Python | List frequency of elements

Sometimes we have the utility in which we require to find the frequency of elements in list and solution to this problem has been discussed many times. But sometimes we come across the task in which we require to find the number of lists that particular elements occur. Let’s discuss certain shorthands in which this can be done.

Method #1 : Using Counter() + set() + list comprehension
The combination of the above functions can be used to perform the task. The Counter function does the grouping, set function extracts the distinct elements as keys of dict and list comprehension check for it’s list occurrences.

# Python3 code to demonstrate 
# list frequency of elements 
# using Counter() + set() + list comprehension 
from collections import Counter 
  
# initializing list 
test_list = [[3, 5, 4], 
             [6, 2, 4], 
             [1, 3, 6]] 
  
# printing original list  
print("The original list : " + str(test_list)) 
  
# using Counter() + set() + list comprehension 
# list frequency of elements 
res = dict(Counter(i for sub in test_list for i in set(sub))) 
  
# printing result 
print("The list frequency of elements is : " + str(res)) 

Output :

The original list : [[3, 5, 4], [6, 2, 4], [1, 3, 6]]
The list frequency of elements is : {1: 1, 2: 1, 3: 2, 4: 2, 5: 1, 6: 2}

Method #2 : Using Counter() + itertools.chain.from_iterable() + map() + set()
The above 4 functionalities can also be combined to achieve this particular task. The set function extracts the dictionary keys formed by Counter, map function performs the task for all sublists and from_iterable function performs using iterators which is faster than list comprehension.

# Python3 code to demonstrate 
# list frequency of elements 
# using Counter() + itertools.chain.from_iterable() + map() + set() 
from collections import Counter 
from itertools import chain 
  
# initializing list 
test_list = [[3, 5, 4], 
             [6, 2, 4], 
             [1, 3, 6]] 
  
# printing original list  
print("The original list : " + str(test_list)) 
  
# using Counter() + itertools.chain.from_iterable() + map() + set() 
# list frequency of elements 
res = dict(Counter(chain.from_iterable(map(set, test_list)))) 
  
# printing result 
print("The list frequency of elements is : " + str(res)) 

Output :

The original list : [[3, 5, 4], [6, 2, 4], [1, 3, 6]]
The list frequency of elements is : {1: 1, 2: 1, 3: 2, 4: 2, 5: 1, 6: 2}

My Personal Notes

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