Python | Extract least frequency element

Last Updated : 13 Apr, 2023

Sometimes, while working with data, we can have a problem in which we need to extract element which is occurring least number of times in the list. Let's discuss certain ways in which this problem can be solved.

Method #1: Using defaultdict() + loop The combination of above functions can be used to perform this task. In this, we extract each element's frequency using defaultdict() and extract the minimum frequency element after traversing the defaultdict.

Python3

# Python3 code to demonstrate working of
# Extract least frequency element
# using defaultdict() + loop
from collections import defaultdict

# initialize list
test_list = [1, 3, 4, 5, 1, 3, 5]

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

# Extract least frequency element
# using defaultdict() + loop
res = defaultdict(int)
for ele in test_list:
  res[ele] += 1
  min_occ = 9999
for ele in res:
    if min_occ > res[ele]:
        min_occ = res[ele]
        tar_ele = ele

# printing result
print("The minimum occurring element is : " + str(tar_ele))

Output

The original list : [1, 3, 4, 5, 1, 3, 5]
The minimum occurring element is : 4

Time Complexity: O(n*n), where n is the length of the input list. This is because we’re using defaultdict() + loop which has a time complexity of O(n*n) in the worst case.
Auxiliary Space: O(1), as we’re using constant additional space

Method #2: Using Counter() This method is similar to above, the difference is that the frequency is stored using Counter() and then we extract element with least frequency.

Python3

# Python3 code to demonstrate working of
# Extract least frequency element
# using Counter()
from collections import Counter

# initialize list 
test_list = [1, 3, 4, 5, 1, 3, 5]

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

# Extract least frequency element
# using Counter
res = Counter(test_list)
tar_ele = res.most_common()[-1][0]

# printing result
print("The minimum occurring element is : " + str(tar_ele))

Output

The original list : [1, 3, 4, 5, 1, 3, 5]
The minimum occurring element is : 4

Time complexity: O(n), where n is the length of the numbers list. The counter() function has a time complexity of O(n)

Auxiliary Space: O(n),where n is the length of the numbers list. This is because the counter() function creates an intermediate result for each element of the numbers list, which requires additional memory.

Method #3 : Using set() and list comprehension
In this method, we can use set() to get the unique elements from the given list and then use list comprehension to find the least occurring element.

Python3

# Python3 code to demonstrate working of
# Extract least frequency element
# using set() and list comprehension

# initialize list 
test_list = [1, 3, 4, 5, 1, 3, 5]

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

# Extract least frequency element
# using set() and list comprehension
unique_elements = set(test_list)
min_occ = min([test_list.count(x) for x in unique_elements])
tar_ele = [x for x in unique_elements if test_list.count(x) == min_occ]

# printing result
print("The minimum occurring element is : " + str(tar_ele))
#this code is contributed by edula vinay kumar reddy

Output

The original list : [1, 3, 4, 5, 1, 3, 5]
The minimum occurring element is : [4]

Time Complexity: O(n) where n is the length of the list.
Auxiliary Space: O(n) where n is the length of the list.

Method #4: Using a dictionary to store the frequency of each element:

Python3

test_list = [1, 3, 4, 5, 1, 3, 5]
freq_dict = {}
for i in test_list:
    if i not in freq_dict:
        freq_dict[i] = 1
    else:
        freq_dict[i] += 1
min_occ = min(freq_dict.values())
tar_ele = [k for k, v in freq_dict.items() if v == min_occ]
# printing original list
print("The original list : " + str(test_list))
 
print("The minimum occurring element is:", tar_ele)
#This code is contributed by Jyothi pinjala.

Output

The original list : [1, 3, 4, 5, 1, 3, 5]
The minimum occurring element is: [4]

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

Method #5: Using heapq library

Use the heapq library to extract the least frequency element. The idea is to iterate over each element in the list and store its frequency in a dictionary. Then, use the heapq library to find the smallest value in the dictionary, which will be the least frequency element.

Python3

import heapq

# initialize list
test_list = [1, 3, 4, 5, 1, 3, 5]

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

# create a dictionary to store the frequency of each element
freq_dict = {}
for elem in test_list:
    if elem in freq_dict:
        freq_dict[elem] += 1
    else:
        freq_dict[elem] = 1

# use heapq library to find the smallest value in the dictionary
min_occ = heapq.nsmallest(1, freq_dict.values())[0]

# find all elements that occur with the minimum frequency
tar_ele = [k for k, v in freq_dict.items() if v == min_occ]

# printing result
print("The minimum occurring element is : " + str(tar_ele))

Output

The original list : [1, 3, 4, 5, 1, 3, 5]
The minimum occurring element is : [4]

Time complexity: O(nlogk), where n is the length of the list and k is the number of distinct elements in the list.
Auxiliary space: O(k), since we need to store the frequency of each element in the dictionary.