Exercise 15.1 years:months:etc. since date - method question

[Robot-Scott]

Ok, I'm having a really hard time wrapping my head around dates.  So far, I've only been able to figure out how to set the timezone, create two dates and display them via NSLog; like this:

Code: (Objective-C)

NSTimeZone *myzone = [NSTimeZone systemTimeZone];
	NSLog(@"The timezone is %@", [myzone description]);
	NSCalendarDate *startDate = [NSCalendarDate dateWithYear:2005 month:5 day:23 hour:13 minute:34 second:45 timeZone:myzone];
	NSLog(@"The first date is %@", [startDate description]);
	NSCalendarDate *endDate = [NSCalendarDate dateWithYear:2009 month:6 day:7 hour:23 minute:27 second:23 timeZone:myzone];
	NSLog(@"The second date is %@", [endDate description]);

What I can't figure out is how to use this method:

Code: (Objective-C)

- (void)years:(NSInteger *)yp months:(NSInteger *)mop days:(NSInteger *)dp hours:(NSInteger *)hp minutes:(NSInteger *)mip seconds:(NSInteger *)sp sinceDate:(NSCalendarDate *)date;

Any hints on plugging the two NSCalendarDates into the preceding method?

Also, how does one get the integer object year from a CalendarDate object (e.g. NSLog(@"%i", [startDate year]) would print "2005")?

Thanks.

[esc]

The method years:months:days:hours:minutes:seconds:sinceDate: returns values in its parameters.

So you set up two dates like this:

Code: (Objective-C)

NSCalendarDate *today = [NSCalendarDate dateWithYear:2009 month:7 day:5 hour:0 minute:0 second:0 timeZone:[NSTimeZone timeZoneWithAbbreviation:@"PST"]];

NSCalendarDate *yesterday = [NSCalendarDate dateWithYear:2009 month:7 day:4 hour:0 minute:0 second:0 timeZone:[NSTimeZone timeZoneWithAbbreviation:@"PST"]]

Then you use years:months:days:hours:minutes:seconds:sinceDate: to find out the difference in years/months/days/hours/minutes/seconds between the receiver and a given date, e.g.

Code: (Objective-C)

NSInteger secs;
[today years:NULL months:NULL days:NULL hours:NULL minutes:NULL seconds:&secs sinceDate:yesterday];
NSLog(@"number of seconds since yesterday=%i", secs);

[Robot-Scott]

Thank you, it worked!

Just a follow-up question: Why do you have to use the '&' character?

[esc]

Thank you, it worked!

Just a follow-up question: Why do you have to use the '&' character?

I figure you might ask that   Because you need to pass in the address of the variable (i.e. pass by reference) so that the method itself can store/change the value of the memory referenced by the variable that you passed in.  A hint you need to pass by reference is in the method parameters:
- (void)years:(NSInteger *)yearsPointer months:(NSInteger *)monthsPointer...

Note that parameter asked for (NSInteger *), not asking for a NSInteger, AND explanation of the parameters "Upon return":
yearsPointer
Upon return, contains the number of years between the receiver and date. Pass NULL to ignore this component.

monthsPointer
Upon return, contains the number of months between the receiver and date. Pass NULL to ignore this component.
...

Check out P. 283 for explanation of the & operator.

[Robot-Scott]

Thanks!

[webwrx]

Here's my answer to 15.1

Code: (Objective-C)

#import <Foundation/Foundation.h>


@interface NSCalendarDate (ElapsedDays)
-(NSInteger) numberOfElapsedDays: (NSCalendarDate *) theDate;
@end


@implementation NSCalendarDate (ElapsedDays)

-(NSInteger) numberOfElapsedDays: (NSCalendarDate *) theDate
{
	NSInteger days;
	
	[self years:NULL months:NULL days:&days  hours:NULL
			   minutes:NULL seconds:NULL sinceDate:theDate];
	
	return days;
}

@end


int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
	
        // I used different time zones as a point of interest.
	// e.g. if you make them both PST you'll get a different result
	
	NSCalendarDate *dateOfBirth = [NSCalendarDate dateWithYear:1938 month:3 day:25 hour:23 minute:30 second:0 timeZone:[NSTimeZone timeZoneWithAbbreviation:@"EST"]];
	
	NSCalendarDate *dateOfDeath = [NSCalendarDate dateWithYear:2009 month:8 day:22 hour:23 minute:5 second:0 timeZone:[NSTimeZone timeZoneWithAbbreviation:@"PST"]];
	
	NSInteger daysSince;

	daysSince = [dateOfDeath numberOfElapsedDays: dateOfBirth];
	NSLog (@"The person lived for %li days.", daysSince);
		
	[pool drain];
    return 0;
}