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Signature Description Parameters 
template<typename T>
void
apply(const char *col_name,
      std::function<bool(const IndexType &,
                         T &)> &&func);
apply is a shortcut for a simple visit. It applies the func to every data point in the named column as long as func returns true. As soon as func returns false apply stops. func must receive a const reference to the index data point and a reference to the named column data point.
 T: Type of the named column
col_name: Name of the column
func: Function to be applied to the named column with the specified signature
 
template<typename T1, typename T2>
void
apply(const char *col_name1, const char *col_name2,
      std::function<bool(const IndexType &,
                         T1 &,
                         T2 &)> &&func);
Same as above but applied to two columns
 T1: Type of the first column
T2: Type of the second column
col_name1: Name of the first column
col_name2: Name of the second column
func: Function to be applied to the named column with the specified signature
 
template<typename T1, typename T2, typename T3>
void
apply(const char *col_name1, const char *col_name2,
      const char *col_name3,
      std::function<bool(const IndexType &,
                         T1 &,
                         T2 &,
                         T3 &)> &&func);
Same as above but applied to three columns
 T1: Type of the first column
T2: Type of the second column
T3: Type of the third column
col_name1: Name of the first column
col_name2: Name of the second column
col_name3: Name of the third column
func: Function to be applied to the named column with the specified signature
 
static void test_apply()  {

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

    MyDataFrame                 df;
    StlVecType<unsigned long>   idxvec = { 1UL, 2UL, 3UL, 10UL, 5UL, 7UL, 8UL, 12UL, 9UL, 12UL, 10UL, 13UL, 10UL, 15UL, 14UL };
    StlVecType<double>          dblvec = { 0.0, 15.0, -14.0, 2.0, 1.0, -12.0, 11.0, 8.0, 7.0, 0.0, 5.0, 4.0, 3.0, 9.0, -10.0 };
    StlVecType<double>          dblvec2 = { 1.0, 0.05, 0.28, 0.31, 0.01, 0.68, 0.12, 1, 0.98, 0.9, 0.81, 0.82, 0.777, 0.34, 0.25 };
    StlVecType<std::string>     strvec = { "zz", "bb", "zz", "ww", "ee", "ff", "gg", "hh", "zz", "jj", "kk", "ll", "mm", "nn", "zz" };

    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));

    auto    softp = [](const unsigned long &, double &val) -> bool  {
        val = std::log(1.0 + std::exp(val * 0.5)) / 0.5;
        return (true);
    };

    df.apply<double>("dbl_col", softp);

    const auto  &dbl_col = df.get_column<double>("dbl_col");

    {
        StlVecType<double>  out_res = { 1.38629, 15.0011, 0.00182293, 2.62652, 1.94815, 0.00495137, 11.0082, 8.0363, 7.0595, 1.38629, 5.15778, 4.25386, 3.40283, 9.0221, 0.0134307 };

        for (const auto &[ans, act] : std::ranges::views::zip(out_res, dbl_col))
            assert(std::fabs(ans - act) < 0.0001);
    }

    auto    haphaz = [](const unsigned long &, double &val1, double &val2, std::string &val3) -> bool  {
        if (val3 == "zz")
            val1 += val2;
        return (true);
    };

    df.apply<double, double, std::string>("dbl_col", "dbl_col_2", "str_col", haphaz);

    {
        StlVecType<double>  out_res = { 2.38629, 15.0011, 0.281823, 2.62652, 1.94815, 0.00495137, 11.0082, 8.0363, 8.0395, 1.38629, 5.15778, 4.25386, 3.40283, 9.0221, 0.263431 };

        for (const auto &[ans, act] : std::ranges::views::zip(out_res, dbl_col))
            assert(std::fabs(ans - act) < 0.0001);
    }

}

C++ DataFrame