Сумма соседних элементов двумерного массива

#include <iostream>
#include <vector>
#include <random>
#include <chrono>
#include <algorithm>
 
const int ROW_SIZE = 3;
const int COL_SIZE = 4;
 
int get_row_index(int index_of_element_in_sequence)
{
    return ((index_of_element_in_sequence + 1) / ROW_SIZE) - 1;
}
 
int get_col_index(int index_of_element_in_sequence)
{
    return ((index_of_element_in_sequence + 1) % COL_SIZE) - 1;
}
 
int main()
{
    const int VALUE_THAT_MUST_BE_FOUND = 3;
 
    int matrix[ROW_SIZE][COL_SIZE];
    std::default_random_engine gen(std::chrono::system_clock::now().time_since_epoch().count());
    std::uniform_int_distribution<int> distribution(0, 3);
 
    for (int i = 0; i < ROW_SIZE; ++i)
        for (int j = 0; j < COL_SIZE; ++j)
            matrix[i][j] = distribution(gen);
 
    for (int i = 0; i < ROW_SIZE; ++i)
    {
        for (int j = 0; j < COL_SIZE; ++j)
            std::cout << matrix[i][j] << "\t";
        std::cout << "\n";
    }
 
    std::vector<int> vec(ROW_SIZE * COL_SIZE);
 
    for (int i = 0, k = 0; i < ROW_SIZE; ++i)
        for (int j = 0; j < COL_SIZE; ++j, ++k)
            vec[k] = matrix[i][j];
 
    std::vector<int>::iterator it;
    it = std::search_n (vec.begin(), vec.end(), 2, VALUE_THAT_MUST_BE_FOUND);
 
 
    if (it != vec.end())
    {
        int row_index = get_row_index(it - vec.begin());
        int col_index = get_col_index(it - vec.begin());
        std::cout << "\ntwo "
                  << VALUE_THAT_MUST_BE_FOUND
                  << "s found at position: matrix["
                  << row_index << "][" << col_index << "]\n";
    }
    else
        std::cout << "\nmatch not found\n";
 
    return 0;
}

    std::default_random_engine gen(std::chrono::system_clock::now().time_since_epoch().count());
    std::uniform_int_distribution<int> distribution(0, 3);