#include <iostream>
#include <algorithm>
#include <vector>
//#include <cstddef>
 
int main() 
{
	std::vector<size_t> m_index_list;
 
	m_index_list.push_back(0);
	m_index_list.push_back(5);
	m_index_list.push_back(9);
 
	for (size_t data_pos = 4; data_pos < 12; ++data_pos)
	{
		// your code goes here
		auto it = lower_bound(m_index_list.begin(), m_index_list.end(), data_pos);
 
		//тут проверка, что fragment_index существует (т.е. it != end(m_index_list)) 
 
		size_t fragment_index = it - m_index_list.begin();
 
		std::cout << "data_pos " << data_pos << ": fragment_index = " << fragment_index << std::endl;
	}
 
	return 0;
}

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