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Counting number of unique values in a Python list

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  • Last Updated : 29 Aug, 2020
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Let us see how to count the number of unique values in a Python list.
Examples :

Input : 10 20 10 30 40 40
Output : 2
Explanation : Only 2 elements, 20 and 30 are unique in the list.

Input : 'geeks' 'for' 'geeks'
Output : 1

Approach 1 : Traversing the list and counting the frequrency of each element using dictionary, finally counting the elements for which the frequency is 1.




# function to count the unique elements
def count_unique(my_list):
  
    # variable to store the unique count
    count = 0
  
    # creating dictionary to count frequency
    freq = {}
  
    # traversing the list
    for x in my_list:
        if (x in freq):
            freq[x] += 1
        else:
            freq[x] = 1
  
    # traversing the dictionary
    for key, value in freq.items():
        if value == 1:
            count += 1
  
    # displaying the count of unique elements
    print(count)
  
# driver function
if __name__ == "__main__":
  
    my_list = [10, 20, 10, 30, 40, 40]
    count_unique(my_list)
  
    my_list = ['geeks', 'for', 'geeks']
    count_unique(my_list)

Output :

2
1

Time complexity : O(N)
Space complexity : O(N)

Approach 2 : Here we will be using the get() method of the dictionary class to count the frequency. This makes the program shorter and demonstrates how get() method is useful instead of if…else.




# function to count the unique elements
def count_unique(my_list):
  
    # variable to store the unique count
    count = 0
  
    # creating dictionary to count frequency
    freq = {}
  
    # traversing the list
    for x in my_list:
        freq[x] = freq.get(x, 0) + 1
  
    # traversing the dictionary
    for key, value in freq.items():
        if value == 1:
            count += 1
  
    # displaying the count of unique elements
    print(count)
  
# driver function
if __name__ == "__main__":
  
    my_list = [10, 20, 10, 30, 40, 40]
    count_unique(my_list)
  
    my_list = ['geeks', 'for', 'geeks']
    count_unique(my_list)

Output :

2
1

Time complexity : O(N)
Space complexity : O(N)

Approach 3 : Here we will be using the count() method of the list class to count the frequency.




# function to count the unique elements
def count_unique(my_list):
  
    # variable to store the unique count
    count = 0
  
    # creating dictionary to count frequency
    freq = {}
  
    # traversing the list
    for x in my_list:
        freq[x] = my_list.count(x)
  
    # traversing the dictionary
    for key, value in freq.items():
        if value == 1:
            count += 1
  
    # displaying the count of unique elements
    print(count)
  
# driver function
if __name__ == "__main__":
  
    my_list = [10, 20, 10, 30, 40, 40]
    count_unique(my_list)
  
    my_list = ['geeks', 'for', 'geeks']
    count_unique(my_list)

Output :

2
1

Time complexity : O(N)
Space complexity : O(N)

Approach 4 : Here we will be using the Counter() method of the collections module to count the frequency.




# importing the module
import collections
  
# function to count the unique elements
def count_unique(my_list):
  
    # variable to store the unique count
    count = 0
  
    # creating dictionary to count frequency
    freq = collections.Counter(my_list)
  
    # traversing the dictionary
    for key, value in freq.items():
        if value == 1:
            count += 1
  
    # displaying the count of unique elements
    print(count)
  
# driver function
if __name__ == "__main__":
  
    my_list = [10, 20, 10, 30, 40, 40]
    count_unique(my_list)
  
    my_list = ['geeks', 'for', 'geeks']
    count_unique(my_list)

Output :

2
1

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