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

template<typename T, typename I = unsigned long>
struct AbsVisitor;
This is a transformer visitor. It means the column(s) passed to this visitor is not read-only and its values may change

This visitor sets negative numeric values to their absolute values.
The get_result() method returns the number of items changed.

 T: Column data type
I: Index type 
static void test_AbsVisitor()  {

    std::cout << "\nTesting AbsVisitor{ } ..." << std::endl;

    MyDataFrame df;

    std::vector<unsigned long>  idxvec =
        { 1UL, 2UL, 3UL, 10UL, 5UL, 7UL, 8UL, 12UL, 9UL, 12UL, 10UL, 13UL, 10UL, 15UL, 14UL };
    std::vector<double>         dblvec =
        { 0.0, -15.0, 14.0, 15.0, -1.0, 12.0, 11.0, -8.0, 15.0, 6.0, -1, 4.0, 14.0, 14.0, -20.0 };
    std::vector<int>            intvec =
        { -1, 2, 3, 4, 5, 8, -6, 7, 11, -14, 9, -3, -5, -4, 9 };
    std::vector<std::string>    strvec =
        { "zz", "bb", "zz", "ww", "ee", "ff", "gg", "zz", "ii", "jj", "kk", "ll", "mm", "ee", "" };

    df.load_data(std::move(idxvec),
                 std::make_pair("dbl_col", dblvec),
                 std::make_pair("int_col", intvec),
                 std::make_pair("str_col", strvec));

    AbsVisitor<double>  abs_v;
    AbsVisitor<int>     abs_v_int;
    auto                result = df.visit<double>("dbl_col", abs_v).get_result();
    auto                result_int = df.single_act_visit<int>("int_col", abs_v_int).get_result();

    assert(result == 5);
    assert(result_int == 6);

    std::vector<double> abs_dblvec =
        { 0.0, 15.0, 14.0, 15.0, 1.0, 12.0, 11.0, 8.0, 15.0, 6.0, 1, 4.0, 14.0, 14.0, 20.0 };
    std::vector<int>    abs_intvec =
        { 1, 2, 3, 4, 5, 8, 6, 7, 11, 14, 9, 3, 5, 4, 9 };

    assert((df.get_column<double>("dbl_col") == abs_dblvec));
    assert((df.get_column<int>("int_col") == abs_intvec));
}
C++ DataFrame