NSDateFormatter crash. How come?

Question

    NSDateFormatter *timeFormatter = [[[NSDateFormatter alloc] init] autorelease];
    [timeFormatter setDateFormat:@"h:mm a"];
    NSString *fullTime = [timeFormatter stringFromDate:someDateHere];
    NSArray *timeParts = [fullTime componentsSeparatedByString:@" "];
    timeLabel.text = [timeParts objectAtIndex:0];
    ampmLabel.text = [timeParts objectAtIndex:1];

The LAST line crashes with

    NSRangeException*** -[__NSArrayI objectAtIndex:]: index 1 beyond bounds [0 .. 0]

How is this possible? There is a nil check on the date that returns just before this code.

Answer 1

From the Data Formatting Guide documentation (section Date Formatters > Use Format Strings to Specify Custom Formats > Fixed Formats):

Although in principle a format string specifies a fixed format, by default NSDateFormater still takes the user’s preferences (including the locale setting) into account. ... In iOS, the user can override the default AM/PM versus 24-hour time setting. This may cause NSDateFormatter to rewrite the format string you set.

In other words, on an iOS device that's set for 24-hour time setting, you won't get "6:02 PM", you'll get "18:02", even though you specified "h:mm a". So when you separate that by spaces, you get back a single value, "18:02", not two values.

Answer 2

There's a caveat in the documentation for NSDateFormatter that says:

Note that although setting a format string (setDateFormat:) in principle specifies an exact format, in practice it may nevertheless also be overridden by a user’s preferences—see Data Formatting Guide for more details.

Could this apply in your case to produce a string without any spaces in it? (That would lead to a length 1 array when split by spaces, giving the exception you see in the place you see it.) Check this by logging the formatted date or attaching a debugger.

Note that the end of the page on date formats does recommend using plain old strftime_l when dealing with unlocalized dates/times. That might be more suitable for you. (Also, you want an AM/PM indicator in data that's bound for a computer? Seriously? The 24-hour clock is way easier to work with usually…)