Counting number of unique values in a Python list

Counting the number of unique values in a Python list involves determining how many distinct elements are present disregarding duplicates.

Using a Set

Using a set to count the number of unique values in a Python list leverages the property of sets where each element is stored only once.

li = [1, 2, 2, 3, 4, 4, 5]
cnt = len(set(li))
print(cnt)

5

Explanation:

• list li is converted to a set, which removes duplicate values, leaving only unique elements.
• len() function is used to count the number of unique elements in the set, resulting in the total count of distinct values in the list

Using collections.Counter

Using collections.Counter allows us to easily count the frequency of each element in a list or string. It creates a dictionary-like object where the keys are the elements and the values are their respective counts.

from collections import Counter

li = [1, 2, 2, 3, 4, 4, 5]
c = Counter(li)
cnt = len(c)
print(cnt)

5

Explanation:

• Counter from the collections module is used to count the occurrences of each element in the list li, resulting in a dictionary-like object c with elements as keys and their frequencies as values.
• len(c) function counts the number of unique keys in the Counter object, which corresponds to the number of distinct elements in the list li.

Using List Comprehension and a Temporary List

Using list comprehension and a temporary list allows for efficiently filtering or modifying elements in a list based on specific conditions. First, a temporary list stores the filtered elements and then list comprehension processes or creates a new list based on that temporary data.

a = [1, 2, 2, 3, 4, 4, 5]
b = []
[b.append(x) for x in l if x not in a]
cnt = len(b)
print(cnt)

5

Explanation:

• List comprehension iterates over each element in l and appends it to the unique_lst if it is not already present, ensuring only unique elements are added.
• len(unique_lst) calculates the number of distinct elements in the unique_lst, representing the count of unique values in the original list l.