Quicksorting in C++

#include <iostream>
#include <fstream>
using namespace std;
void swap(int *a, int i, int j) {
    int t;
    if (i!=j) {
        t = a[i];
        a[i] = a[j];
        a[j] = t;
    }
}

int partition(int *arr, int left, int right) {
    int i, j;
    int pivot = (left + right) / 2;
    swap(arr, pivot, right);
    for (i=j=left; i<right; i++) {
        if (arr[i]<arr[right]) {
            swap(arr, i, j);
            j++;
        }
    }
    swap(arr,j,right);
    return j;
}

void quickSort(int *arr, int left, int right) {
      int pivot = partition(arr, left, right);

      if (left < pivot-1)
            quickSort(arr, left, pivot-1);
      if (pivot+1 < right)
            quickSort(arr, pivot+1, right);
}

int main() {
    int a[100], n, i;
    ifstream num_file;
    num_file.open("numbers.txt");
    if (num_file.is_open())
    {
        n=0;
        while (!num_file.eof())
            num_file >> a[n++];
        //n--;
        num_file.close();
        cout << "Given array: ";
        for (i=0; i<n; i++)
            cout << a[i] << " ";
        quickSort(a, 0, n-1);
        cout << "\nSorted array: ";
        for (i=0; i<n; i++)
            cout << a[i] << " ";
        cout << endl;
    } else
        cout << "Error opening file!";
    return 0;
}