Python – Values frequencies of key

Last Updated : 05 May, 2023

Sometimes, while working with Python dictionary lists, one can have a problem in which we require to find the frequency of all the values occurring in a specific key in dictionary list. This can have application across many domains including web development. Let’s discuss certain ways in which this task can be performed.

Input : test_list = [{‘gfg’: [1]}, {‘good’: [5, 78, 10], ‘gfg’: [5, 6, 7, 8]}] Output : {8: 1, 1: 1, 5: 1, 6: 1, 7: 1} Input : test_list = [{‘gfg’: [1, 5, 6, 7]}] Output : {1: 1, 5: 1, 6: 1, 7: 1}

Method #1 : Using Counter() + list comprehension The combination of above functionalities is used to solve this problem. In this, we perform the task of finding frequency using Counter() and computation is done using list comprehension.

Python3

# Python3 code to demonstrate working of 
# Values frequencies of key
# Using Counter() + list comprehension
from collections import Counter
 
# initializing list
test_list = [{'gfg' : [1, 5, 6, 7], 'is' : [9, 6, 2, 10]},
             {'gfg' : [5, 6, 7, 8], 'good' : [5, 7, 10]},
             {'gfg' : [7, 5], 'best' : [5, 7]}]
 
# printing original list
print("The original list is : " + str(test_list))
 
# frequency key 
freq_key = "gfg"
 
# Values frequencies of key
# Using Counter() + list comprehension
res = Counter([idx for val in test_list for idx in val[freq_key]])
 
# printing result 
print("The frequency dictionary : " + str(dict(res))) 

Output :

The original list is : [{‘gfg’: [1, 5, 6, 7], ‘is’: [9, 6, 2, 10]}, {‘gfg’: [5, 6, 7, 8], ‘good’: [5, 7, 10]}, {‘gfg’: [7, 5], ‘best’: [5, 7]}] The frequency dictionary : {8: 1, 1: 1, 5: 3, 6: 2, 7: 3}

Time Complexity: O(n*n) where n is the number of elements in the list “test_list”.
Auxiliary Space: O(n) where n is the number of elements in the list “test_list”.

Method #2 : Using chain.from_iterables() + Counter() The combination of above methods can be used to solve this problem. In this, we perform the task of iteration and binding using chain.from_iterables().

Python3

# Python3 code to demonstrate working of 
# Values frequencies of key
# Using chain.from_iterables() + Counter()
from collections import Counter
import itertools
 
# initializing list
test_list = [{'gfg' : [1, 5, 6, 7], 'is' : [9, 6, 2, 10]},
             {'gfg' : [5, 6, 7, 8], 'good' : [5, 7, 10]},
             {'gfg' : [7, 5], 'best' : [5, 7]}]
 
# printing original list
print("The original list is : " + str(test_list))
 
# frequency key 
freq_key = "gfg"
 
# Values frequencies of key
# Using chain.from_iterables() + Counter()
res = Counter(itertools.chain.from_iterable([sub[freq_key] for sub in test_list]))
 
# printing result 
print("The frequency dictionary : " + str(dict(res))) 

Output :

The original list is : [{‘gfg’: [1, 5, 6, 7], ‘is’: [9, 6, 2, 10]}, {‘gfg’: [5, 6, 7, 8], ‘good’: [5, 7, 10]}, {‘gfg’: [7, 5], ‘best’: [5, 7]}] The frequency dictionary : {8: 1, 1: 1, 5: 3, 6: 2, 7: 3}

Time Complexity: O(n*n) where n is the number of elements in the dictionary. The chain.from_iterables() + Counter() is used to perform the task and it takes O(n*n) time.
Auxiliary Space: O(n) additional space of size n is created where n is the number of elements in the dictionary.

Method #3 : Using extend(),list(),set(),count() methods

Approach

Extracting all values of a particular key in list of dictionaries using for loop and extend() method
Create a new list with all unique values of above values list
Now create a dictionary with unique value as key and count of this value in the values list using count() method
Display dictionary

Python3

# Python3 code to demonstrate working of
# Values frequencies of key
 
# initializing list
test_list = [{'gfg' : [1, 5, 6, 7], 'is' : [9, 6, 2, 10]},
            {'gfg' : [5, 6, 7, 8], 'good' : [5, 7, 10]},
            {'gfg' : [7, 5], 'best' : [5, 7]}]
 
