Python – Convert Frequency dictionary to list

Sometimes, while working with Python dictionaries, we can have a problem in which we need to construct the list out of the values of dictionary. This task is reverse of finding frequency and has application in day-day programming and web development domain. Let’s discuss certain ways in which this task can be performed.

Input : test_dict = {‘gfg’ : 3, ‘ide’ : 2} 
Output : [‘gfg’, ‘gfg’, ‘gfg’, ‘ide’, ‘ide’] 

Input : test_dict = {‘practice’ : 1, ‘write’ : 2, ‘ide’ : 4} 
Output : [‘practice’, ‘write’, ‘write’, ‘ide’, ‘ide’, ‘ide’, ‘ide’]

Method #1: Using loop
This is brute way to solve this problem. In this, we iterate for dictionary and extract the frequency and replicate the elements at that frequency. 

Python3

# Python3 code to demonstrate working of
# Convert Frequency dictionary to list
# Using loop

# initializing dictionary
test_dict = {'gfg': 4, 'is': 2, 'best': 5}

# printing original dictionary
print("The original dictionary : " + str(test_dict))

# Convert Frequency dictionary to list
# Using loop
res = []
for key in test_dict:
    for idx in range(test_dict[key]):
        res.append(key)

# printing result
print("The resultant list : " + str(res))

Output

The original dictionary : {'gfg': 4, 'is': 2, 'best': 5}
The resultant list : ['gfg', 'gfg', 'gfg', 'gfg', 'is', 'is', 'best', 'best', 'best', 'best', 'best']

Time Complexity: O(n*n), where n is the length of the list test_list 
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the res list 

Method #2: Using list comprehension
This method is similar to above method with respect to working. This is shorthand to above method. 

Python3

def construct_list(dictionary):
    # Use list comprehension to repeat the keys of the dictionary
    # value number of times and return the result as a list
    return [key for key, value in dictionary.items() for _ in range(value)]


# initializing dictionary
test_dict = {'gfg' : 4, 'is' : 2, 'best' : 5}

# printing original dictionary
print("The original dictionary : " + str(test_dict))

# printing result
print("The resultant list : " + str(construct_list(test_dict)))

Output

The original dictionary : {'gfg': 4, 'is': 2, 'best': 5}
The resultant list : ['gfg', 'gfg', 'gfg', 'gfg', 'is', 'is', 'best', 'best', 'best', 'best', 'best']

Method 3: Using the built-in itertools.repeat() and itertools.chain.from_iterable()

Step-by-step approach

• Import the itertools module. This module provides functions for creating iterators for efficient looping.
• Define a function construct_list that takes a dictionary as input.
• Create a list iterators using a list comprehension that iterates through the items in the dictionary. For each key-value pair in the dictionary, create an iterator using the itertools.repeat() function. The iterator repeats the key value times.
• Concatenate all the iterators in the iterators list using the itertools.chain.from_iterable() function. This function takes an iterable of iterables as input and returns a single iterable that concatenates all the input iterables.
• Convert the concatenated iterable into a list using the list() constructor and return the result.
• Define a dictionary test_dict and initialize it with key-value pairs.
• Print the original dictionary using the print() function and the str() constructor to convert the dictionary to a string.
• Call the construct_list() function with test_dict as input and print the resultant list using the print() function and the str() constructor to convert the list to a string.

Python3

import itertools

def construct_list(dictionary):
    # Create a list of iterators that produce the keys of the dictionary
    iterators = [itertools.repeat(key, value) for key, value in dictionary.items()]

    # Concatenate all the iterators and return the result as a list
    return list(itertools.chain.from_iterable(iterators))

# initializing dictionary
test_dict = {'gfg' : 4, 'is' : 2, 'best' : 5}

# printing original dictionary
print("The original dictionary : " + str(test_dict))

# printing result
print("The resultant list : " + str(construct_list(test_dict)))

Output

The original dictionary : {'gfg': 4, 'is': 2, 'best': 5}
The resultant list : ['gfg', 'gfg', 'gfg', 'gfg', 'is', 'is', 'best', 'best', 'best', 'best', 'best']

Time complexity: O(n), where n is the total number of values in the dictionary.
Auxiliary space: O(n), where n is the total number of values in the dictionary.