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Find size of a list in Python

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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.


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:

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).


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


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.


# 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



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.


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)


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:


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


The size of the size lst: 3

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

Last Updated : 10 Aug, 2023
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