Python: Flatten a given nested list structure

Last update on April 19 2025 13:02:18 (UTC/GMT +8 hours)

Flatten Nested List Structure

Write a Python program to flatten a given nested list structure.

Sample Solution-1:

Python Code:

# Define a function 'flatten_list' that takes a nested list 'n_list' as input
def flatten_list(n_list):
    # Initialize an empty list 'result_list' to store the flattened elements
    result_list = []

    # Check if 'n_list' is empty; if so, return an empty result list
    if not n_list:
        return result_list

    # Create a stack to keep track of nested lists; start with 'n_list' as the initial item in the stack
    stack = [list(n_list)]

    # Iterate while the stack is not empty
    while stack:
        # Pop the current list 'c_num' from the stack
        c_num = stack.pop()

        # Pop the next item from 'c_num'
        next = c_num.pop()

        # If 'c_num' is not empty, push it back onto the stack
        if c_num:
            stack.append(c_num)

        # Check if 'next' is a list
        if isinstance(next, list):
            # If 'next' is a non-empty list, push it onto the stack
            if next:
                stack.append(list(next))
        else:
            # If 'next' is not a list, add it to 'result_list'
            result_list.append(next)

    # Reverse 'result_list' to maintain the original order of elements
    result_list.reverse()

    # Return the flattened list
    return result_list

# Define a nested list 'n_list' with various sublists
n_list = [0, 10, [20, 30], 40, 50, [60, 70, 80], [90, 100, 110, 120]]

# Print a message indicating the purpose of the following output
print("Original list:")

# Print the original nested list 'n_list'
print(n_list)

# Print a message indicating the purpose of the following output
print("\nFlatten list:")

# Call the 'flatten_list' function with 'n_list' as an argument and print the flattened result
print(flatten_list(n_list))

Sample Output:

Original list:
[0, 10, [20, 30], 40, 50, [60, 70, 80], [90, 100, 110, 120]]

Flatten list:
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]

Flowchart:

Sample Solution-2:

Loop over elements, use list.extend() if the element is a list, list.append() otherwise.

Python Code:

# Define a function 'flatten_list' that takes a nested list 'nums' as input
def flatten_list(nums):
    # Initialize an empty list 'result' to store the flattened elements
    result = []

    # Iterate through each element 'i' in the input list 'nums'
    for i in nums:
        # Check if 'i' is a list; if so, extend 'result' with its elements, otherwise, append 'i' to 'result'
        result.extend(i) if isinstance(i, list) else result.append(i)

    # Return the flattened list
    return result

# Define a nested list 'n_list' with various sublists
n_list = [0, 10, [20, 30], 40, 50, [60, 70, 80], [90, 100, 110, 120]]

# Print a message indicating the purpose of the following output
print("Original list:")

# Print the original nested list 'n_list'
print(n_list)

# Print a message indicating the purpose of the following output
print("\nFlatten list:")

# Call the 'flatten_list' function with 'n_list' as an argument and print the flattened result
print(flatten_list(n_list))

Sample Output:

Original list:
[0, 10, [20, 30], 40, 50, [60, 70, 80], [90, 100, 110, 120]]

Flatten list:
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120]

Flowchart:

For more Practice: Solve these Related Problems:

Write a Python program to flatten a list containing nested lists and tuples.
Write a Python program to flatten a nested list up to a specified depth.
Write a Python program to flatten a list while preserving non-list elements as-is.
Write a Python program to convert a nested list into a single-level dictionary.

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