Bubble Sort Algorithm in DSA Go

Mental Model

Bubble sort repeatedly swaps adjacent items if they are in the wrong order, pushing the largest unsorted item to the end each pass.

Analogy: Imagine bubbles rising in water: the biggest bubble slowly moves up to the surface by swapping places with smaller bubbles below it.

Index: 0   1   2   3   4
Array: [5]->[3]->[8]->[4]->[2]

Dry Run Walkthrough

Input: array: [5, 3, 8, 4, 2]

Goal: Sort the array in ascending order using bubble sort

Step 1: Compare 5 and 3, swap because 5 > 3

[3] -> [5] -> [8] -> [4] -> [2]

Why: Smaller number should come before larger, so swap to start sorting

Step 2: Compare 5 and 8, no swap because 5 < 8

[3] -> [5] -> [8] -> [4] -> [2]

Why: Order is correct, no need to swap

Step 3: Compare 8 and 4, swap because 8 > 4

[3] -> [5] -> [4] -> [8] -> [2]

Why: 8 is bigger, so it moves towards the end

Step 4: Compare 8 and 2, swap because 8 > 2

[3] -> [5] -> [4] -> [2] -> [8]

Why: 8 bubbles up to the last position

Step 5: Start second pass: Compare 3 and 5, no swap

[3] -> [5] -> [4] -> [2] -> [8]

Why: 3 < 5, order is correct

Step 6: Compare 5 and 4, swap because 5 > 4

[3] -> [4] -> [5] -> [2] -> [8]

Why: 5 moves towards the end

Step 7: Compare 5 and 2, swap because 5 > 2

[3] -> [4] -> [2] -> [5] -> [8]

Why: 5 moves closer to its correct position

Step 8: Start third pass: Compare 3 and 4, no swap

[3] -> [4] -> [2] -> [5] -> [8]

Why: 3 < 4, order is correct

Step 9: Compare 4 and 2, swap because 4 > 2

[3] -> [2] -> [4] -> [5] -> [8]

Why: 4 moves towards the end

Step 10: Start fourth pass: Compare 3 and 2, swap because 3 > 2

[2] -> [3] -> [4] -> [5] -> [8]

Why: 2 moves to the front, array is sorted now

Result:

[2] -> [3] -> [4] -> [5] -> [8]

Annotated Code

DSA Go

package main

import "fmt"

func bubbleSort(arr []int) {
    n := len(arr)
    for i := 0; i < n-1; i++ {
        swapped := false
        for j := 0; j < n-1-i; j++ {
            if arr[j] > arr[j+1] {
                arr[j], arr[j+1] = arr[j+1], arr[j] // swap adjacent elements
                swapped = true
            }
        }
        if !swapped {
            break // stop if no swaps in this pass
        }
    }
}

func main() {
    arr := []int{5, 3, 8, 4, 2}
    bubbleSort(arr)
    for i, v := range arr {
        if i > 0 {
            fmt.Print(" -> ")
        }
        fmt.Print(v)
    }
    fmt.Println()
}

for i := 0; i < n-1; i++ {

outer loop controls passes over the array

swapped := false

track if any swaps happen this pass

for j := 0; j < n-1-i; j++ {

inner loop compares adjacent pairs up to last sorted element

if arr[j] > arr[j+1] {

compare adjacent elements to decide if swap needed

arr[j], arr[j+1] = arr[j+1], arr[j]

swap elements to bubble larger one up

swapped = true

mark that a swap occurred this pass

if !swapped { break }

stop early if no swaps, array is sorted

Output

2 -> 3 -> 4 -> 5 -> 8

Complexity Analysis

Time: O(n^2) because in worst case we compare and swap pairs for each element multiple times

Space: O(1) because sorting is done in place without extra storage

vs Alternative: Compared to more advanced sorts like quicksort (O(n log n)), bubble sort is slower but simpler to understand and implement

Edge Cases

empty array

function returns immediately without error

DSA Go

for i := 0; i < n-1; i++ {

array with one element

no swaps needed, array remains same

DSA Go

for i := 0; i < n-1; i++ {

already sorted array

early break after first pass because no swaps occur

DSA Go

if !swapped { break }

When to Use This Pattern

When you see a problem asking to sort a small list with simple swaps and no extra space, bubble sort is a good first approach because it repeatedly swaps adjacent out-of-order items.

Common Mistakes

Mistake: Not reducing inner loop range each pass, causing unnecessary comparisons
Fix: Use 'n-1-i' as inner loop limit to skip already sorted elements

Mistake: Not breaking early when no swaps happen, leading to extra passes
Fix: Add a swapped flag and break if no swaps in a pass

Summary

Bubble sort repeatedly swaps adjacent elements to sort the array.

Use it for small or nearly sorted lists where simplicity matters more than speed.

The key insight is that each pass pushes the largest unsorted element to its correct place at the end.