我应该使用哪种数据类型作为 STL 映射的键？

Question

I'm creating a program that reads some records from a .csv file and stores it in a map. My key is supposed to have two attributes, year and month. But in some cases, I'm asked to print the data of the entire year and in that case, I will not have the option to enter the month (ie month=0). So which data type should be best in this case? At first, I thought of integers but, it would not work if I'm asked to print the details of the entire year. I think probably using a user-defined type object would work, however, I'm a little confused about iterating the map.

Answer 1

Sounds like your Month and Year are two independent keys, and that sometimes you are looking for non-unique results (eg year specified, month not specified). Simple solution is to store your records in std::map<Year, std::map<Month, std::vector<Records> > my_data ; Then

• to get the records from the particular Year , you can iterate through the result of my_data[Year("2020")]
• to get the records from the particular Month (regardless the Year ), for (const auto& el: my_data) doSomething(el.second[Month("September")]); = to get the results from the particular Year and Month my_data[Year("2010")][Month("December")]

Since C++20 you can use year and month from the standard C++ library; Before that you can just define Year and Month structures and provide "less than" operator.