Shrink List for Repeating Elements - Python

We are given a list of elements that may contain repeated values. Our task is to shrink the list by ensuring that each repeated element appears only once while maintaining the order of their first occurrence. For example, if the input list is [2, 3, 2, 5, 3, 5], the output should be [2, 3, 5].

Using set

This method uses a set to track elements that have already been added to the output. It is efficient because sets provide fast membership checks.

a = [2, 3, 2, 5, 3, 5]
s = set()
res = []
for x in a:
    if x not in s:
        s.add(x)
        res.append(x)
print(res)

[2, 3, 5]

Explanation:

• A set is used to keep track of elements that have already been added to the result list.
• For each element in the input list, we check if it is in the set. If not, we add it to both the set and the result list.
• The order of elements is preserved as we only append elements to the result list when encountering them for the first time.

Let's explore some more methods and see how we can shrink given list for repeating elements.

Using dictionary from collections

This method uses the OrderedDict class from the collections module to maintain the order of unique elements.

from collections import OrderedDict
a = [2, 3, 2, 5, 3, 5]
res = list(OrderedDict.fromkeys(a))
print(res)

[2, 3, 5]

Explanation:

• The OrderedDict.fromkeys function is used to create an ordered dictionary where each key is a unique element from the input list.
• The keys of the ordered dictionary are then converted back to a list, preserving the order.
• This method is concise and avoids the need for a loop and an explicit set.

Using list comprehension with set

This method combines list comprehension and a set to achieve the same result but in a single line of code.

a = [2, 3, 2, 5, 3, 5]
s = set()
res = [x for x in a if x not in s and not s.add(x)]
print(res)

[2, 3, 5]

Explanation:

• A set is used to track seen elements during list comprehension.
• The not s.add(element) ensures that each element is added to the set only once while constructing the result list.

Using for loop and in operator

This method checks for the presence of each element in the result list using the in operator and for loop before adding it.

a = [2, 3, 2, 5, 3, 5]
res = []
for x in a:
    if x not in res:
        res.append(x)
print(res)

[2, 3, 5]

Explanation:

• 'in' operator checks if an element is already in the result list.
• If not, the element is appended to the result list.
• This method is less efficient compared to the others due to the repeated membership checks on the list, which are slower than set operations.