如何从C ++中的tm对象获取星期几？

Question

I'm trying to write a program which parses a string representing a date formatted as YYYYMMDD (using strptime() ) and prints it in the form of dayOfWeek, Month Day, Year (using put_time() ). Here's what I have so far:

#include <iostream>
#include <sstream>
#include <ctime>
#include <iomanip>

using namespace std;

int main() {
    struct tm tm;
    string s("20131224");
    if (strptime(s.c_str(), "%Y%m%e", &tm)) {
        cout << put_time(&tm, "%A, %B %e, %Y") << endl;
    }
}

The problem is that the day of the week is always a Sunday, regardless of the date.

It appears to be a problem with strptime() not populating the day of the week information if only given a year, month, and day, and then put_time() not filling in this information either.

According to this documentation for strftime() , "missing fields in the tm structure may be filled in by strftime() if given enough information." I haven't found the same information regarding put_time() (which strftime() appears to be based on), so perhaps I'm expecting too much of the function.

Can strptime() automatically fill in the day of the week ( tm_wday ) on input, given a year, month, and day? Or can put_time() automatically fill this information on output? If not, is there another way to add this information to a tm object?

Answer 1

Here's a way to do it without using the C API, but instead the C++11/14 <chrono> facilities and this free, open-source, header-only library .

#include "date.h"
#include <iostream>
#include <sstream>

using namespace std;

int main() {
    istringstream s("20131224");
    date::sys_days tp;
    s >> date::parse("%Y%m%e", tp);
    if (!s.fail())
        cout << date::format("%A, %B %e, %Y", tp) << endl;
}

Output:

Tuesday, December 24, 2013

date::sys_days above is just a typedef for a std::chrono::system_clock::time_point but with a precision of days instead of whatever your platform provides (microseconds, nanoseconds, whatever). And that means you can easily add other durations to it, such as std::chrono::hours, minutes, seconds, milliseconds, etc.

cout << date::format("%A, %B %e, %Y %H:%M", tp + 2h + 35min ) << endl;

Tuesday, December 24, 2013 02:35

You can paste the above code into this wandbox demo and try it out for yourself for various versions of clang and gcc:

http://melpon.org/wandbox/permlink/PodYB3AwdYNFKbMv

Answer 2

Try this (compiled in Mac)

#include <iostream>

using namespace std;

int main(int argc, const char * argv[]) {
    const string DAY[]={"Sun","Mon","Tue",
        "Wed","Thu","Fri","Sat"};

    time_t rawtime;
    tm * timeinfo;
    time(&rawtime);
    timeinfo=localtime(&rawtime);

    int weekday=timeinfo->tm_wday;
    cout << "Today is: " << DAY[weekday] << "\n" << endl;
    return 0;
}