How to check for negative number and what is expected output for negative numbers

#1

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

#define MAX_SIZE 1000005

void SieveOfEratosthenes(vector &primes)
{
bool IsPrime[MAX_SIZE];
memset(IsPrime, true, sizeof(IsPrime));

for (int p = 2; p * p < MAX_SIZE; p++)
{
    if (IsPrime[p] == true)
    {

        for (int i = p * p; i <  MAX_SIZE; i += p)
            IsPrime[i] = false;
    }
}


for (int p = 2; p < MAX_SIZE; p++)
   if (IsPrime[p])
        primes.push_back(p);

}

int main()
{ int n;
vector primes;
cin>>n;
// Function call
SieveOfEratosthenes(primes);

cout << primes[n-1];

return 0;

}

#3

The numbers cannot be -ve since the smallest prime is 2
and the i/p are designed in a way that they do not give out of range values.

So we are not required to be worried of -ve input case

#4

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