Signature Description Parameters
#include <DataFrame/DataFrameStatsVisitors.h>

template<typename T, typename I = unsigned long,
         std::size_t A = 0>
struct MedianVisitor;

// -------------------------------------

template<typename T, typename I = unsigned long,
         std::size_t A = 0>
using med_v = MedianVisitor<T, I, A>;
        
This is a “single action visitor”, meaning it is passed the whole data vector in one call and you must use the single_act_visit() interface.

This functor class finds the median of the given column, using the above Kth element visitor. It computes in linear time.
T: Column data type
I: Index type
A: Memory alignment boundary for vectors. Default is system default alignment
static void test_median()  {

    std::cout << "\nTesting Median ..." << std::endl;

    std::vector<unsigned long>  idx =
        { 123450, 123451, 123452, 123453, 123454, 123455, 123456, 123457, 123458, 123459, 123460, 123461, 123462, 123466,
          123467, 123468, 123469, 123470, 123471, 123472, 123473 };
    std::vector<double>         d1 = { 1.0, 10, 8, 18, 19, 16, 21, 17, 20, 3, 2, 11, 7.0, 5, 9, 15, 14, 13, 12, 6, 4 };
    std::vector<double>         d2 = { 1.0, 10, 8, 18, 19, 16, 17, 20, 3, 2, 11, 7.0, 5, 9, 15, 14, 13, 12, 6, 4 };
    std::vector<int>            i1 = { 1, 10, 8, 18, 19, 16, 21, 17, 20, 3, 2, 11, 7, 5, 9, 15, 14, 13, 12, 6, 4 };
    std::vector<int>            i2 = { 1, 10, 8, 18, 19, 16, 17, 20, 3, 2, 11, 7, 5, 9, 15, 14, 13, 12, 6, 4 };
    MyDataFrame                 df;

    df.load_data(std::move(idx), std::make_pair("dblcol_1", d1), std::make_pair("intcol_1", i1));
    df.load_column("dblcol_2", std::move(d2), nan_policy::dont_pad_with_nans);
    df.load_column("intcol_2", std::move(i2), nan_policy::dont_pad_with_nans);

    MedianVisitor<double>   med_visit;
    double                  result = df.single_act_visit<double>("dblcol_1", med_visit).get_result();

    assert(result == 10.0);

    result = df.single_act_visit<double>("dblcol_2", med_visit).get_result();
    assert(result == 10.50);

    MedianVisitor<int>  med_visit2;
    int                 result2 = df.single_act_visit<int>("intcol_1", med_visit2).get_result();

    assert(result2 == 10);

    result2 = df.single_act_visit<int>("intcol_2", med_visit2).get_result();
    assert(result2 == 10);
}
C++ DataFrame