Subtraction of dictionaries
res = {key: test_dict2[key] - test_dict1.get(key, 0)
                       for key in test_dict2.keys()}

print("The difference dictionary is : " + str(res))

Output:

The original dictionary 1 : {'gfg': 6, 'is': 4, 'best': 7}
The original dictionary 2 : {'gfg': 10, 'is': 6, 'best': 10}
The difference dictionary is : {'gfg': 4, 'is': 2, 'best': 3}

Subtracting values between two dictionaries using Counter() + “-” operator

The combination of the above methods can be used to perform this particular task. In this, the Counter function converts the dictionary into the form in which the minus operator can perform the task of subtraction. 

Python3

from collections import Counter

# Initialize dictionaries
test_dict1 = {'gfg': 6, 'is': 4, 'best': 7}
test_dict2 = {'gfg': 10, 'is': 6, 'best': 10}

# printing original dictionaries
print("The original dictionary 1:", test_dict1)
print("The original dictionary 2:", test_dict2)

# Using Counter() + "-" operator
# Subtraction of dictionaries
temp1 = Counter(test_dict1)
temp2 = Counter(test_dict2)
res = temp2 - temp1

print("The difference dictionary is:", dict(res))

Output

The original dictionary 1: {'gfg': 6, 'is': 4, 'best': 7}
The original dictionary 2: {'gfg': 10, 'is': 6, 'best': 10}
The difference dictionary is: {'gfg': 4, 'is': 2, 'best': 3}

Dictionary Subtraction using the built-in function set() and dictionary comprehension

This method uses the built-in function set() to iterate through the keys of both dictionaries. The keys that are common in both dictionaries are subtracted and the result is stored in a new dictionary.

Python3

test_dict1 = {'gfg' : 6, 'is' : 4, 'best' : 7}
test_dict2 = {'gfg' : 10, 'is' : 6, 'best' : 10}

# printing original dictionaries
print("The original dictionary 1 : ", test_dict1)
print("The original dictionary 2 : ", test_dict2)

# Using set() and dictionary comprehension
# Subtraction of dictionaries
res = {key: test_dict2[key] - test_dict1[key] for key in set(test_dict1) & set(test_dict2)}

print("The difference dictionary is : ", res)

Output

The original dictionary 1 :  {'gfg': 6, 'is': 4, 'best': 7}
The original dictionary 2 :  {'gfg': 10, 'is': 6, 'best': 10}
The difference dictionary is :  {'best': 3, 'is': 2, 'gfg': 4}

Time complexity: O(n)
Auxiliary space: O(n) where n is the number of common keys in both dictionaries.

Subtract Values from two Dictionaries using a for loop and dict.get()

By using a for loop along with the dict.get() method. This method helps iterate through the keys of one dictionary and, using, dict.get()check if the same key exists in the other dictionary. If it does, the corresponding key-value pair can be removed.

Python3

test_dict1 = {'gfg' : 6, 'is' : 4, 'best' : 7}
test_dict2 = {'gfg' : 10, 'is' : 6, 'best' : 10}
diff_dict = {}

# printing original dictionaries
print("The original dictionary 1 : ", test_dict1)
print("The original dictionary 2 : ", test_dict2)
for key in test_dict2.keys():
    diff_dict[key] = test_dict2[key] - test_dict1.get(key, 0)

print("The difference dictionary is : ", diff_dict)

Output

The original dictionary 1 :  {'gfg': 6, 'is': 4, 'best': 7}
The original dictionary 2 :  {'gfg': 10, 'is': 6, 'best': 10}
The difference dictionary is :  {'gfg': 4, 'is': 2, 'best': 3}

Time Complexity: O(N)
Auxiliary Space:  O(N)

Subtracting values between two dictionaries using items() Method

We can use the items() method to iterate over the key-value pairs of both dictionaries and check for differences in values.

Initialize the two dictionaries test_dict1 and test_dict2. Initialize an empty dictionary diff_dict to store the difference between the two dictionaries. Iterate over the key-value pairs of test_dict1 using the items() method. For each key:

• If the key is present in test_dict2, compute the difference in values and add it to diff_dict.
• If the key is not present in test_dict2, set the difference value to the negative of the value in test_dict1.

Iterate over the key-value pairs of test_dict2 using the items() method. For each key:

• If the key is not present in test_dict1, set the difference value to the value in test_dict2.

Print the original dictionaries and the different dictionaries.

Python3

test_dict1 = {'gfg' : 6, 'is' : 4, 'best' : 7}
test_dict2 = {'gfg' : 10, 'is' : 6, 'best' : 10}

# Initialize an empty dictionary to store the difference
diff_dict = {}

# Iterate over the key-value pairs of both dictionaries
for key, value in test_dict1.items():
    if key in test_dict2:
        # If the key is present in both dictionaries, compute the difference in values
        diff_dict[key] = test_dict2[key] - value
    else:
        # If the key is not present in test_dict2, set the difference value to the value in test_dict1
        diff_dict[key] = -value

# Iterate over the key-value pairs of test_dict2
for key, value in test_dict2.items():
    if key not in test_dict1:
        # If the key is not present in test_dict1, set the difference value to the value in test_dict2
        diff_dict[key] = value

# Print the original dictionaries and the difference dictionary
print("The original dictionary 1 : ", test_dict1)
print("The original dictionary 2 : ", test_dict2)
print("The difference dictionary is : ", diff_dict)

Output

The original dictionary 1 :  {'gfg': 6, 'is': 4, 'best': 7}
The original dictionary 2 :  {'gfg': 10, 'is': 6, 'best': 10}
The difference dictionary is :  {'gfg': 4, 'is': 2, 'best': 3}

Time Complexity: O(n), where n is the size of the larger dictionary.
Auxiliary Space: O(n), where n is the size of the larger dictionary.