Quicksort does not work with large arrays - c++

Question

I'm trying to sort large arrays with Quicksort and Mergesort to evaluate performances.
I've a problem: if I impose a large number of elements in an array, the program does not start to generate values randomly. In the code below, if N=500000, it works very well. If N > 500000, for example 1000000, it does not work. With MergeSort the limit is 200000. I tried on multiple devices, C++ on Eclipse IDE.
Someone knows how to solve the problem?
#define N 800000
#include <iostream>
#include <cstdlib>
#include <time.h>
#include <chrono>
using namespace std;
void Exchange(int *a, int *b) {
int temp;
temp = *a;
*a = *b;
*b = temp;
}
int Partition(int A[], int p, int r) {
int x = A[r];
int i = p - 1;
for (int j = p; j <= r; j++) {
if (A[j] < x) {
i++;
Exchange(&A[i], &A[j]);
}
}
Exchange(&A[i + 1], &A[r]);
return i + 1;
}
int RPartition(int A[], int p, int r) {
srand(time(NULL));
int i = p + rand() % (p - r);
Exchange(&A[i], &A[r]);
return Partition(A, p, r);
}
void QuickSort(int A[], int p, int r) {
if (p < r) {
int q = RPartition(A, p, r);
QuickSort(A, p, q - 1);
QuickSort(A, q + 1, r);
}
}
void Stampa(int A[], int n) {
for (int i = 0; i < n; i++) {
cout << A[i] << "\n";
}
}
int main() {
srand(50000);
int A[N];
for (int i = 0; i < N; i++) {
A[i] = rand();
}
cout << "Array non ordinato\n";
Stampa(A, N);
auto start = std::chrono::system_clock::now();
QuickSort(A, 0, N - 1);
auto end = std::chrono::system_clock::now();
cout << "\nArray ordinato\n";
Stampa(A, N);
std::chrono::duration<double> elapsed = end - start;
cout << "Elapsed time: " << elapsed.count() << "s";
}

Answer 1

The explanation is very simple: you allocate the array as a local variable with automatic storage (aka on the stack), hence if the size is too large, you get a stack overflow.
You should either allocate the array from the heap or define it as static data.
Here is a modified version:
int main() {
srand(time(NULL));
int *A = new int[N];
for (int i = 0; i < N; i++) {
A[i] = rand();
}
cout << "Array non ordinato\n";
Stampa(A, N);
auto start = std::chrono::system_clock::now();
QuickSort(A, 0, N - 1);
auto end = std::chrono::system_clock::now();
cout << "\nArray ordinato\n";
Stampa(A, N);
std::chrono::duration<double> elapsed = end - start;
cout << "Elapsed time: " << elapsed.count() << "s";
delete[] A;
}