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Скачать или смотреть Does an Empty List Have a Length? Understanding List Operations in Python

  • vlogize
  • 2025-05-27
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Does an Empty List Have a Length? Understanding List Operations in Python
Does empty list have a length?pythonpython 3.x
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Описание к видео Does an Empty List Have a Length? Understanding List Operations in Python

Discover how Python handles empty lists, their lengths, and understand list assignment with practical examples in dot product functions.
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This video is based on the question https://stackoverflow.com/q/68267174/ asked by the user 'MR Li' ( https://stackoverflow.com/u/15682132/ ) and on the answer https://stackoverflow.com/a/68267248/ provided by the user 'Pedro Rodrigues' ( https://stackoverflow.com/u/3343753/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: Does empty list have a length?

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The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
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Does an Empty List Have a Length? Understanding List Operations in Python

When working with Python, one common question that arises is: Does an empty list have a length? At first glance, this may seem straightforward, but it can lead to confusion when you're manipulating lists, especially when writing functions that perform operations like calculating dot products of two arrays.

In this post, we’ll explore the nuances of list lengths in Python, particularly regarding empty lists, and clarify how to work with them effectively while avoiding common errors.

The Problem: Understanding Empty Lists and Lengths

Let’s start with the basic premise: an empty list does, indeed, have a length, which is always 0. You can verify this using the built-in len() function in Python, as shown below:

[[See Video to Reveal this Text or Code Snippet]]

However, the confusion often arises when you try to perform list operations, particularly when initializing lists intended to hold values after a specific calculation or assignment. This can lead to an error: IndexError: list assignment index out of range while attempting to assign values to the list without sufficient size.

Example Scenario: Dot Product Function

Imagine you want to create a function that takes two arrays a and b of length n, and returns an array c where each element is the product of corresponding elements in a and b. Initially, you might write something like this:

[[See Video to Reveal this Text or Code Snippet]]

When given the arrays a = [1, 2, 3] and b = [2, 4, 5], the output would correctly give:

[[See Video to Reveal this Text or Code Snippet]]

The Issue: Assignment Error with Empty Lists

However, if you change the initialization of c to c = [], you will encounter an IndexError at runtime when trying to assign a value at an index that does not exist:

[[See Video to Reveal this Text or Code Snippet]]

Why Does This Error Occur?

The key point here is that Python does not automatically resize lists for indexed assignments. When you create an empty list with c = [], it has no indexed positions to assign values to, hence the index you'll be attempting to write to is out of range.

The Solution: How to Properly Populate Lists

There are two straightforward alternatives to avoid this error and successfully populate your list for calculating the dot product:

Option 1: Use append() Method

You can initialize an empty list and use the append() method to populate it dynamically:

[[See Video to Reveal this Text or Code Snippet]]

Option 2: List Comprehension

Another elegant and concise method is to use list comprehension to simultaneously create and populate your list:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion: Mastering List Operations

Understanding how list lengths and assignments work in Python is crucial, especially when you’re creating dynamic functions. Remember that an empty list is valid but must be handled correctly to avoid common pitfalls. With a good grasp of list operations, you can effectively write functions that perform various computations without running into errors.

By using methods like append() or list comprehensions, you can easily and cleanly populate lists without having to worry about pre-allocating sizes or running into index errors.

Happy coding!

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