Market Analysis I solution and explanation

import pandas as pd

users = pd.DataFrame({
    'user_id': [1, 2, 3, 4],
    'join_date': ['2018-01-01', '2018-02-09', '2018-01-19', '2018-05-21'],
    'favorite_brand': ['Lenovo', 'Samsung', 'LG', 'HP']
})

orders = pd.DataFrame({
    'order_id': [1, 2, 3, 4, 5, 6],
    'order_date': pd.to_datetime(['2019-08-01', '2018-08-02', '2019-08-03', '2018-08-04', '2018-08-04', '2019-08-05']),
    'item_id': [4, 2, 3, 1, 1, 2],
    'buyer_id': [1, 1, 2, 4, 3, 2],
    'seller_id': [2, 3, 3, 2, 4, 4]
})

# Filter orders for the year 2019
orders_2019 = orders[orders['order_date'].dt.year == 2019]

# Count orders per user
order_counts_2019 = orders_2019.groupby('buyer_id').size().reset_index(name='orders_in_2019')

# Merge with users data
total = pd.merge(users, order_counts_2019, how='left', left_on='user_id', right_on='buyer_id').fillna(0)

total['orders_in_2019'] = total['orders_in_2019'].astype(int)

# Select relevant columns
result = total[['user_id', 'join_date', 'orders_in_2019']]

print(result)

/* C solution is typically not used directly for SQL tasks but if simulating: initialize structures for Users and Orders, then proceed with filtering and counting orders per user in C code. */

SELECT u.user_id AS buyer_id, u.join_date, COUNT(o.order_id) AS orders_in_2019 FROM Users u LEFT JOIN (SELECT buyer_id, order_id FROM Orders WHERE YEAR(order_date) = 2019) o ON u.user_id = o.buyer_id GROUP BY u.user_id, u.join_date;

import java.util.*;

class Solution {
    public List<Map<String, Object>> analyzeMarket(List< Map<String, Object>> users, List<Map<String, Object>> orders) {
        Map<Integer, Integer> orderCountMap = new HashMap<>();

        for (Map<String, Object> order : orders) {
            Date orderDate = (Date) order.get("order_date");
            Calendar cal = Calendar.getInstance();
            cal.setTime(orderDate);
            int year = cal.get(Calendar.YEAR);

            if (year == 2019) {
                int buyerId = (Integer) order.get("buyer_id");
                orderCountMap.put(buyerId, orderCountMap.getOrDefault(buyerId, 0) + 1);
            }
        }

        List<Map<String, Object>> result = new ArrayList<>();

        for (Map<String, Object> user : users) {
            int userId = (Integer) user.get("user_id");
            String joinDate = (String) user.get("join_date");

            Map<String, Object> entry = new HashMap<>();
            entry.put("buyer_id", userId);
            entry.put("join_date", joinDate);
            entry.put("orders_in_2019", orderCountMap.getOrDefault(userId, 0));

            result.add(entry);
        }

        return result;
    }
}

#include <iostream>
#include <vector>
#include <unordered_map>
#include <string>
#include <ctime>

void analyzeMarket(const std::vector<std::tuple<int, std::string, std::string>>& users,
                   const std::vector<std::tuple<int, std::string, int, int, int>>& orders) {
    std::unordered_map<int, int> orderCountMap;

    for (const auto& order : orders) {
        std::string orderDateStr;
        int buyerId;        
        std::tie(std::ignore, orderDateStr, std::ignore, buyerId, std::ignore) = order;

        std::tm tm = {};
        std::istringstream ss(orderDateStr);
        ss >> std::get_time(&tm, "%Y-%m-%d");

        if (tm.tm_year + 1900 == 2019) {
            orderCountMap[buyerId]++;
        }
    }

    for (const auto& user : users) {
        int userId;
        std::string joinDate;
        std::tie(userId, joinDate, std::ignore) = user;

        std::cout << "buyer_id: " << userId << " join_date: " << joinDate << " orders_in_2019: " << orderCountMap[userId] << std::endl;
    }
}