Python | Remove elements of list that are repeated less than k times

Last Updated : 15 Apr, 2019

Given a list of integers (elements may be repeated), write a Python program to remove the elements that are repeated less than k times.

Examples:

Input : lst = ['a', 'a', 'a', 'b', 'b', 'c'], k = 2
Output : ['a', 'a', 'a', 'b', 'b']

Input : lst = [1, 1, 1, 1, 2, 2, 3, 3, 3, 4, 4], k = 3
Output : [1, 1, 1, 1, 3, 3, 3]

Approach #1 : Pythonic naive

Counter() from collections module construct a dictionary mapping values to counts and save them in ‘counted’. Then we make use of ‘temp_lst’ to store the elements that need to be removed. Finally, we traverse through the given list and append all elements that are not in ‘temp_lst’ to ‘res_lst’ containing the required output.

# Python3 program to Remove elements of  
# list that repeated less than k times 
from collections import Counter 
  
def removeElements(lst, k): 
    counted = Counter(lst) 
      
    temp_lst = [] 
    for el in counted: 
        if counted[el] < k: 
            temp_lst.append(el) 
              
    res_lst = [] 
    for el in lst: 
        if el not in temp_lst: 
            res_lst.append(el) 
              
    return(res_lst) 
      
# Driver code 
lst = ['a', 'a', 'a', 'b', 'b', 'c'] 
k = 2
print(removeElements(lst, k)) 

Output:

['a', 'a', 'a', 'b', 'b']

Approach #2 : Efficient Approach

The efficient approach to use Counter method is to construct a dictionary mapping value to counts and then use a list comprehension to filter for counts larger than a specified value. This approach is both time and space efficient.

# Python3 program to Remove elements of  
# list that repeated less than k times 
from collections import Counter 
  
def removeElements(lst, k): 
    counted = Counter(lst) 
    return [el for el in lst if counted[el] >= k] 
      
# Driver code 
lst = ['a', 'a', 'a', 'b', 'b', 'c'] 
k = 2
print(removeElements(lst, k)) 

Output:

['a', 'a', 'a', 'b', 'b']

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