Python - Maximum N repeated Elements

Given a List of elements, we need to remove an element if it's occurrence in the list increases more than N. For example,

Input : a = [6, 4, 6, 3, 6], N = 1
Output : [6, 4, 3]
Explanation : The occurrence 2nd onwards of 6 are removed.

Input : a = [6, 4, 6, 3, 6], N = 2
Output : [6, 4, 6, 3]
Explanation : The occurrence 3rd onwards of 6 are removed.

Let's discuss some of the methods to achieve this.

Using loop + count()

The combination of the above functionalities can be used to solve this problem. In this, we perform an iteration of elements and check if the count is more than N of that element using count(), if yes, then we remove that element.

a = [5, 7, 7, 2, 5, 5, 7, 2, 2]

n = 2

# Using loop + count()
res = []
for ele in a:

    # checking of elements occurrence is not greater than N
    if res.count(ele) < n:
        res.append(ele)

print("Extracted elements : " + str(res))

Extracted elements : [5, 7, 7, 2, 5, 2]

Explanation: res.count(ele) checks an element's frequency in res and appends it if its count is less than 'N'.

Using defaultdict and loop

We can use defaultdict to count element frequencies in a single pass. Then, iterate through the dictionary and append each element to the result list up to N times if its count is at least N.

from collections import defaultdict

a = [5, 7, 7, 2, 5, 5, 7, 2, 2]

n = 2

# Using defaultdict and loop
freq_dict = defaultdict(int)

for elem in a:
    freq_dict[elem] += 1
    
res = []

for key, value in freq_dict.items():
    if value >= n:
        res += [key] * n

print("Extracted elements : " + str(res))

Extracted elements : [5, 5, 7, 7, 2, 2]

Explanation: defaultdict(int) is used to count element frequencies and appends each element to the result list up to N times if its count is at least N.

Using Counter() + loop

We can use Counter() to count elements, and then append its size elements if less than N, if greater, list comprehension is used to extend elements till N. Doesn't preserve order.

from collections import Counter

a = [5, 7, 7, 2, 5, 5, 7, 2, 2]

n = 2

# Using Counter() + loop
res = []
temp = Counter(a)
for key, ele in temp.items():

    # Conditional check for size decision during append
    if ele <= n:
        res.extend([key for i in range(ele)])
    else:
        res.extend([key for i in range(n)])

print("Extracted elements : " + str(res))

Extracted elements : [5, 5, 7, 7, 2, 2]

Explanation: Counter() is used to count element frequencies and .items() to iterate through them. It then uses extend() to add each element to res up to N times, ensuring efficient list construction.

Using set() and list comprehension

We can also solve this problem by first finding the set of elements that occur at least N times in the list, and then using a list comprehension to extract all occurrences of those elements.

a = [5, 7, 7, 2, 5, 5, 7, 2, 2]

n = 2

# Using set() and list comprehension
res = [elem for elem in set(a) if a.count(elem) >= n for i in range(n)]

print("Extracted elements : " + str(res))

The original list : [5, 7, 7, 2, 5, 5, 7, 2, 2]
Extracted elements : [2, 2, 5, 5, 7, 7]

Explanation: This code uses set() to get unique elements and list comprehension to include each element up to N times if its frequency (checked using count()) is at least N.