Python – Extract elements with equal frequency as value

Given a list, extract all the elements having same frequency as its value.

Examples:

Input : test_list = [4, 3, 2, 2, 3, 4, 1, 3, 2, 4, 4] 
Output : [1, 3, 4] 
Explanation : All elements occur equal times as their value.

Input : test_list = [4, 3, 2, 2, 3, 4, 1, 3, 2, 4] 
Output : [1, 3] 
Explanation : All elements occur equal times as their value. 

Method #1 : Using list comprehension + count()

In this, task of getting frequency is done using count(), list comprehension is used to iterate for each element, compare and extract.

Python3

# Python3 code to demonstrate working of
# Extract elements with equal frequency as value
# Using list comprehension + count()

# initializing list
test_list = [4, 3, 2, 2, 3, 4, 1, 3, 2, 4, 4]

# printing original list
print("The original list is : " + str(test_list))

# removing duplicates using set()
# count() for computing frequency
res = list(set([ele for ele in test_list if test_list.count(ele) == ele]))

# printing result
print("Filtered elements : " + str(res))

Output

The original list is : [4, 3, 2, 2, 3, 4, 1, 3, 2, 4, 4]
Filtered elements : [1, 3, 4]

Method #2 : Using filter() + lambda + count()

In this, we perform task of filtering elements using filter() and lambda, count() again is used to get count of all the elements.

Python3

# Python3 code to demonstrate working of
# Extract elements with equal frequency as value
# Using filter() + lambda + count()

# initializing list
test_list = [4, 3, 2, 2, 3, 4, 1, 3, 2, 4, 4]

# printing original list
print("The original list is : " + str(test_list))

# removing duplicates using set()
# count() for computing frequency
# filter used to perform filtering
res = list(set(list(filter(lambda ele : test_list.count(ele) == ele, test_list))))

# printing result
print("Filtered elements : " + str(res))

Output

The original list is : [4, 3, 2, 2, 3, 4, 1, 3, 2, 4, 4]
Filtered elements : [1, 3, 4]