Question

Update: modified condition in the merge function, now everything works fine

I’m trying to implement a merge sort algorithm in c++ which works with intege vectors. So here is my piece of code:

#include <vector>
#include <iostream>

using namespace std;

void Vout (vector <int> V){
    for (int i = 0; i<V.size(); i++)
        cout << V[i] << "\t";
    cout << endl;
}

vector <int> merge (vector <int> B, vector <int> C){
    vector <int> D;
    int n = B.size() + C.size();
    int i=0, j=0;

    for (int k = 0; k<n; k++){
        if( ( B[i]<C[j] || j == C.size() ) && i<B.size() ){  //was like this: if( B[i]<C[j] ){
            D.push_back(B[i]);
            i++;
        }
        else{
            D.push_back(C[j]);
            j++;
        }
    }
    return D;
}

vector <int> merge_sort(vector <int> A){
    int n = A.size();
    if (n<=1) return A;
    vector <int> B, C;
    for (int i = 0; i<n/2; i++)
        B.push_back(A[i]);
    for (int i = n/2; i<n; i++)
        C.push_back(A[i]);

    B = merge_sort(B);
    C = merge_sort(C);
    vector <int> D = merge(B, C);
    return D;
}


int main() {
    vector <int> data;
    freopen("input.txt", "rt", stdin);
    int n;
    while (cin >> n){
        data.push_back(n);}
    Vout (data);
    Vout (merge_sort(data));
}

However with this input

5 4 6 7 9 1 2 8 3 12 10

it gives the following output:

1 2 3 0 4 5 6 7 8 9 0

I don’t quite understand where all these zeroes came from. Please help.

Answer 1

Your merge() function doesn’t handle running off the end of the B or C vectors; when it reaches the end of either, your comparison (B[i]<C[j]) will be comparing against one past the end of the array.