Count occurrences of an item in a List in Python

In this post, we will explore different ways to count the occurrences of an item in a Python list.

1. Using `list.count()` function

One of the simplest method to get the occurrences of an item in a list is to use the list.count() function. This function is a built-in function of a list object that returns the number of times an item appears in a list.

ints = [1, 2, 3, 4, 2, 1]

item = 2

freq = ints.count(item)

print(freq) # 2

Download Run Code

2. Using `operator.countOf()` function

Another way to count the occurrences of an item in a Python list is to use the operator.countOf() function from the operator module. This function returns the number of occurrences of a value in an iterable. It is similar to the list.count() function but it can be used with other types of iterables as well.

import operator

ints = [1, 2, 3, 4, 2, 1]

item = 2

freq = operator.countOf(ints, item)

print(freq) # 2

Download Run Code

3. Using `collections.Counter()` function

If we need the count of multiple items from the list, we can use the collections.Counter() function from the collections module. This function returns a dictionary-like object that contains counts of unique items in an iterable. The keys are the items and the values are their counts. We can then access the count for a specific item by passing the key to the get() function. Here is an example of how to use this function:

from collections import Counter

ints = [1, 2, 3, 4, 5]

item = 3

freq = Counter(ints).get(item)

print(freq) # 1

Download Run Code

4. Using `dictionary`

Alternatively, we can create a frequency map using a dictionary to get the total number of occurrences of each item in a list. The following example demonstrates this by using the set() function to remove the duplicates and then use the count() function to get the frequency of each item:

ints = [1, 2, 3, 4, 5, 3]

item = 3

freq = {i: ints.count(i) for i in set(ints)}

print(freq.get(item)) # 2

Download Run Code

However, this approach is not recommended for large lists. The time complexity of it is quadratic since the count() function is called once for each distinct element in the list. We can improve the performance by creating the frequency map manually, as shown below:

ints = [1, 2, 3, 4, 5, 3]

item = 3

freq = {}

for i in ints:

freq[i] = freq.setdefault(i, 0) + 1

print(freq.get(item)) # 2

Download Run Code

5. Using `pandas.Series.value_counts()` function

We can also use the pandas.Series.value_counts() function to count the occurrences of an item in a list. This function returns a Series object that contains the counts of each unique value in the original list. Here is a simple example of its usage:

import pandas as pd

ints = [1, 2, 3, 4, 2, 1]

item = 2

# convert the list to a pandas Series

series = pd.Series(ints)

# count the occurrences of each value in the Series

print(series.value_counts())

'''

Output:

1 2

2 2

3 1

4 1

Name: count, dtype: int64

'''

Download Code

That’s all about counting occurrences of an item in a list in Python.

Count occurrences of an item in a List in Python

1. Using list.count() function

2. Using operator.countOf() function

3. Using collections.Counter() function

4. Using dictionary

5. Using pandas.Series.value_counts() function

Share this:

1. Using `list.count()` function

2. Using `operator.countOf()` function

3. Using `collections.Counter()` function

4. Using `dictionary`

5. Using `pandas.Series.value_counts()` function