MarkXLII MarkXLII - 1 month ago 11
C++ Question

Passing vector by reference: Segmentation fault

I'm trying to get the following code for merge sort work with vectors instead of C style arrays and I'm having a hard time figuring out why it crashes with segmentation fault. Can someone help me out in understanding the problem here?

#include <bits/stdc++.h>
using namespace std;

void merge(vector<int>& a, int l, int m, int r) {
vector<int> L;
vector<int> R;

for (int i = 0; i <= m; i++)
L.push_back(a[i]);
for (int i = m+1; i <= r; i++)
R.push_back(a[i]);

int i = 0, j = 0; // Initial index of first and second subarray
int k = l; // Initial index of merged subarray
while (i < L.size() && j < R.size()) {
if (L[i] <= R[j]) {
a[k] = L[i];
i++;
}
else {
a[k] = R[j];
j++;
}
k++;
}

// Filling leftovers
while (i < L.size()) {
a[k] = L[i];
k++;
i++;
}
while (j < R.size()) {
a[k] = R[j];
k++;
j++;
}
}

void merge_sort(vector<int>& a, int l, int r) {
if (l < r) {
int m = l + (r-l) / 2; // Avoids integer overflow.
merge_sort(a, l, m);
merge_sort(a, m+1, r);
merge(a, l, m, r);
}
}

int main()
{
vector<int> a = {2, 4, 1, 5, 3, 9};
int size = a.size();
merge_sort(a, 0, size-1);
for (int i = 0; i < size; i++)
cout << a[i] << ' ';
cout << endl;
return 0;
}

Answer

You need to go from l to m when filling vector L:

void merge(vector<int>& a, int l, int m, int r) {
    vector<int> L;
    vector<int> R;

    for (int i = l; i <= m; i++)
        L.push_back(a[i]);
    ...
}

Otherwise L contains additional elements outside the range of interest. Then, index k steps off the end of vector a when iterating for the size of L.

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