c++ - 使用 STL multimap 为选择性项目赋值

Question

typedef std::pair<int, bool> acq_pair; //edge, channel_quality
typedef std::pair<int, acq_pair> ac_pair;
typedef std::multimap<int, acq_pair> ac_map;
typedef ac_map::iterator It_acq;


int bits = acq_map.size();
std::cout << "bits = " << bits << std::endl;

std::vector<std::vector<bool> > c_flags (1 << bits);

for (i = 0; i < c_flags.size(); ++i)
{
    for (j = 0; j < bits; ++j)
    {
        c_flags[i].push_back( (i & (1 << j)) > 0);      
    }           
}

std::cout << "c_flags.size() = " << c_flags.size() << std::endl;

for(i = 0; i < c_flags.size(); ++i)
{
    for(j = 0; j < bits; ++j)
    {
        std::cout << c_flags[i][j] << std::endl;

        for(It_acq itc = acq_map.begin(); itc!= acq_map.end(); ++itc)
        {
            acq_pair it1 = itc->second;
            itc->second.second = c_flags[i][j];
            std::cout << itc->first << " : " << it1.first << " : " << it1.second << std::endl;
        }
    }   
    std::cout << "\n" << std::endl;
}

如何一次只访问多 map 容器中的一项？我只想更新 map 中的第 j 个值，但是当我遍历 map 时，所有 bool 值都发生了变化。是否有选择性的方式来访问 map 容器值？

Answer 1

行 itc->second.second = c_flags[i][j]; performed in a loop with itc from begin() to end() 确实对映射的每个值执行赋值.如果目标是只修改映射中的第 j 个值，则不需要遍历整个映射:

    for(size_t j = 0; j < bits; ++j)
    {
        std::cout << c_flags[i][j] << std::endl;

        It_acq itc = acq_map.begin(); // itc points at the beginning
        advance(itc, j); // itc points at the j'th element
        itc->second.second = c_flags[i][j]; // the assignment

        for(It_acq itc = acq_map.begin(); itc!= acq_map.end(); ++itc)
        {
            acq_pair it1 = itc->second;
//            itc->second.second = c_flags[i][j];   // no assignment here
            std::cout << itc->first << " : " << it1.first << " : " << it1.second << std::endl;
        }
    }

如果此映射以这种方式用于索引访问，则可能值得考虑切换到 vector 。