Find size of a list in Python

A list is a collection data type that is ordered and changeable. A list can have duplicate entries as well. Here, the task is to find the number of entries in a list in Python.

Examples:

Input: a = [1, 2, 3, 1, 2, 3]
Output: 6
Explanation: The output is 6 because the number of entries in the list a is also 6.

Find the Length of a List in Python

Below are the methods that we will cover in this article:

• Using len() method
• Using sum() function
• Using for loop
• Using the length_hint() method

Find the size of the list using the len() method

The len() works in O(1) time as the list is an object and has a member to store its size. Below is a description of len() from Python docs.

Return the length (the number of items) of an object. The argument may be a sequence (such as a string, bytes, tuple, list, or range) or a collection (such as a dictionary, set, or frozen set).

Python3

# Python program to demonstrate working of len()
a = []
a.append("Hello")
a.append("Geeks")
a.append("For")
a.append("Geeks")
print("The length of list is: ", len(a))

Output

The length of list is:  4

Find the length of a list using the sum() function

Another approach is to use the built-in sum() function in combination with a generator expression. This allows you to find the size of a list by summing the number of elements in the list that meet a certain condition.

Python3

# list of numbers
numbers = [1, 2, 3, 1, 2, 3]

# find the size of the list
size = sum(1 for num in numbers)

# print the size of the list
print(size)

Output

6

This will output 6 because the list contains 6 elements.

Time complexity: The time complexity of the approach using the sum() function and a generator expression is O(n), where n is the length of the list.
Space complexity: The auxiliary space complexity of this approach is O(1) because the generator expression only requires a single variable to store the current element being processed.

Find the length of the list using for loop

In this way, we initialize a variable count and then we increment the variable through the loop, and by the end of the loop we get the length of the list in our count variable.

Python3

lst = [1,1,2,5,1,5,2,4,5]
count = 0
for i in lst:
  count += 1
print("The length of the lst is :",count)

Output

The length of the lst is : 9

Time Complexity: O(n)
Space Complexity: O(1)

Find the size of the list using the length_hint() method

The length_hint() function from the operator module to estimate the length of a list. However, please note that this function is not guaranteed to give you the exact size of the list, especially for standard Python lists.

The length_hint() the function provides a hint about the expected length of an iterable, but it might not be accurate for all types of iterables. Here’s your example:

Python3

from operator import length_hint

lst = ['Geeks', 'For', 'Geeks']
size = length_hint(lst)
print('The size of the size lst:',size)

Output

The size of the size lst: 3

Time Complexity: O(n)
Space Complexity: O(1)