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Signature Description Parameters 
template<typename ... Ts>
DataFrame<T, H>
change_freq(size_type new_freq,
            time_frequency time_unit =
                time_frequency::not_valid) const;
This returns a new DataFrame with the same index type. But the frequency of the index in the new DataFrame is according to the parameters of this call. If the frequency has increased the data is forward-filled from self. If the frequency has decreased, the last data point is copied from self.

NOTE: The index type must be either a numeric or DateTime or there will be a compilation assert
NOTE: Self must be already sorted by index in ascending order, or the behavior is undefined
 Ts: List all the types of all data columns. A type should be specified in the list only once.
new_freq: The new frequency expressed in a positive integer value. The Unit depends on the index type and time_unit parameter
time_unit: Time unit based on time_frequency enum. If index time is not DateTime, time_unit must be not_valid. If index time is DateTime, time_unit must not be not_valid. Otherwise, the behavior is undefined.
 
static void test_change_freq()  {

    std::cout << "\nTesting change_freq( ) ..." << std::endl;

    //
    // This test increases the frequnecy
    //

    MyDataFrame                 df;
    StlVecType<unsigned long>   idxvec = { 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 };
    StlVecType<double>          dblvec = { 0.0, 15.0, 14.0, 2.0, 1.0, 12.0, 11.0, 8.0, 7.0, 6.0 };
    StlVecType<double>          dblvec2 = { 100.0, 101.0, 102.0, 103.0 };
    StlVecType<int>             intvec = { 1, 2, 3, 4, 5 };
    StlVecType<std::string>     strvec = { "aaa", "bbb", "ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii", "jjj" };

    df.load_data(std::move(idxvec),
                 std::make_pair("dbl_col", dblvec),
                 std::make_pair("dbl_col_2", dblvec2),
                 std::make_pair("str_col", strvec));
    df.load_column("int_col", std::move(intvec), nan_policy::dont_pad_with_nans);

    const auto  df2 = df.change_freq<double, int, std::string>(1);

    assert(df2.get_index().size() == 46);
    assert(df2.get_column<double>("dbl_col").size() == 46);
    assert(df2.get_column<double>("dbl_col_2").size() == 46);
    assert(df2.get_column<int>("int_col").size() == 46);
    assert(df2.get_column<std::string>("str_col").size() == 46);
    assert(df2.get_index()[0] == 0);
    assert(df2.get_index()[45] == 45);

    assert(df2.get_column<double>("dbl_col")[0] == 0);
    assert(df2.get_column<double>("dbl_col")[4] == 0);
    assert(df2.get_column<double>("dbl_col")[7] == 15.0);
    assert(df2.get_column<double>("dbl_col")[28] == 12.0);
    assert(df2.get_column<double>("dbl_col")[44] == 7.0);
    assert(df2.get_column<double>("dbl_col")[45] == 6.0);

    assert(df2.get_column<double>("dbl_col_2")[0] == 100.0);
    assert(df2.get_column<double>("dbl_col_2")[4] == 100.0);
    assert(df2.get_column<double>("dbl_col_2")[7] == 101.0);
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[28]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[44]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[45]));

    assert(df2.get_column<std::string>("str_col")[0] == "aaa");
    assert(df2.get_column<std::string>("str_col")[4] == "aaa");
    assert(df2.get_column<std::string>("str_col")[7] == "bbb");
    assert(df2.get_column<std::string>("str_col")[28] == "fff");
    assert(df2.get_column<std::string>("str_col")[44] == "iii");
    assert(df2.get_column<std::string>("str_col")[45] == "jjj");

    assert(df2.get_column<int>("int_col")[0] == 1);
    assert(df2.get_column<int>("int_col")[4] == 1);
    assert(df2.get_column<int>("int_col")[7] == 2);
    assert(df2.get_column<int>("int_col")[28] == 5);
    assert(df2.get_column<int>("int_col")[44] == 5);
    assert(df2.get_column<int>("int_col")[45] == 5);
}

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

static void test_change_freq_2()  {

    std::cout << "\nTesting change_freq_2( ) ..." << std::endl;

    //
    // This test increases the frequnecy
    //

    DTDataFrame             df;
    StlVecType<DateTime>    idxvec =
        { DateTime(20240702, 0), DateTime(20240702, 5), DateTime(20240702, 10), DateTime(20240702, 15),
          DateTime(20240702, 20), DateTime(20240703, 1), DateTime(20240703, 6), DateTime(20240703, 11), DateTime(20240703, 16), DateTime(20240703, 21) };
    StlVecType<double>      dblvec = { 0.0, 15.0, 14.0, 2.0, 1.0, 12.0, 11.0, 8.0, 7.0, 6.0 };
    StlVecType<double>      dblvec2 = { 100.0, 101.0, 102.0, 103.0 };
    StlVecType<int>         intvec = { 1, 2, 3, 4, 5 };
    StlVecType<std::string> strvec = { "aaa", "bbb", "ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii", "jjj" };

    df.load_data(std::move(idxvec),
                 std::make_pair("dbl_col", dblvec),
                 std::make_pair("dbl_col_2", dblvec2),
                 std::make_pair("str_col", strvec));
    df.load_column("int_col", std::move(intvec), nan_policy::dont_pad_with_nans);

    const auto  df2 = df.change_freq<double, int, std::string>(30, time_frequency::minutely);

