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Signature Description Parameters 
template<typename ... Ts>
DataFrame
get_data_every_n(size_type n, size_type starting_idx = 0) const;
It returns a new DataFrame (including the index and data columns) starting from starting_idx and including every other n datapoint. If a data column is shorter than the index column, it will be also shorter in the returned DataFrame.
 Ts: List all the types of all data columns. A type should be specified in the list only once.
n: Include the data every other n index
starting_idx: Starting point of selection
 
template<typename ... Ts>
PtrView
get_view_every_n(size_type n, size_type starting_idx = 0);
It behaves like get_data_every_n, but it returns a View. A view is a DataFrame that is a reference to the original DataFrame. So if you modify anything in the view the original DataFrame will also be modified.

NOTE: There are certain operations that you cannot do with a view. For example, you cannot add/delete columns, etc.
NOTE: The columns in the result are not padded with NaN.
NOTE: Views could not be const, because you can change original data through views.
 Ts: List all the types of all data columns. A type should be specified in the list only once.
n: Include the data every other n index
starting_idx: Starting point of selection
 
template<typename ... Ts>
ConstPtrView
get_view_every_n(size_type n, size_type starting_idx = 0) const;
It behaves like get_view_every_n, but it returns a const View.
 Ts: List all the types of all data columns. A type should be specified in the list only once.
n: Include the data every other n index
starting_idx: Starting point of selection
 
static void test_get_data_every_n()  {

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

    MyDataFrame                 df;
    std::vector<unsigned long>  idxvec = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21 };
    std::vector<double>         dblvec = { 0, 15, -14, 2, 1, 12, 11, 8, 7, 6, 5, 9, 10, 1, 2, 3, 4, 5 };
    std::vector<double>         dblvec2 = { 100, 101, 102, 103, 104, 105, 106.55, 107.34, 1.8, 111, 112, 113, 114, 115, 116, 50, 51, 52, 53, 54, 55 };
    std::vector<int>            intvec = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };
    std::vector<std::string>    strvec = { "zz", "bb", "ww", "ee", "ff", "gg", "hh", "ii", "jj", "kk", "ll", "oo" };

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

    auto    df2 = df.get_data_every_n<double, std::string, int>(3, 1);

    assert(df2.get_index().size() == 7);
    assert(df2.get_index()[0] == 2);
    assert(df2.get_index()[3] == 11);
    assert(df2.get_index()[6] == 20);

    assert(df2.get_column<double>("dbl_col").size() == 7);
    assert(df2.get_column<double>("dbl_col")[0] == 15.0);
    assert(df2.get_column<double>("dbl_col")[3] == 5.0);
    assert(std::isnan(df2.get_column<double>("dbl_col")[6]));

    assert(df2.get_column<double>("dbl_col_2").size() == 7);
    assert(df2.get_column<double>("dbl_col_2")[0] == 101.0);
    assert(df2.get_column<double>("dbl_col_2")[3] == 112.0);
    assert(df2.get_column<double>("dbl_col_2")[6] == 54.0);

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

    assert(df2.get_column<std::string>("str_col").size() == 4);
    assert(df2.get_column<std::string>("str_col")[0] == "bb");
    assert(df2.get_column<std::string>("str_col")[1] == "ff");
    assert(df2.get_column<std::string>("str_col")[3] == "ll");

    const auto  view = df.get_view_every_n<double, std::string, int>(3, 1);

    assert(view.get_index().size() == 7);
    assert(view.get_index()[0] == 2);
    assert(view.get_index()[3] == 11);
    assert(view.get_index()[6] == 20);

    assert(view.get_column<double>("dbl_col").size() == 7);
    assert(view.get_column<double>("dbl_col")[0] == 15.0);
    assert(view.get_column<double>("dbl_col")[3] == 5.0);
    assert(std::isnan(view.get_column<double>("dbl_col")[6]));

    assert(view.get_column<double>("dbl_col_2").size() == 7);
    assert(view.get_column<double>("dbl_col_2")[0] == 101.0);
    assert(view.get_column<double>("dbl_col_2")[3] == 112.0);
    assert(view.get_column<double>("dbl_col_2")[6] == 54.0);

    assert(view.get_column<int>("int_col").size() == 3);
    assert(view.get_column<int>("int_col")[0] == 2);
    assert(view.get_column<int>("int_col")[1] == 5);
    assert(view.get_column<int>("int_col")[2] == 8);

    assert(view.get_column<std::string>("str_col").size() == 4);
    assert(view.get_column<std::string>("str_col")[0] == "bb");
    assert(view.get_column<std::string>("str_col")[1] == "ff");
    assert(view.get_column<std::string>("str_col")[3] == "ll");
}

C++ DataFrame