Input year, then print calendar

Question

Hi, I am stuck on the following problem from my C programming class:

Write a program that prompts the user to input a year, and then outputs the calendar(for the entire year).

I have no idea how to approach this problem. I can usually start my homework problems (this is an optional challenge problem), but I am really lost. We've worked through chapters 1-10 of Deitel and Deitel (loops, arrays, pointers, I/O, etc), but I don't know how to approach this at all. Any hints or suggestsions would be appreciated.

Thanks! Alexis

Answer 1

A good start may be the localtime(3) and mktime(3) functions. Alternatively, you can implement the relevant date arithmetic from scratch. Then, simply, generate the first line of the calendar (find the weekday that corresponds to January 1st, then print taht in the right place, followed by the rest of the week), then print all but the last lines, then print the last line.

Depending on if you want a calendar paginated by month or not, this MAY be better done ona per-month instead of a per-year basis.

Answer 2

In general, when you have a big problem like this one, you want to break it down into little problems that are easier to solve.

Here's one possible little problem to start with: if you know how many days there are in a month, and what weekday the first of the month falls on, could you output a calendar for that month?

Answer 3

The hardest part is determining which day of the week the year starts on.

http://en.wikipedia.org/wiki/Calculating_the_day_of_the_week

But even without that knowledge, when I first implemented this, I used a reference date (for example, you know that today, January 11, 2010 is a Monday) and counted days from there. (Just keep in mind that leap years have an extra day, and that leap years are every 4 years except every 100 years except every 400 years.)

http://en.wikipedia.org/wiki/Leap_year

Answer 4

Well, first figure out the algorithmic part of your problem - given a year, find what day Jan 1st is.

After this, just note the number of days in each month (store it in an array, say num_days[]), and the note the number of months in a year and an array of strings for the months.

For e.g. the outermost loop iterates over the months. Say, iterate for(i=0;i<NumMonths;++i). Then, for each month, print the string, e.g. month[i], then a newline.

Then, with simple tabs, print Sun Mon Tue ..., and another newline.

Then using the day Jan 1st corresponds to (call it FirstDay), insert spaces, and start with that day. Keep printing the dates and newlines till you hit max_month[i] which is 31 (for January). Store the name of day of the last day of the previous month and just reiterate treating that day as FirstDay.

Answer 5

You need a couple pieces to start. First, you need a formula that computes the day of the week for January 1 of whatever year is entered. You'll also need a formula to determine if the year is a leap year. Both of these formulae are easily found with a simple Google search. The third item you need is a simple array containing the number of days in each of the 12 months for a non-leap year.

Once you have these things, its trivial to determine the week day for each month of the year. Make sure to account for February 29 in a leap year. From there, you just need to create a function that primts out the monthly calendar in a form that looks similar to the calendar hanging on the wall. Try sketching out the desired layout on paper first and use that as a template for creating the appropriate format statements.

Answer 6

Might check out the doomsday algorithm. This would get you certain "dooms days" like Jan 31 is a doomsday, for 2008, that was a saturday. You can work backwords from there

Answer 7

Does this code qualify? :-)

char command[]="cal 2010";
sprintf(command,"cal %d",argv[1]);
system(command);

It assumes a Unix machine with cal in the path.

Answer 8

Basically, there are two approaches:

1. The easy / pragmatical way: Solve the task, and forget about everything else. Here you may check documentation for mktime() (you'll find an example based on mktime below..).

2. The scientific / engineering way: Learn to know how it works! You can start at the great wikipedia article about the gregorian calendar. Read it, understand it, and write code that implements the underlying algorithms (which are known, no rocket science, it is possible). This will improve your skills a lot (in fact, you really should do such a thing, maybe not the calender but another topic, it will give you a big leap in understanding all things).

Now some pragmatic code to start with. mktime() has a great feature: it knows the calender details, and it accepts e.g. a date "2010-01-60" and will convert it into February 29th, 2010. But, this will only work for dates after 1970. It will not work for earlier dates (though I am not 100% sure, but it shouldn't work, because unix time starts at January 1st, 1970, but try with other dates, maybe mktime() is not restricted to unix time).

Pseudo-Code, this prints each day on a single line (YYYY-MM-DD):

void print_cal( int year ) {
    static char weekdays[] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };
    struct tm tm;
    for( int day=0; day<365; ++day ) {
        memset( &tm, 0, sizeof(tm) );
        tm.tm_year = year - 1900;
        tm.tm_mday = day;
        mktime( &tm ); // modifies tm

        printf( "%04d-%02d-%02d, %s\n", tm.tm_year, tm.tm_mon+1, tm.tm_mday, weekdays[tm.tm_wday] );
    }
}

This code ignores leap years. You still have to adjust it to be correct for leap years! Also the result is not very pretty yet, just one line per day.

EDIT: added output of weekdays.