    assert(df2.get_index().size() == 91);
    assert(df2.get_column<double>("dbl_col").size() == 91);
    assert(df2.get_column<double>("dbl_col_2").size() == 91);
    assert(df2.get_column<int>("int_col").size() == 91);
    assert(df2.get_column<std::string>("str_col").size() == 91);
    assert(df2.get_index()[0] == DateTime(20240702, 0));
    assert(df2.get_index()[89] == DateTime(20240703, 20, 30));
    assert(df2.get_index()[90] == DateTime(20240703, 21));

    assert(df2.get_column<double>("dbl_col")[0] == 0);
    assert(df2.get_column<double>("dbl_col")[9] == 0);
    assert(df2.get_column<double>("dbl_col")[17] == 15.0);
    assert(df2.get_column<double>("dbl_col")[55] == 12.0);
    assert(df2.get_column<double>("dbl_col")[80] == 7.0);
    assert(df2.get_column<double>("dbl_col")[90] == 6.0);

    assert(df2.get_column<double>("dbl_col_2")[0] == 100.0);
    assert(df2.get_column<double>("dbl_col_2")[9] == 100.0);
    assert(df2.get_column<double>("dbl_col_2")[17] == 101.0);
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[55]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[80]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[90]));

    assert(df2.get_column<std::string>("str_col")[0] == "aaa");
    assert(df2.get_column<std::string>("str_col")[9] == "aaa");
    assert(df2.get_column<std::string>("str_col")[17] == "bbb");
    assert(df2.get_column<std::string>("str_col")[55] == "fff");
    assert(df2.get_column<std::string>("str_col")[80] == "iii");
    assert(df2.get_column<std::string>("str_col")[90] == "jjj");

    assert(df2.get_column<int>("int_col")[0] == 1);
    assert(df2.get_column<int>("int_col")[9] == 1);
    assert(df2.get_column<int>("int_col")[17] == 2);
    assert(df2.get_column<int>("int_col")[55] == 5);
    assert(df2.get_column<int>("int_col")[80] == 5);
    assert(df2.get_column<int>("int_col")[90] == 5);
}

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

static void test_change_freq_3()  {

    std::cout << "\nTesting change_freq_3( ) ..." << std::endl;

    //
    // This test decreases the frequnecy
    //

    MyDataFrame                 df;
    StlVecType<unsigned long>   idxvec = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 };
    StlVecType<double>          dblvec = { 0.0, 15.0, 14.0, 2.0, 1.0, 12.0, 11.0, 8.0, 7.0, 6.0, 20.0, 21.0, 22.0, 23.0, 24.0, 25.0, 26.0 };
    StlVecType<double>          dblvec2 = { 100.0, 101.0, 102.0, 103.0, 104.0, 105.0, 106.0, 107.0 };
    StlVecType<int>             intvec = { 1, 2, 3, 4, 5, 6, 7, 8 };
    StlVecType<std::string>     strvec = { "aaa", "bbb", "ccc", "ddd", "eee", "fff", "ggg", "hhh", "iii", "jjj", "kkk", "lll", "mmm", "nnn", "ooo", "ppp", "qqq" };

    df.load_data(std::move(idxvec),
                 std::make_pair("dbl_col", dblvec),
                 std::make_pair("dbl_col_2", dblvec2),
                 std::make_pair("str_col", strvec));
    df.load_column("int_col", std::move(intvec), nan_policy::dont_pad_with_nans);

    const auto  df2 = df.change_freq<double, int, std::string>(3);

    assert(df2.get_index().size() == 7);
    assert(df2.get_column<double>("dbl_col").size() == 7);
    assert(df2.get_column<double>("dbl_col_2").size() == 7);
    assert(df2.get_column<int>("int_col").size() == 7);
    assert(df2.get_column<std::string>("str_col").size() == 7);
    assert(df2.get_index()[0] == 0);
    assert(df2.get_index()[6] == 16);

    assert(df2.get_column<double>("dbl_col")[0] == 0);
    assert(df2.get_column<double>("dbl_col")[1] == 2);
    assert(df2.get_column<double>("dbl_col")[2] == 11.0);
    assert(df2.get_column<double>("dbl_col")[3] == 6.0);
    assert(df2.get_column<double>("dbl_col")[4] == 22.0);
    assert(df2.get_column<double>("dbl_col")[5] == 25.0);
    assert(df2.get_column<double>("dbl_col")[6] == 26.0);

    assert(df2.get_column<double>("dbl_col_2")[0] == 100.0);
    assert(df2.get_column<double>("dbl_col_2")[1] == 103.0);
    assert(df2.get_column<double>("dbl_col_2")[2] == 106.0);
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[3]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[4]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[5]));
    assert(std::isnan(df2.get_column<double>("dbl_col_2")[6]));

    assert(df2.get_column<std::string>("str_col")[0] == "aaa");
    assert(df2.get_column<std::string>("str_col")[1] == "ddd");
    assert(df2.get_column<std::string>("str_col")[2] == "ggg");
    assert(df2.get_column<std::string>("str_col")[3] == "jjj");
    assert(df2.get_column<std::string>("str_col")[4] == "mmm");
    assert(df2.get_column<std::string>("str_col")[5] == "ppp");
    assert(df2.get_column<std::string>("str_col")[6] == "qqq");

    assert(df2.get_column<int>("int_col")[0] == 1);
    assert(df2.get_column<int>("int_col")[1] == 4);
    assert(df2.get_column<int>("int_col")[2] == 7);
    assert(df2.get_column<int>("int_col")[3] == 8);
    assert(df2.get_column<int>("int_col")[4] == 8);
    assert(df2.get_column<int>("int_col")[5] == 8);
    assert(df2.get_column<int>("int_col")[6] == 8);
}

C++ DataFrame