this is the header file .h:, note the std::string& means to modify the string value, while removing the ampersand meaning you create a copy to modify. note 2, std::vector<std::string> means the type is vector and components are string, the components are always angle bracketed, another example // Modern C++ std::vector<std::vector<int>>  

#pragma once
#include <string>
#include <vector>
#include <map>
#include <unordered_map>
#include <DataFrame/DataFrame.h>

class GlobalMonopoly {
public:
    global Monopoly(const std::string& start_date, int periods);
    void processMonopolies();
    void saveToExcel(const std::string& filename);
    void verifyData();

private:
    struct MonopolyData {
        DataFrame<double> market_share;
        DataFrame<double> monopolies;
    };

    std::vector<std::string> dates;
    std::vector<std::string> leel_list{"L6", "L5"};
    std::unordered_map<std::string, std::map<std::string, MonopolyData>> history_dfs;
    std::map<std::string, MonopolyData> monopoly_dfs;

    DataFrame<double> getMarketShareData(const std::srting& day, const std::string& level);
    DataFrame<double> getApiData(const std::vector<std::string>& idList, const std::string& day);
    void capWeight(DataFrame<double>& df, double cap);
    std::vector<std::string> generateDates(const std::string& start_date, int periods);
  };

here is the cpp source files: note when to use curly bracket in C++? curly brackets {} are used for initialization in several common scenarios. Initializer Lists – Direct initialization of arrays, vectors, or other containers, for example std::vector<std::string> level_list{“L6”, “L5”};  // Modern C++ style or Class/Struct Member Initialization: struct Person {std::string name{“John”};  // Default member initialization   int age{25}; };

#include "global_monopoly.h"
#include <iostream>
#include <iomanip>
#include <cmath>
#include <xlnt/xlnt.hpp>  // Using xlnt for Excel operations

GlobalMonopoly:: globalMonopoly(const std::string& start_date, int periods) {
    dates = generateDates(start_date, periods);
}
void GlobalMonopoly::processMonopolies() {
    for (const auto& day : dates) {
        std::map<std::string, MonopolyData> day_monopolies;
        for (const auto& level : level_list) {
            std::cout << "Processing " << day << "  and " << level << std::endl;
            //get market share data
           auto market_share_df = getMarketShareData(day, level);
           //store results
          day_monopolies[level] = MonopolyData{market_share_df, monopolies_df};
          std::cout << "number of monopolies in"" << level << ": "
                        << monopolies_df.rows() << std::endl;
      }
      history_dfs[day] = day_monopolies;
      if (day == dates.back()) {
          monopoly_dfs = day_monopolies;
       }
  }
}
void globalMonopoly::saveToExccel(const std::string& filename) {
    xlnt::workbook wb;

    std::map<std::string DataFrame<double>> histories;
    for (const auto& level : {"L6", "L5"}) {
        DataFrame<double> history_df;

        for (const auto& date : dates) {
          auto df = history_dfs[date][level].monopolies;
       }
      histories[level[ = history_df;
      //create worksheet and write data
      auto ws = wb.create_sheet("Level " + std::string(level));
   }
   wb.save(filename);
 }
void GlobalMonopoly::verifyData() {
      for (const auto& [level, history_df] : histories) {
         std::cout << "\nchecking " << level << " history:\n";
      //check size