Sorting

Bubble Sort

Repeatedly walk the array comparing adjacent pairs and swapping any that are out of order. After pass k, the k largest elements are in their final positions at the end. Stop early when a full pass makes zero swaps.

Algorithm

Canonical input [5, 1, 4, 2, 8] finishes after three passes: two with swaps, then a clean pass that triggers the early exit. Final array [1, 2, 4, 5, 8].

adjacent-pair compare and swap Inner loop walks `j` from `0` to `n - i - 2` comparing `arr[j]` and `arr[j + 1]`.
early exit A `swapped` flag set false at the start of each pass. If no swap happened, break out of the outer loop.

Basic Implementation

basic.c
Replay: real traced execution (multi-file project)
#include <stdio.h>
#include <stdbool.h>

int main(void) {
    int arr[] = {5, 1, 4, 2, 8};
    int n = (int)(sizeof(arr) / sizeof(arr[0]));
    for (int i = 0; i < n - 1; ++i) {
        bool swapped = false;
        for (int j = 0; j < n - i - 1; ++j) {
            if (arr[j] > arr[j + 1]) {
                int tmp = arr[j];
                arr[j] = arr[j + 1];
                arr[j + 1] = tmp;
                swapped = true;
            }
        }
        if (!swapped) {
            break;
        }
    }
    printf("[");
    for (int i = 0; i < n; ++i) {
        if (i > 0) printf(", ");
        printf("%d", arr[i]);
    }
    printf("]\n");
    return 0;
}

  1. arr ← [5, 1, 4, 2, 8]

    4int main(void) {5    int arr[] = {5, 1, 4, 2, 8};6    int n = (int)(sizeof(arr) / sizeof(arr[0]));
    values this step[5, 1, 4, 2, 8]arr

  2. n ← 5

    5int arr[] = {5, 1, 4, 2, 8};6int n = (int)(sizeof(arr) / sizeof(arr[0]));7for (int i = 0; i < n - 1; ++i) {
    values this step5n[5, 1, 4, 2, 8]arr

  3. swapped ← false

    7for (int i = 0; i < n - 1; ++i) {8    bool swapped = false;9    for (int j = 0; j < n - i - 1; ++j) {
    values this stepfalseswapped

  4. arr[j] > arr[j+1] ← true

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this steptruearr[j] > arr[j+1][5, 1, 4, 2, 8]arr0j5arr[j]1arr[j+1]

  5. arr ← [1, 5, 4, 2, 8], swapped ← true

    12arr[j] = arr[j + 1];13arr[j + 1] = tmp;14swapped = true;
    values this step[5, 1, 4, 2, 8] → [1, 5, 4, 2, 8]arrtrueswapped

  6. arr[j] > arr[j+1] ← true

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this steptruearr[j] > arr[j+1][1, 5, 4, 2, 8]arr1j5arr[j]4arr[j+1]

  7. arr ← [1, 4, 5, 2, 8], swapped ← true

    12arr[j] = arr[j + 1];13arr[j + 1] = tmp;14swapped = true;
    values this step[1, 5, 4, 2, 8] → [1, 4, 5, 2, 8]arrtrueswapped

  8. arr[j] > arr[j+1] ← true

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this steptruearr[j] > arr[j+1][1, 4, 5, 2, 8]arr2j5arr[j]2arr[j+1]

  9. arr ← [1, 4, 2, 5, 8], swapped ← true

    12arr[j] = arr[j + 1];13arr[j + 1] = tmp;14swapped = true;
    values this step[1, 4, 5, 2, 8] → [1, 4, 2, 5, 8]arrtrueswapped

  10. arr[j] > arr[j+1] ← false

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this stepfalsearr[j] > arr[j+1][1, 4, 2, 5, 8]arr3j5arr[j]8arr[j+1]

  11. swapped ← false

    7for (int i = 0; i < n - 1; ++i) {8    bool swapped = false;9    for (int j = 0; j < n - i - 1; ++j) {
    values this stepfalseswapped

  12. arr[j] > arr[j+1] ← false

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this stepfalsearr[j] > arr[j+1][1, 4, 2, 5, 8]arr0j1arr[j]4arr[j+1]

  13. arr[j] > arr[j+1] ← true

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this steptruearr[j] > arr[j+1][1, 4, 2, 5, 8]arr1j4arr[j]2arr[j+1]

  14. arr ← [1, 2, 4, 5, 8], swapped ← true

    12arr[j] = arr[j + 1];13arr[j + 1] = tmp;14swapped = true;
    values this step[1, 4, 2, 5, 8] → [1, 2, 4, 5, 8]arrtrueswapped

  15. arr[j] > arr[j+1] ← false

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this stepfalsearr[j] > arr[j+1][1, 2, 4, 5, 8]arr2j4arr[j]5arr[j+1]

  16. swapped ← false

    7for (int i = 0; i < n - 1; ++i) {8    bool swapped = false;9    for (int j = 0; j < n - i - 1; ++j) {
    values this stepfalseswapped

  17. arr[j] > arr[j+1] ← false

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this stepfalsearr[j] > arr[j+1][1, 2, 4, 5, 8]arr0j1arr[j]2arr[j+1]

  18. arr[j] > arr[j+1] ← false

    9for (int j = 0; j < n - i - 1; ++j) {10    if (arr[j] > arr[j + 1]) {11        int tmp = arr[j];
    values this stepfalsearr[j] > arr[j+1][1, 2, 4, 5, 8]arr1j2arr[j]4arr[j+1]

  19. loop ← break

    17if (!swapped) {18    break;19}
    values this stepbreakloopfalseswapped

  20. stdout ← [1, 2, 4, 5, 8]

    25}26printf("]\n");27return 0;
    values this step[1, 2, 4, 5, 8]stdout[1, 2, 4, 5, 8]arr

Complexity

  • Time: O(n^2) worst and average; O(n) best (already sorted with early exit)
  • Space: O(1)
  • Stable: yes

Implementation notes

  • C: nested for loops with the early-exit bool swapped flag from <stdbool.h>. C's standard qsort would hide the comparison-and-swap the lesson is teaching.
  • A raw int tmp swap keeps the swap step visible; calling out to a helper would obscure the three-step pattern the lesson highlights.
  • The replay distinguishes compare frames from swap frames so the moving pivot value is visible. The pass number and swapped flag appear in the trace.