Question

Basically I want to know I a user chooses from my settings what day they want a week to start on (This would have a value in the format, @”Mon” for example) and I have an NSMutableArray that holds NSDates (users add theses dates throughout the life of the app) ordered in a chronologically ascending manner, but not necessarily consecutive (there could be day’s that users missed and did not add the date to the array), how could I determine that a particular NSDate in the array is part of a different week (relative to the date the user chose for a week to start on)?

Answer 1

This will give you the difference between two dates in weeks:

NSDateComponents *dateDifference = [gregorian components:NSWeekCalendarUnit fromDate:day1 toDate:day2 options:0];
NSUInteger weeksDiff = [dateDifference week];

So if(!weekDiff){ /*same week*/}

Complete example:

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

NSDateComponents *comps1 = [[NSDateComponents alloc] init];
NSDateComponents *comps2 = [[NSDateComponents alloc] init];
[comps1 setDay:5];
[comps2 setDay:12];

NSDate *day1 = [gregorian dateByAddingComponents:comps1 toDate:[NSDate date] options:0];
NSDate *day2 = [gregorian dateByAddingComponents:comps2 toDate:[NSDate date] options:0];


NSDateComponents *dateDifference = [gregorian components:NSWeekCalendarUnit fromDate:day1 toDate:day2 options:0];

NSLog(@"\n%@ \n%@\n%d", day1, day2, [dateDifference week]);
[comps1 release];
[comps2 release];
[gregorian release];

For converting strings to NSDates, see NSDateFormatter

edit as response to comment
if you want to have a week that starts with the first date you pass in, you could instead count the days and check if they have more than 7 days difference.

NSDateComponents *dateDifference = [gregorian components:NSDayCalendarUnit fromDate:day1 toDate:day2 options:0];
NSUInteger daysDiff = [dateDifference day];
if(weekDiff < 7){ /*same week*/}