Question

I have an NSArray containing date/time NSStrings in the following format:

2/2/2011 2:46:39 PM
2/4/2011 11:59:47 AM
…

where the date is represented as month/day/year.

How do I sort this NSArray making sure the newest date/times are at the top?

Answer 1

When you’re dealing with dates, use NSDate instead of NSString. Also, it’s important to consider the time zone — does the Web service provide dates in UTC or some other time zone?

You should first convert your array of strings into an array of dates. Otherwise, you’d be converting a string to a date whenever it is used for comparison, and there will be more comparisons than the number of strings.

For example:

NSDateFormatter *formatter = [[[NSDateFormatter alloc] init] autorelease];
[formatter setDateFormat:@"MM/DD/YYYY hh:mm:ss a"];

NSMutableArray *dateArray = [NSMutableArray array];
for (NSString *dateString in array) {
    NSDate *date = [formatter dateFromString:dateString];
    if (date) [dateArray addObject:date];
    // If the date is nil, the string wasn't a valid date.
    // You could add some error reporting in that case.
}

This converts array, an array of NSStrings, to dateArray, a mutable array of NSDates. The date formatter uses the system time zone. If you want to use UTC as the time zone:

NSDateFormatter *formatter = [[[NSDateFormatter alloc] init] autorelease];
[formatter setDateFormat:@"MM/DD/YYYY hh:mm:ss a"];
[formatter setTimeZone:[NSTimeZone timeZoneWithAbbreviation:@"UTC"]];

Having done that, sorting the array is trivial:

[dateArray sortUsingSelector:@selector(compare:)];