How to keep those specify number of lagest component? [closed]

edit

This is my test image

.

I wnat to keep those 4th largest component. For this target, I have write a ton code:

#include<opencv.hpp>
#include<vector>
using namespace std;
using namespace cv;

bool mygreater(const vector<int> a, const vector<int> b) {
    if (a[1] < b[1])
        return false;
    else
        return true;
}

int main() {
    Mat mask, img = imread("blobn.png", 0);

    threshold(img, mask, 0, 255, THRESH_OTSU);
    Mat labels;
    int n;
    n = connectedComponents(img, labels);
    vector<vector<int>> countCom;
    for (int i=0; i < n; i++) {
        vector<int> temp{ i,0 };
        countCom.push_back(temp);
    }
    //std::cout << labels.type();
    for (int i = 0; i < labels.rows; i++) {
        int* data = labels.ptr<int>(i);
        for (int j = 0; j < labels.cols; j++) {
            countCom[data[j]][1]++;
        }
    }

    countCom.erase(countCom.begin());


    sort(countCom.begin(),  countCom.end(), mygreater);

    vector<int> resultCom;
    for (int i = 0; i < 4; i++)
        resultCom.push_back(countCom[i][0]);

    cout << endl;

    for (int i = 0; i < 4; i++)
        cout << resultCom[i]<<"\t";
    for (int i = 0; i < labels.rows; i++) {
        int* data = labels.ptr<int>(i);
        for (int j = 0; j < labels.cols; j++)
            if (find(resultCom.begin(), resultCom.end(), data[j]) == resultCom.end())
                data[j] = 0;
    }

    return 0;
}

Is there any easy method can do this in our opencv?