I have written this Merge Sort program in C. It shows "Segmentation fault (core dumped)" after implementation of arrays

Question

#include <stdio.h>

void mergesort();
void merge();

int main()
{
    int a[40], n;
    printf("\nEnter the number of elements:");
    scanf("%d", &n);
    printf("\nEnter the %d elements:", n);
    for (int i = 0; i < n; i++)
    {
        scanf("%d", &a[i]);
    }
    mergesort(a, 0, n - 1);
    printf("\nThe Sorted array is: ");
    for (int i = 0; i < n; i++)
    {
        printf("%d ", a[i]);
    }
    return 0;
}

void mergesort(int a[], int first, int last)
{
    int mid;
    if (first < last)
    {
        mid = (mid + last) / 2;
        mergesort(a, first, mid);
        mergesort(a, mid + 1, last);
        merge(a, first, mid, last);
    }
}

void merge(int a[], int first, int mid, int last)
{
    int b[50];
    int i, j, k;
    i = first;
    j = mid + 1;
    k = first;
    while (i <= mid && j <= last)
    {
        if (a[i] <= a[j])
            b[k++] = a[i++];
        else
            b[k++] = a[j++];
    }
    if (i > mid)
    {
        while (j <= last)
            b[k++] = a[j++];
    }
    else
    {
        while (i <= mid)
            b[k++] = a[i++];
    }
    for (i = first; i <= last; i++)
        a[i] = b[i];
}

I have written this Merge Sort program in C. It shows "Segmentation fault (core dumped)" after implementation of arrays. I don't know what is the issue. So please help me out i have exam on the next day. I get output like this

Enter the number of elements:5

Enter the 5 elements:1 2 3 5 4 Segmentation fault (core dumped)

It was found that I made a mistake while implementing mid variable. The correct one is:

mid=(first+last)/2;

Thank you all for your helps. Each and every of your replies were useful in identifying the error.

Answer 1

The program has undefined behavior since you read mid when it's uninitialized:

void mergesort(int a[], int first, int last) {
    int mid;
    if (first < last) {
        mid = (mid + last) / 2;
//             ^^^

You probably want:

        mid = (first + last) / 2;

Answer 2

void mergesort(int a[],int first,int last)
{
 int mid;
 if(first<last)
 {
  mid=(mid+last)/2;
  mergesort(a,first,mid);
  mergesort(a,mid+1,last);
  merge(a,first,mid,last);
 }
}

When this runs the first time, mid is not guaranteed to be 0 or any other number; mid is some integer that may change every time. Thus, the number you are getting may be well outside the bounds of the array. Initializing mid to 0 would fix that, but as the answers above me point out you actually want to do mid = (first + last) / 2