Reading & Checking Arrays C++

#include <iostream>
#include <vector>

#define NELEM 6     /* constant for correct number of elements */

int main (void) {

    int i, last = 0;                        /* int read and last seen */
    std::vector<std::vector<int>> v {};     /* vector of vector<int> */
    std::vector<int> tmp {};                /* temporary vector<int> to fill */

    while (std::cin >> i) {                 /* read each integer in file */
        last = i;                           /* save last i to test on loop exit */
        if (i == -1) {                      /* if -1 array ended with last int */
            if (tmp.size() < NELEM)         /* is size less than NELEM? */
                std::cerr << "\nthe array does not have enough integers.\n";
            else if (tmp.size() > NELEM)    /* otherwise if greater than NELEM */
                std::cerr << "\nyou have too many numbers in this array.\n";
            else {
                v.push_back(tmp);           /* add tmp to vector<vector<int>> */
                std::cout << "\nthe size of the array is correct.\n";
            }
            tmp.clear();                    /* clear temporary vector<int> */
            continue;
        }
        std::cout << " " << i;              /* output element */
        if (i < 0) {                        /* check if number is negative */
            std::cerr << "\nthis array has negative element.\n";
            while (std::cin >> i && (last = i) != -1) {}/* read/discard rest */
            tmp.clear();                    /* clear temp array */
            continue;
        }
        tmp.push_back(i);                   /* add int to temporary vector */
    }
    if (last != -1)                         /* validate last read is -1 */
        std::cerr << "error: last input not -1.\n";

    std::cout << "\ncorrect arrays:\n";     /* output good arrays */
    for (auto r : v) {                      /* loop over good vectors */
        for (auto c : r)                    /* loop over integers */
            std::cout << " " << c;          /* output values */
        std::cout << '\n';                  /* tidy up with newline */
    }
}

/*

    echo "1 2 3 4 5 6 -1 1 2 3 4 -1 1 2 -3 4 5 6 7 8 -1 1 2 3 4 5 6 7 -1 2 3 4 5 6 7 -1" | ./bin/readmultiarray
     1 2 3 4 5 6
    the size of the array is correct.
     1 2 3 4
    the array does not have enough integers.
     1 2 -3
    this array has negative element.
     1 2 3 4 5 6 7
    you have too many numbers in this array.
     2 3 4 5 6 7
    the size of the array is correct.

    correct arrays:
     1 2 3 4 5 6
     2 3 4 5 6 7


*/