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# Counting number of unique values in a Python list

• Last Updated : 29 Aug, 2020

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 ifâ€¦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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