我的程序不断崩溃，我不知道为什么

Question

I am new to all this but I am wondering if I can receive a little help. I am trying to get the days left from a startDate and endDate. I thought I found the answer in listing #12 in the Apple docs here ---> link

After scouring stackoverflow, this is what I came up with.

    //My Code (This is code I found on stack overflow and modified
    //Start Date
    //Creating a date from a string then converting it into an NSDate Object
    NSString *startDateString = @"2016-01-01 00:00:00";
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    dateFormatter.dateFormat = @"YYYY-MM-dd HH:mm:ss";
    NSDate *startDate = [dateFormatter dateFromString:startDateString];

    //End Date
    NSString *endDateString = @"2016-12-25 00:00:00";
    NSDateFormatter *endDateFormatter = [[NSDateFormatter alloc] init];
    dateFormatter.dateFormat = @"YYYY-MM-dd HH:mm:ss";
    NSDate *endDate = [endDateFormatter dateFromString:endDateString];


    //This is Apple's code.

    NSCalendar *gregorian = [[NSCalendar alloc]
                             initWithCalendarIdentifier:NSCalendarIdentifierGregorian];

    NSUInteger unitFlags = NSCalendarUnitMonth | NSCalendarUnitDay;

    NSDateComponents *components = [gregorian components:unitFlags
                                                fromDate:startDate
                                                  toDate:endDate options:0];
    NSInteger months = [components month];
    NSInteger days = [components day];


    NSLog(@"there are %ld months and %ld days left till Christmas", months, days);

The program keeps crashing at,

 NSDateComponents *components = [gregorian components:unitFlags
                                                fromDate:startDate
                                                  toDate:endDate options:0];

The question,

Can anyone guide me on how to resolve this issue?

Answer 1

Simples. What dateFormat did you send for endDateFormatter?

I'm not asking what dateFormat you think you did set, but which format you did set. Two different questions with entirely different answers.

Now you ask if we can guide you how to resolve this:

Learn to use the debugger. The obvious thing to do is step through the code line by line, and examine each value that is produced. You would then notice before components:fromDate:toDate: crashes, that your endDate is nil.

Most importantly: Learn the first rule of debugging: You did something wrong, and you need to find out what it is. That reduces the possibilities so much. There are only 11 lines of code before the crash. One of them is wrong. One of them you wrote wrong. Just go through them one by one, with the firm conviction in your mind that one of the lines is wrong , and it would be obvious.

(That's how I found the bug. I looked at the code with the conviction that there is a mistake, and searched for the mistake, and with that attitude it's easy to find. Beginners are convinced their code is correct, and as a result they can't see the most obvious bugs).

Or examine your code variable by variable. Check how dateFormatter gets created and changed. Check how endDateFormatter gets created and changed. Anything unusual that you can see?

BTW. The YYYY vs yyyy bug produces lots of laughter among the more experienced developers and creates enormous frustration among the less experienced at the beginning of each year. Up to three days around New Year every year that bug causes many applications to display years wrong.

BTW. A better question than "how to find this problem" would be "how to avoid the problem". Maddy remarked (and I'd say 99% chance the it is correct) that this is a copy / paste error.

Ignoring the fact that two date formatters were not needed: After a copy / paste like that, I use the features of the editor. Obviously you need to change uses of startDateString, dateFormatter and startDate. The compiler even tells you, because immediately after the paste operation you have three re-declared variables.

What I do: Double-click on startDateString giving the first error. Command-Shift-E (make it the search string). Type in the new name endDateString, double-click on it and Command-C (copy). Command-G to find the next occurence of startDateString, Command-V (paste) to replace it with the new name, Command-G again until all uses of the name are changed. Then the same with the second error. Practice using your editor.