Sum of primes - from https://www.hackerrank.com/contests/projecteuler/challenges/euler010/copy-from/1302201972

HR_cpp_Euler10.cpp

using namespace std;

int main(){
    int t;
    cin >> t;
                                                    // Using sieve of Eratosthenes, from Wikipedia
bool primes[1000001];                               // Let A be an array of Boolean values, 
fill_n(primes, 1000001, true);                       // indexed by integers 2 to n, initially all set to true. 
for (long i = 2; i <= 1000; ++i)                    // i = 2, 3, 4, ..., not exceeding root n:
    if (primes[i])                                  // if A[i] is true:
        for (long long j = i*i; j <= 1000000; j += i)    //   for j = i2, i2+i, i2+2i, i2+3i, ..., not exceeding n:
            primes[j] = false;                      // A[j] := false.
    
    long long answers[t];
    long questions[t];
    for(int a0 = 0; a0 < t; a0++){
        long n;
        cin >> n;
        questions[a0] = n;
        long istart = 2;
        long long sum = 0;
        for(int l = 0; l < a0; ++l){
            if (questions[l] < n) {
                sum = answers[l];
                istart = questions[l] + 1;
            }
        }
        for (long x = istart; x <= n; ++x)
            sum += primes[x] ? x : 0 ;
        answers[a0] = sum;
        cout << sum << endl;
    }
    return 0;
}