#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.

CodePudding user response：

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;

CodePudding user response：

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