Suggestions for beginning steps to build a calendar program in C [closed]

问题

Closed. This question needs to be more focused. It is not currently accepting answers.

---

Want to improve this question? Update the question so it focuses on one problem only by editing this post.

Closed 2 years ago.

Improve this question

I am working on a practice project of my school, which is to build a C program that resembles a calendar in linux or Mac command lines. The program doesn't have to implement all the methods that a calendar has, but it should produce the same output for these arguments

$ cal -m 11 2010
$ cal -m 13 2011
$ cal -m mar 2012
$ cal -m maRc 2013
$ cal -m MARCH 2014
$ cal -m MARCHY 2015
$ cal -m 5
$ cal 18
$ cal 2018
$ cal

How would I approach this? Is there any suggestion for beginning steps?

I would really appreciate any help.

回答1:

This is a good challenge, and quite difficult if you approach it from first principles. You asked for suggestions, so I'll be happy to give you a few.

First, write code to display a calendar at all. You probably want it to look something like this:

Su Mo Tu We Th Fr Sa
 1  2  3  4  5  6  7
 8  9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30

For your first implementation, assume that the first of the month is a Sunday. For your first implementation, either have the length of the month be hardwired at 30 days, or have it be a command-line argument. Make sure the code works for months of length 28, 29, 30, or 31 days.

This first subproblem obviously just involves some well-chosen loops and printf calls. But this will keep you busy for a while, especially if you're a beginner.

Next, figure out how to make it work for months where the first day of the month isn't Sunday. You'll probably want to make it look something like this:

Su Mo Tu We Th Fr Sa
          1  2  3  4
 5  6  7  8  9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30

Again, this obviously just involves loops and printf calls. Again (for now), have the first-day indication hardwired, or passed in from the command line. Make sure it works for all seven possibilities. Depending on how you wrote your first program (the one that assumed Sunday), you might be able to make a few incremental modifications, but if there's no clean way to add this variability, don't be afraid to throw just about everything away and start over.

Finally, we come to the hard part. (But don't worry too much if you thought the preceding work was plenty hard -- this next step is hard in a different way.) Obviously to make a real calendar program, we're going to have to stop hardwiring things, and start computing the actual length of an actual month, and the day of the week on which it starts.

There's no function (in C and Unix, at least) that directly tells you how many days there are in a given month. There's no function that directly tells you the day of the week for the first of the month.

It's straightforward to hardwire a 12-element array containing the number of days in each month, although you obviously have to do something special for February in leap years. (See here for tips on accurately computing which years are leap years.) But how to figure out which day of the week a month starts on? This is the same problem as "How can I find the day of the week given the date?", where the date is January 1, or February 1, or whatever month it is you're working on. I think there are two general approaches for solving this problem:

1. Work it out from first principles. My go-to algorithm for this problem is an old chestnut called "Zeller's congruence", although there are a few alternative chestnuts as well. The old C FAQ list has several of them in question 20.31.

2. Let the machine do the dirty work. To find the day of the week given the date, construct a struct tm containing the desired date in the tm_year, tm_mon, and tm_mday fields. (Remember that tm_mon is 0-based, and tm_year is 1900-based.) Fill in tm_hour, tm_min, and tm_sec with 12:00:00. Fill in tm_isdst with 0 or -1. Call mktime. On output, tm_wday will contain the day of the week for that date.

Lastly, if you've gotten this far, you can worry about the command line input format. If it was me, I'd settle for just parsing

cal 7 2018

to accept the month and year number as integers. If you want to accept all the other variants you mentioned, that's going to be a certain amount of somewhat tedious, fussy work. There are lots of ways to do it; I'll let you discover some techniques that work for you.

Finally, as an extra-credit challenge (and this is a real challenge!), as Bathsheba suggested in a comment, try making your program do the same thing as the standard Unix/Linux cal command does if you invoke it as

cal 9 1752

来源：https://stackoverflow.com/questions/51188738/suggestions-for-beginning-steps-to-build-a-calendar-program-in-c