# printing original list
print("The original list is : " + str(test_list))
 
# frequency key
freq_key = "gfg"
 
# Values frequencies of key
res = dict()
x=[]
for i in test_list:
    x.extend(i[freq_key])
y=list(set(x))
for i in y:
    res[i]=x.count(i)
# printing result
print("The frequency dictionary : " + str(res))

Output

The original list is : [{'gfg': [1, 5, 6, 7], 'is': [9, 6, 2, 10]}, {'gfg': [5, 6, 7, 8], 'good': [5, 7, 10]}, {'gfg': [7, 5], 'best': [5, 7]}]
The frequency dictionary : {1: 1, 5: 3, 6: 2, 7: 3, 8: 1}

Time Complexity : O(N)

Auxiliary Space : O(N)

Method #4:Using Numpy

Algorithm

Initialize the list of dictionaries.
Print the original list.
Initialize the frequency key for which we want to find the values frequency.
Initialize an empty dictionary for storing the frequency of values of the frequency key.
For each dictionary in the list, extend the values corresponding to the frequency key to a list x.
Get the unique values of list x using set().
For each unique value in the list of unique values, count the frequency of the value in the list x using count() function and store the frequency in the resultant dictionary.
Print the frequency dictionary.

Python3

import numpy as np
 
# initializing list
test_list = [{'gfg': [1, 5, 6, 7], 'is': [9, 6, 2, 10]},
            {'gfg': [5, 6, 7, 8], 'good': [5, 7, 10]},
            {'gfg': [7, 5], 'best': [5, 7]}]
 
# printing original list
print("The original list is:", test_list)
 
# frequency key
freq_key = "gfg"
 
# Values frequencies of key
res = {}
flat_arr = np.concatenate([d[freq_key] for d in test_list])
unique, counts = np.unique(flat_arr, return_counts=True)
for i in range(len(unique)):
    res[unique[i]] = counts[i]
 
# printing result
print("The frequency dictionary:", res)
#This code is contributed by Vinay Pinjala.

Output

The original list is: [{'gfg': [1, 5, 6, 7], 'is': [9, 6, 2, 10]}, {'gfg': [5, 6, 7, 8], 'good': [5, 7, 10]}, {'gfg': [7, 5], 'best': [5, 7]}]
The frequency dictionary: {1: 1, 5: 3, 6: 2, 7: 3, 8: 1}

The time complexity of the given code is O(N*M), where N is the number of dictionaries in the list and M is the maximum length of the list associated with the given frequency key.

The space complexity of the given code is also O(N*M), since we are creating a list to store all the values associated with the given frequency key, and then creating a dictionary with keys as unique values and their count as values.

Method 5 : Using a loop

step-by-step approach:

Initialize an empty dictionary called “freq_dict” to store the frequency count of each value in the “gfg” key.
Loop through each dictionary in the list “test_list”.
Access the values of the “gfg” key in the current dictionary using the syntax “current_dict[‘gfg’]”.
Loop through each value in the “gfg” key using a for loop.
For each value in the “gfg” key, check if it is already a key in the “freq_dict” dictionary. If it is, increment its value by 1. If it is not, add it as a new key with value 1.
After iterating through all the dictionaries in “test_list”, “freq_dict” will contain the frequency count of each value in the “gfg” key.
Print the “freq_dict” dictionary.

Python3

# initializing list
test_list = [{'gfg': [1, 5, 6, 7], 'is': [9, 6, 2, 10]},
            {'gfg': [5, 6, 7, 8], 'good': [5, 7, 10]},
            {'gfg': [7, 5], 'best': [5, 7]}]
 
# printing original list
print("The original list is:", test_list)
 
# frequency key
freq_key = "gfg"
 
# initialize frequency dictionary
freq_dict = {}
 
# loop through each dictionary in test_list
for d in test_list:
     
    # loop through each value in the "gfg" key
    for val in d[freq_key]:
         
        # update frequency count
        if val in freq_dict:
            freq_dict[val] += 1
        else:
            freq_dict[val] = 1
 
# printing result
print("The frequency dictionary:", freq_dict)

Output

The original list is: [{'gfg': [1, 5, 6, 7], 'is': [9, 6, 2, 10]}, {'gfg': [5, 6, 7, 8], 'good': [5, 7, 10]}, {'gfg': [7, 5], 'best': [5, 7]}]
The frequency dictionary: {1: 1, 5: 3, 6: 2, 7: 3, 8: 1}

Time complexity: O(N*M), where N is the number of dictionaries in “test_list” and M is the average length of the “gfg” key in each dictionary.

Auxiliary space: O(K), where K is the number of unique values in the “gfg” key across all dictionaries in “test_list”.

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