Segmentation Fault: 11 on small input for array/vector

Question

There are several questions related to this error on stackoverflow, and I understand that its related to excess memory usage by the array, or when using pointers (I tried this with vectors aswell) but using a small array, it still shows this error. The same code earlier was running fine (for merge sorting an array).

My input was as follows:

5

9 8 1 2 4

Output:

Segmentation fault: 11

#include<iostream>
#include<vector>
using namespace std;

void merge(vector <int> ar, int l, int m, int r){
    int n1 = m-l+1;
    int n2 = r-m;

    int L[n1];
    int R[n2];

    for (int i = 0; i < n1; ++i)
    {
        L[i]=ar[l+i];
    }

    for (int j = 0; j < n2; ++j)
    {
        R[j]=ar[m+j+1];
    }

    int i,j;
    i = j = 0;
    int k = i;


    while(i<n1 && j<n2){

        if (L[i]<R[j])
        {
            ar[k]=L[i];
            i++;
        }
        else if (R[j]<L[i])
        {
            ar[k]=R[j];
            j++;
        }
        k++;

    }

    while(i<n1){
        ar[k]=L[i];
        i++;
        k++;
    }
    while(j<n2){
        ar[k]=R[j];
        j++;
        k++;
    }


}


void mergesort(vector <int> ar, int l, int r){
    int m;
    m=r+(l-r)/2;
    if (l<r)
    {


        mergesort(ar, l, m);
        mergesort(ar, m+1, r);
        merge(ar, l, m, r);
    }
}

void print(vector <int> ar, int size){

    for (int i = 0; i < size; ++i)
    {
        cout<<ar[i]<< " ";
    }


}

int main()
{
    int n;
    cin>>n;
    vector <int> ar;

    for (int i = 0; i < n; ++i)
    {
        cin>>ar[i];
    }

    print(ar,n);
    mergesort(ar, 0, n-1);
    print(ar, n);



    return 0;
}

Answer 1

The problem is in part with m=r+(lr)/2 . When l is 0 and r is 1 , (lr)/2 is 0 . This makes m equal to 1 , l equal to 0 , and r equal to 1 and the mergesort(ar, l, m); call identical to the one it just worked through. The stack grows unbounded until you have a segmentation fault. One way to fix this which will also make your code more efficient is to merge the lists when the difference between l and r is below some threshold. Or, you can just swap the two elements when you get to the point where l and r differ by one, like so:

if (l - r <= 1) {
    int temp = ar[l];
    ar[l] = ar[r];
    ar[r] = temp;
    return;
}