Python – Restrict Elements Frequency in List

When it is required to restrict elements frequency in a list, we can use a simple iteration along with the append method and defaultdict to track element counts.

Example

Below is a demonstration of the same ?

from collections import defaultdict

my_list = [11, 14, 15, 14, 11, 14, 14, 15, 15, 16]

print("The list is :")
print(my_list)

my_dict = {14: 3, 11: 1, 16: 1, 15: 2}

print("The restrict dictionary is :")
print(my_dict)

my_result = []
my_def_dict = defaultdict(int)
for element in my_list:
    my_def_dict[element] += 1
    
    if my_def_dict[element] > my_dict[element]:
        continue
    else:
        my_result.append(element)

print("The result is :")
print(my_result)

Output

The list is :
[11, 14, 15, 14, 11, 14, 14, 15, 15, 16]
The restrict dictionary is :
{14: 3, 11: 1, 16: 1, 15: 2}
The result is :
[11, 14, 15, 14, 14, 15, 16]

How It Works

The algorithm works by tracking the frequency of each element as we iterate through the list:

• A defaultdict keeps count of how many times each element has been seen

• For each element, we increment its count in the defaultdict

• If the current count exceeds the allowed frequency from our restriction dictionary, we skip the element using continue

• Otherwise, we add the element to our result list using append

Alternative Approach Using Counter

Here's another way to achieve the same result using Counter ?

from collections import Counter

my_list = [11, 14, 15, 14, 11, 14, 14, 15, 15, 16]
restrictions = {14: 3, 11: 1, 16: 1, 15: 2}

result = []
element_count = Counter()

for element in my_list:
    element_count[element] += 1
    if element_count[element] <= restrictions[element]:
        result.append(element)

print("The result is :")
print(result)

The result is :
[11, 14, 15, 14, 14, 15, 16]

Conclusion

Use defaultdict or Counter to track element frequencies while iterating through a list. This approach allows you to restrict the number of times each element appears in the resulting list based on predefined limits.