Code in Merge Sort in C not working properly

Question

int lb = 0, ub = 8, mid;   //ub=upper bound,lb=lower bound.
int i = 0, j = 0, k = 0;
int a[] = { 15, 5, 24, 8, 1, 3, 16, 10, 20 };
int b[10];

void mergeSort(int a[], int lb, int ub) {
    if (lb < ub) {
        mid = ((lb + ub) / 2);
        mergeSort(a, lb, mid);
        mergeSort(a, mid + 1, ub);
        merge(a, lb, mid, ub);
    }
}

void merge(int a[], int lb, int mid, int ub) {
    i = lb;
    j = mid + 1;
    k = lb;

    while (i <= mid && j <= ub) {
        if (a[i] <= a[j]) {
            b[k] = a[i];
            i++;
        } else {
            b[k] = a[j];
            j++;
        }
        k++;
    }

    if (i > mid) {
        while (j <= ub) {
            b[k] = a[j];
            j++;
            k++;
        }
    } else {
        while (i <= mid) {
            b[k] = a[i];
            i++;
            k++;
        }
    }

    for (k = lb; k <= ub; k++) {
        a[k] = b[k];
    }
}

void printList(int A[]) {
    for (k = 0; k <= 8; k++) {
        printf("%d\n", A[k]);
    }
    printf("done\n");
}

int main() {
    printList(a);
    mergeSort(a, 0, 8);
    printList(a);
}

I think the code of mergesort and merge is not the problem, the problem is how I have checked the code many times but I cannot find the mistakes, so I hope someone could explain to me where is the problems, thanks for everyone who try to help!!

the output when I run the code:

5
15
8
1
3
16
10
20
24
done

which is not a sorted list.

Answer 1

The problem is mid is a global variable, so it gets modified by the recursive call mergeSort(a, lb, mid); and a different value is used for the second recursive call mergeSort(a, mid + 1, ub);and yet a different value is used for the final call merge(a, lb, mid, ub); producing unexpected results, and potentially causing undefined behavior.

Do not use global variables. Define both arrays in main , make i , j , k and mid local variables in merge and mergeSort and pass the auxiliary array b to merge and mergeSort .

Here is a modified version:

#include <stdio.h>

void merge(int a[], int lb, int mid, int ub, int b[]) {
    int i = lb;
    int j = mid + 1;
    int k = lb;

    while (i <= mid && j <= ub) {
        if (a[i] <= a[j]) {
            b[k] = a[i];
            i++;
        } else {
            b[k] = a[j];
            j++;
        }
        k++;
    }
    if (i > mid) {
        while (j <= ub) {
            b[k] = a[j];
            j++;
            k++;
        }
    } else {
        while (i <= mid) {
            b[k] = a[i];
            i++;
            k++;
        }
    }
    for (k = lb; k <= ub; k++) {
        a[k] = b[k];
    }
}

void mergeSort(int a[], int lb, int ub, int b[]) {
    if (lb < ub) {
        int mid = (lb + ub) / 2;
        mergeSort(a, lb, mid, b);
        mergeSort(a, mid + 1, ub, b);
        merge(a, lb, mid, ub, b);
    }
}

void printList(int A[], int len) {
    for (int k = 0; k < len; k++) {
        printf(" %d", A[k]);
    }
    printf("\n");
}

int main() {
    int a[] = { 15, 5, 24, 8, 1, 3, 16, 10, 20 };
    int len = sizeof(a) / sizeof(a[0]);
    // make b a temporary array the same size as a
    int b[len];

    printf("before:");
    printList(a, len);
    mergeSort(a, 0, len - 1, b);
    printf("after:");
    printList(a, len);
    return 0;
}

Output:

before: 15 5 24 8 1 3 16 10 20
after: 1 3 5 8 10 15 16 20 24

Note that is it less error prone to pass ub as the index to the element after the last one of the slice, this way the slice length is ub - lb and you do not need any confusing +1 / -1 adjustments. Furthermore, there is no need to copy the remaining elements from the right half as they are already in the proper position in the destination array.

Here is a modified version with these simplifications:

#include <stdio.h>

void merge(int a[], int lb, int mid, int ub, int b[]) {
    int i = lb;
    int j = mid;
    int k = lb;

    while (i < mid && j < ub) {
        if (a[i] <= a[j]) {
            b[k++] = a[i++];
        } else {
            b[k++] = a[j++];
        }
    }
    while (i < mid) {
        b[k++] = a[i++];
    }
    for (i = lb; i < k; i++) {
        a[i] = b[i];
    }
}

void mergeSort(int a[], int lb, int ub, int b[]) {
    if (ub - lb >= 2) {
        int mid = lb + (ub - lb) / 2;
        mergeSort(a, lb, mid, b);
        mergeSort(a, mid, ub, b);
        merge(a, lb, mid, ub, b);
    }
}

void printList(int A[], int len) {
    for (int k = 0; k < len; k++) {
        printf(" %d", A[k]);
    }
    printf("\n");
}

int main() {
    int a[] = { 15, 5, 24, 8, 1, 3, 16, 10, 20 };
    int len = sizeof(a) / sizeof(a[0]);
    // make b a temporary array the same size as a
    int b[len];

    printf("before:");
    printList(a, len);
    mergeSort(a, 0, len, b);
    printf("after:");
    printList(a, len);
    return 0;
}