Flatten a Nested List in Python Using Nested for Loops

Flattening a nested list is often used in Python when working with complex data structures. It involves converting a nested list, which contains sublists within it, into a single, flat list. This process simplifies the structure and makes it easier to manipulate and analyze the data.

In this tutorial, we will explore how to flatten a nested list in Python using nested for loops. We will explain the logic and step-by-step operation of the code, which takes a nested list as input and produces a flat list as output. Understanding this process will enable you to effectively handle nested lists and extract the individual elements for further processing or analysis.

Whether you are working on data manipulation, algorithmic problem-solving, or any other Python project involving nested lists, mastering the technique of flattening them will greatly enhance your programming skills. So let’s dive in and compile a Python program to flatten a nested list using nested for loops.

Python Program to Flatten a Nested List Using Nested for Loops

#Flatten a Nested List in python Using Nested for Loops

my_list = [[1], [2, 3], [4, 5, 6, 7], [8,9,10,11,12,13]]

# Create an empty list
flat_list = []

# Access each element of the sublist using a nested loop 
for sublist in my_list:
    for num in sublist:
        # append that element to flat_list

# Output

Code Logic Explanation

  • Initialising the nested list

We start by defining a nested list called my_list that contains sublists with different numbers of elements.

  • Creating an empty list

Next, we create an empty list called flat_list that will store the flattened elements of the nested list.

  • Accessing elements of the sublist using nested loops

To flatten the nested list, we use nested for loops. The outer loop iterates over each sublist in my_list, while the inner loop iterates over each element in the current sublist.

  • Appending each element to flat_list

Inside the inner loop, we append each element (num) to the flat_list using the append() method. This adds the element to the end of the list.

  • Printing the flattened list

After the nested loops complete, we have a flattened list containing all the elements from the nested list. We print the flat_list to display the flattened result.


The output of the code is the flattened list. In this case, the output is:

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]

The alternative approaches to flattening a list are:

  • Using List Comprehension: It allows us to iterate over the sublists and their elements in a nested manner and generate a flattened list in a single line of code. It offers efficient and faster execution compared to nested loops.
  • Using the itertools module: It provides a powerful set of tools for iterating and manipulating iterables. The itertools.chain() function can be used to chain the sublists together and create a flattened list. 

While the alternatives mentioned above provide more concise and often faster solutions, there are cases where using nested for loops may be preferable:

  • Readability and Simplicity: This approach is straightforward and easy to understand, especially for beginners or individuals who may not be familiar with list comprehensions or the itertools module. 
  • Control and Customization: With nested for loops, you have more control over the iteration process and can easily add additional conditions or logic within the loops. This flexibility allows for customization based on specific requirements or modifications needed during the flattening process.

Flattening a nested list is a common task in Python programming. By using nested for loops, we can iterate over the sublists and access each element, appending them to a new list to create the flattened structure. This approach allows us to transform complex nested lists into a single, flat list, making it easier to work with the data.

By understanding the step-by-step logic and operation of the provided code, you can now confidently flatten nested lists in Python using nested for loops.

FAQs – Flatten a Nested List in Python Using Nested for Loops

Can the code handle arbitrarily nested lists?

Yes, the code can handle nested lists of any depth. The nested for loops will traverse through each level of nesting, ensuring that all elements from the sublists are appended to the final flat list.

Is the order of elements preserved when flattening a nested list?

Yes, the order of elements is preserved when flattening a nested list using the nested for loop approach. The elements are added to the flat list in the order they appear in the original nested list, maintaining their relative positions.

What happens if the nested list contains non-numeric or non-iterable elements?

The code can handle any type of element in the nested list, including non-numeric or non-iterable elements such as strings or objects. It will append each individual element to the flat list regardless of its type.

Can I modify the code to remove duplicates from the flat list?

Yes, if you want to remove duplicates from the flat list, you can use the set() function or convert the flat list to a set and then back to a list to eliminate duplicates. However, this will change the order of elements.

How can I adapt the code to handle irregularly shaped nested lists?

If the nested list is irregularly shaped, meaning the sublists have varying lengths, the code using nested for loops will still work correctly. It will flatten the nested list, regardless of the varying lengths of the sublists.

Are there any performance considerations when flattening large or deeply nested lists?

The nested for loop approach has a time complexity of O(n), where n is the total number of elements in the nested list. However, for extremely large or deeply nested lists, other methods like recursion or iterative algorithms might be more efficient. It’s important to consider the size and complexity of your data to choose the most suitable approach.

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