Common Mistakes in Bubble Sort and How to Avoid Them

Bubble Sort is a simple and intuitive sorting algorithm, often one of the first ones you’ll encounter when learning about sorting. Because it’s easy to understand, it’s a great starting point. However, even with its simplicity, it’s easy to make mistakes when you’re writing the code for it. Whether you’re a beginner or just need a quick refresher, understanding these common pitfalls can save you a lot of time and frustration. In this article, we’ll explore the most frequent mistakes made while implementing Bubble Sort and show you how to avoid them.

Forgetting the Core Idea

One of the biggest mistakes is losing sight of Bubble Sort’s fundamental concept: repeatedly comparing adjacent elements and swapping them if they are in the wrong order.

Mistake:

Let’s say you have an array [5, 1, 4, 2, 8] and you try to sort it, but you only compare elements once without looping through the array multiple times.

def try_bubble_sort(arr):
  n = len(arr)
  for j in range(0, n-1):
    if arr[j] > arr[j+1]:
      arr[j], arr[j+1] = arr[j+1], arr[j]

Why It’s Wrong:

If you only loop once, the largest element will “bubble” to the end, but the other elements won’t be in the correct order.

Fix:

Remember that Bubble Sort needs to make multiple passes through the array. You’ll need an outer loop to ensure that each element is compared with every other element until no more swaps are needed.

def bubble_sort(arr):
  n = len(arr)
  for i in range(n): # Outer loop for multiple passes
    for j in range(0, n-i-1):
      if arr[j] > arr[j+1]:
        arr[j], arr[j+1] = arr[j+1], arr[j]

Incorrect Loop Boundaries

Another common mistake is getting the loop boundaries wrong, which can lead to incomplete sorting or even runtime errors. The code below might look okay at first, but it’s not fully secure.

Mistake:

def bubble_sort_incorrect(arr):
  n = len(arr)
  for i in range(n):
    for j in range(0, n): # Incorrect inner loop range
      if arr[j] > arr[j+1]: # potential index error
        arr[j], arr[j+1] = arr[j+1], arr[j]

Why It’s Wrong:

The inner loop iterates one element too far, which causes an “IndexError: list index out of range” because it tries to access arr[n] when the valid indices are only up to arr[n-1].

Fix:

The inner loop should stop one element short of the end of the array, and also account for the elements that are already in their sorted position:

def bubble_sort_correct(arr):
  n = len(arr)
  for i in range(n):
    for j in range(0, n-i-1): # Correct inner loop range
      if arr[j] > arr[j+1]:
        arr[j], arr[j+1] = arr[j+1], arr[j]

In this corrected code, the inner loop condition j < n - i - 1 prevents accessing elements beyond the array bounds and accounts for the sorted elements at the end of the array.

Inefficient Implementation

Bubble sort can be made more efficient by stopping early if no swaps occur during a pass. Forgetting this optimization leads to unnecessary iterations, especially if the array is already sorted or nearly sorted.

Mistake:

def bubble_sort_inefficient(arr):
  n = len(arr)
  for i in range(n):
    for j in range(0, n-i-1):
      if arr[j] > arr[j+1]:
        arr[j], arr[j+1] = arr[j+1], arr[j]
  # No check for early termination

Why It’s Wrong:

Without checking for swaps, the algorithm always performs all n passes, even if the array becomes sorted before the last pass.

Fix:

Add a flag to check if any swaps occurred during a pass and break the loop if no swaps were made:

def bubble_sort_optimized(arr):
  n = len(arr)
  for i in range(n):
    swapped = False # Flag to optimize
    for j in range(0, n-i-1):
      if arr[j] > arr[j+1]:
        arr[j], arr[j+1] = arr[j+1], arr[j]
        swapped = True
    if swapped == False:
      break # No swaps, so the array is sorted

Incorrect Comparison

Another common mistake is using the wrong comparison operator, which results in the array being sorted in the wrong order (e.g., descending instead of ascending).

Mistake:

def bubble_sort_descending(arr):
  n = len(arr)
  for i in range(n):
    for j in range(0, n-i-1):
      if arr[j] < arr[j+1]: # Sorts in descending order (incorrect)
        arr[j], arr[j+1] = arr[j+1], arr[j]

Why It’s Wrong:

The < operator sorts the array in descending order, which is not what’s intended if you want ascending order.

Fix:

Use the > operator to sort in ascending order:

def bubble_sort_ascending(arr):
  n = len(arr)
  for i in range(n):
    for j in range(0, n-i-1):
      if arr[j] > arr[j+1]: # Corrected comparison
        arr[j], arr[j+1] = arr[j+1], arr[j]

Key Takeaways for Error-Free Bubble Sort

  1. Multiple Passes: Ensure the algorithm iterates through the array multiple times to compare each element with every other element.
  2. Correct Loop Boundaries: Pay close attention to the loop ranges to avoid IndexError and ensure all elements are compared.
  3. Check Comparison Operators: Verify that you are using the correct comparison operator (> or <) to sort in the desired order.
  4. Early Termination: Implement the optimization to stop early if no swaps occur during a pass to improve efficiency.

By avoiding these common mistakes, you can implement Bubble Sort correctly and efficiently.

What’s Next?

Now that you understand common mistakes in Bubble Sort implementation, you might want to explore: