使用 C++ 打开动态 URL

Question

Got the following URL - https://barbaraperes.com/2017/08/31/2017_08_31.csv , being 2017 the year, 08 the month and 31 the day.

Currently I'm building a script that goes through all days, months and years from a given moment to the present time. For every day, the script changes a url (string) inserting the days, months and years, and opens it .

This is the current code

#include <time.h>
#include <string>
#include <iostream>

int main() {
    struct tm date;

    date.tm_year = 2007 - 1900; // tm_year (int) years since 1900
    date.tm_mon = 6; // tm_mon (int) months since January (0-11)
    date.tm_mday = 1; // tm_mday (int) day of the month (1-31)

    time_t end_date = time(NULL);

    std::cout << " =========================================================== \n";

    std::cout << "Let's go!\n";

    std::cout << " =========================================================== \n";

    for (; mktime(&date) < end_date; ++date.tm_mday) {

        std::cout << "Defining the new date... \n";

        char year[16];
        char month[16];
        char day[16];

        strftime(year, sizeof(year), "%Y", &date);
        strftime(month, sizeof(month), "%m", &date); // %m writes month as a decimal number (01-12)
        strftime(day, sizeof(day), "%d", &date);

        std::cout << "New date defined! \n";

        std::cout << "Year: " << year << "\n";
        std::cout << "Month: "<< month << "\n";
        std::cout << "Day: "<<  day << "\n\n";

        std::cout << "Make the url dynamic: \n";

        std::string url = "https://barbaraperes.com////__.csv";
        // https://barbaraperes.com/2017/08/31/2017_08_31.csv

        std::string str1 = year;
        std::string str2 = month;
        std::string str3 = day;

        url.insert(25, str1);
        url.insert(30, str2);
        url.insert(33, str3);
        url.insert(36, str1);
        url.insert(41, str2);
        url.insert(44, str3);

        std::cout << "This is the dynamic url: " << url << "\n\n";

    std::cout << " =========================================================== \n";
    }

    std::cout << " =========================================================== \n";

}   

It works fine until the day reaches 31, as we can see in the next image

Once the day reaches 31 (end of the first month which is July), it will start printing empty days and the month won't change to 08 (August)

How can we solve this?

Answer 1

Here's how you could step your days forward. This prints 400 days, starting at 2007/07/01.

#include <cstring>
#include <ctime>
#include <iostream>

int main() {
    std::tm date{};

    date.tm_year = 2007 - 1900; // tm_year (int) years since 1900
    date.tm_mon = 6;            // tm_mon (int) months since January (0-11)
    date.tm_mday = 1;           // tm_mday (int) day of the month (1-31)
    date.tm_hour = 12;          // mid day

    std::time_t current = std::mktime(&date);

    for(int i = 0; i < 400; ++i) {
        std::memcpy(&date, std::localtime(&current), sizeof(date));

        std::cout << date.tm_year + 1900 << " " << date.tm_mon + 1 << " " << date.tm_mday << "\n";

        current += 60 * 60 * 24; // add one day
    }
}

And here's a fancier version with support to create your URL:s on the fly:

#include <cstring>
#include <ctime>
#include <iostream>

struct mytm : std::tm {        // inherit "tm" to extend it a little
    mytm(int year, int month, int day) : tm{}, current{} {
        tm_year = year - 1900;
        tm_mon = month - 1;
        tm_mday = day;
        tm_hour = 12;
        current = std::mktime(this);
    }

    void add_days(int days) {
        current += days * 60 * 60 * 24;
        std::memcpy(this, std::localtime(&current), sizeof(std::tm));
    }

    std::string strftime(const char* format, std::size_t bufsize = 64) {
        std::string retval(bufsize, ' ');
        std::size_t len = std::strftime(&retval[0], retval.size(), format, this);
        retval.resize(len);
        return retval;
    }

    std::time_t current;
};

int main() {
    mytm date(2007, 7, 1);

    for(int i = 0; i < 400; ++i) {
        std::string url =
            date.strftime("https://barbaraperes.com/%Y/%m/%d/%Y_%m_%d.csv", 51);
        std::cout << url << "\n";
        date.add_days(1);
    }
}

Answer 2

You are incrementing the day after the loop condition is evaluated and yet you are relying on the call to mktime() in the loop condition to normalize your time.

The first thing you should do after incrementing the day is call mktime to normalize the time.

   for (; mktime(&date) < end_date; ++date.tm_mday) {
      mktime(&date);