Python Program to Split a List into Two Halves

Last Updated : 19 Dec, 2024

Splitting a list into two halves is a common operation that can be useful in many cases, such as when dealing with large datasets or when performing specific algorithms (e.g., merge sort). In this article, we will discuss some of the most common methods for splitting a list into two halves.

Using List Slicing

List slicing is a highly efficient way to split a list into two halves. By using the slicing technique, we can directly access the first and second halves of the list. This method does not modify the original list, and it preserves the order of elements.

Python

a = [1, 2, 3, 4, 5, 6]  # Integer division to find the middle index mid = len(a) // 2  # Elements from the start to the middle x = a[:mid]  # Elements from the middle to the end y= a[mid:]  print(x) print(y)

Output

[1, 2, 3] [4, 5, 6]

Explanation:

a[:mid] extracts the first half of the list, from index 0 to mid-1.
a[mid:] extracts the second half of the list, from index mid to the end of the list.
The list is split efficiently in a single step.

Let's see some more methods and see how we can split a list into two halves.

Table of Content

Using List Comprehension

List comprehension can also be used to extract the first half and second half based on index ranges, especially if we need to apply some conditions or transformations while splitting.

Python

a = [1, 2, 3, 4, 5, 6]  mid = len(a) // 2  # Create the first half using a list comprehension x = [a[i] for i in range(mid)]  # Create the second half using a list comprehension y = [a[i] for i in range(mid, len(a))]  print(x) print(y)

Output

First Half: [1, 2, 3] Second Half: [4, 5, 6]

Explanation:

[a[i] for i in range(mid)] is used to extract the first half of the list. It loops through the list from index 0 to mid - 1 and appends those elements to the new list first_half.
Similarly, [a[i] for i in range(mid, len(a))] extracts the second half of the list by iterating from index mid to the end of the list and appending those elements to second_half.

Using a Loop

This method involves using a for loop to manually split the list into two parts. It can be useful if we want more control over how the list is split.

Python

a = [1, 2, 3, 4, 5, 6] mid = len(a) // 2  # Initialize empty lists to hold the two halves x = [] y = []  # Populate the first half using a loop for i in range(mid):     x.append(a[i])  # Populate the second half using a loop for i in range(mid, len(a)):     y.append(a[i])  print(x) print(y)

Output

[1, 2, 3] [4, 5, 6]

Explanation:

The loop iterates over the first half of the list and adds each element to first_half.
A second loop iterates over the second part of the list and adds each element to second_half.

Using numpy.array_split()

For working with numeric data or using the numpy library, numpy.array_split() provides a clean way to split lists (or arrays) into two or more parts. This function automatically handles splitting the list and is efficient for large datasets.

Python

import numpy as np  a = [1, 2, 3, 4, 5, 6]  # Creates two sub-arrays, dividing the list evenly split_lst = np.array_split(a, 2)  # Assign the first half to x x = split_lst[0]  # Assign the second half to y y = split_lst[1]  print(x) print(y)

Output

[1 2 3] [4 5 6]

Explanation:

np.array_split(a, 2) splits the list 'a' into two parts and returns them as a list of numpy arrays.
It also works with lists that cannot be evenly divided, automatically adjusting the size of the parts