How to count the occurrences of an item in a list using Python

Using count()

Lists in python have a built-in function called count() which accepts an element as a parameter, searches the list for the occurrences of this item and returns back the result as an integer. For example,

['a', 'b', 'c', 'a', 'a', 'c'].count('c')
# 2

The count() function runs in a O(n) time. Therefore, if you want to find the occurrences of just one element then this function is suitable and is the most idiomatic in my opinion.

Using the collections.Counter class

From the python documentation, a Counter is a dict subclass for counting hashable objects. The result is a dictionary of the list elements as keys and their counts and values. For example,

from collections import Counter

my_list = ['a', 'b', 'c', 'a', 'a', 'c']

Counter(my_list)
Counter({'a': 3, 'b': 1, 'c': 2})

The results of the Counter class are stable meaning that the order of the list will be preserved.  The Counter class has O(n) complexity. Therefore, it's better to be used if you have to count the occurrences of more than one elements in a list.

Using a for loop

If you just want to practise python and you want to do this on your own you can do something similar to the code below:

my_list = ['a', 'b', 'c', 'a', 'a', 'c']
count = 0
search_element = 'c'

for element in my_list:
    if element == search_element:
        count += 1


count
# 2

In this example, we iterate through the list and every time we find the element we are searching for we increase the count variable. The result will be in the count variable in the end. I would recommend to avoid writing your own solution since built-in solutions exist. This way you avoid making trivial bugs that otherwise you would't make. Always rely on built-in functionality if it's available.

Other ways

There are other ways to do this as well using different libraries such as pandas and numpy. However, I will not be showing these examples here since it's outside of the scope of this tutorial.

Stack overflow answers

You can check the stack overflow answers here as well.

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