向量对的第一个元素的范围

Question

I need to find the range of the first elements of a vector pair. I need this range for a map, which counts the duplicate entries in this vector. Here is a code snipped and how I managed it. Maybe there is another, better solution?

unordered_map<int, int> frequency;
vector<pair<unsigned int,Point>> Roi_Num_Koord;
vector<int> Roi_first_Element;

int main()
{
    // Part1: fill the Vector pair
    Roi_Num_Koord.emplace_back(make_pair(0,Point(3.6));
    Roi_Num_Koord.emplace_back(make_pair(1,Point(4,8));
    Roi_Num_Koord.emplace_back(make_pair(2,Point(8.3));
    Roi_Num_Koord.emplace_back(make_pair(3,Point(4,6));

    // Part 2: now copy the first element to another vector 

    for (int i = 0; i < Roi_Num_Koord.size(); i++)
    {
        Roi_first_Element.emplace_back(Roi_Num_Koord[i].first);
    }

    // Part 3: now do the duplicate search (Code was taken out of the internet)
    for (int i : Roi_first_Element)
    {
        ++frequency[i];
        cout << "freque "<<frequency[i] << endl;        
    }
    for (const auto& e : frequency)
    {       
        if (e.second == 5)
        {
            std::cout << "Roi " << e.first << " encountered " << e.second << " times\n";
        }
    }
}

So is there a possibility to remove Part 2 and find out the range of the first Element of Roi_Num_Koord ?, so that I don't have to copy the first elements of this vector to the other vector ( Roi_first_Element )

Answer 1

Yes the second step is completely redundant. You just iterate through the container and whenever you need first element of the pair you say it explicitly pretty much like you do in Step 2.

for(const pair<unsigned int,Point>& element : Roi_Num_Koord)
{
    ++frequency[element.first];
    cout << "freque " << frequency[element.first] << endl;        
}