Question

The following program calculates all primes for really large numbers (eg. 600,851,475,143). Everything works right so far except when I put in large numbers the destructor is crashing the application. Can anyone see something wrong with my application?
After rechecking my solution the answer is wrong but the question still is valid.

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
#include <cmath>
#include <stdexcept>
#include <climits>

typedef std::vector<unsigned long long>::const_iterator prime_it;

#define MAX_COL 900000

struct large_vector
{
public:
  large_vector(unsigned long long size, unsigned int row) :
    m_Row(row)
  {
    m_RowVector.reserve(size);
  }
  std::vector<bool> m_RowVector;
  unsigned int m_Row;
};

struct prime_factor
{
public:
  prime_factor(unsigned long long N);
  ~prime_factor() {}
  void print_primes();
private:
  std::vector<bool> m_Primes;
  std::vector<large_vector>m_Vect_Primes;
  unsigned long long m_N;
};

prime_factor::prime_factor(unsigned long long N) :
  m_N(N)
{
  // If number is odd then we need the cieling of N/2 / MAX_COL
  int number_of_vectors = (m_N % MAX_COL == 0) ? (m_N / MAX_COL) : ((m_N / MAX_COL) + 1);
  std::cout << "There will be " << number_of_vectors << " rows";
  if (number_of_vectors != 0) {
    for (int x = 0; x < number_of_vectors; ++x) {
      m_Vect_Primes.push_back(large_vector(MAX_COL, x));
    }

    m_Vect_Primes[0].m_RowVector[0] = false;
    m_Vect_Primes[0].m_RowVector[1] = false;
    unsigned long long increment = 2;
    unsigned long long index = 0;
    while (index < m_N) {
      for (index = 2*increment; index < m_N; index += increment) {
        unsigned long long row = index/MAX_COL;
        unsigned long long col = index%MAX_COL;
        m_Vect_Primes[row].m_RowVector[col] = true;
      }
      while (m_Vect_Primes[increment/MAX_COL].m_RowVector[increment%MAX_COL]) {
        increment++;
      }
    }
  }
}

void prime_factor::print_primes()
{
  for (int index = 0; index < m_N; ++index) {
    if (m_Vect_Primes[index/MAX_COL].m_RowVector[index%MAX_COL] == false) {
      std::cout << index << " ";
    }
  }
}

/*!
 * Driver
 */
int main(int argc, char *argv[])
{
  static const unsigned long long N = 600851475143;
  prime_factor pf(N);
  pf.print_primes();
}

---

Update
I am pretty sure this is a working version:

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
#include <cmath>
#include <stdexcept>
#include <climits>

typedef std::vector<unsigned long long>::const_iterator prime_it;

#define MAX_COL 900000

struct large_vector
{
public:
  large_vector(unsigned long long size, unsigned int row) :
    m_Row(row)
  {
    m_RowVector.resize(size);
  }
  std::vector<bool> m_RowVector;
  unsigned int m_Row;
};

struct prime_factor
{
public:
  prime_factor(unsigned long long N);
  ~prime_factor() {}
  void print_primes();
private:
  std::vector<bool> m_Primes;
  std::vector<large_vector>m_Vect_Primes;
  unsigned long long m_N;
};

prime_factor::prime_factor(unsigned long long N) :
  m_N(N)
{
  // If number is odd then we need the cieling of N/2 / MAX_COL
  int number_of_vectors = (m_N % MAX_COL == 0) ? ((m_N/2) / MAX_COL) : (((m_N/2) / MAX_COL) + 1);
  std::cout << "There will be " << number_of_vectors << " rows";
  if (number_of_vectors != 0) {
    for (int x = 0; x < number_of_vectors; ++x) {
      m_Vect_Primes.push_back(large_vector(MAX_COL, x));
    }

    m_Vect_Primes[0].m_RowVector[0] = false;
    m_Vect_Primes[0].m_RowVector[1] = false;
    unsigned long long increment = 2;
    unsigned long long index = 0;
    while (index < m_N) {
      for (index = 2*increment; index < m_N/2; index += increment) {
        unsigned long long row = index/MAX_COL;
        unsigned long long col = index%MAX_COL;
        m_Vect_Primes[row].m_RowVector[col] = true;
      }
      increment += 1;
      while (m_Vect_Primes[increment/MAX_COL].m_RowVector[increment%MAX_COL]) {
        increment++;
      }
    }
  }
}

void prime_factor::print_primes()
{
  for (unsigned long long index = 0; index < m_N/2; ++index) {
    if (m_Vect_Primes[index/MAX_COL].m_RowVector[index%MAX_COL] == false) {
      std::cout << index << " ";
    }
  }
}

/*!
 * Driver
 */
int main(int argc, char *argv[])
{
  static const unsigned long long N = 400;
  prime_factor pf(N);
  pf.print_primes();
}

Answer 1

Your usage of reserve is incorrect.

m_RowVector.reserve(size);

Here m_RowVector has space reserved so that the vector can grow without being re-allocated. BUT the size of m_RowVector is still 0 and thus accessing any elements is still undefined. You must change the size of the array with either resize() or push_back() to put elements into the vector.

I can’t see anything wrong but I am sure that you have other index beyond the end of vector problems. I would change the use of operator[] into the method at() this will throw an exception when you access elements of the end of the vector and give you a clue to the actual location of the error.