Sort NSArray of date strings or objects

Question

I have an NSArray that contains date strings (i.e. NSString) like this: "Thu, 21 May 09 19:10:09 -0700"

I need to sort the NSArray by date. I thought about converting the date string to an NSDate object first, but got stuck there on how to sort by the NSDate object.

Thanks.

Solution 1

Store the dates as NSDate objects in an NS(Mutable)Array, then use -[NSArray sortedArrayUsingSelector: or -[NSMutableArray sortUsingSelector:] and pass @selector(compare:) as the parameter. The -[NSDate compare:] method will order dates in ascending order for you. This is simpler than creating an NSSortDescriptor, and much simpler than writing your own comparison function. (NSDate objects know how to compare themselves to each other at least as efficiently as we could hope to accomplish with custom code.)

Solution 2

If I have an NSMutableArray of objects with a field "beginDate" of type NSDate I am using an NSSortDescriptor as below:

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"beginDate" ascending:TRUE];
[myMutableArray sortUsingDescriptors:[NSArray arrayWithObject:sortDescriptor]];
[sortDescriptor release];

Solution 3

You may also use something like the following:

//Sort the array of items by  date
    [self.items sortUsingComparator:^NSComparisonResult(id obj1, id obj2){
        return [obj2.date compare:obj1.date];
    }];

But this does assume that the date is stored as a NSDate rather a NString, which should be no problem to make/do. Preferably, I recommend also storing the data in it's raw format. Makes it easier to manipulate in situations like this.

Solution 4

You can use blocks to sort in place:

sortedDatesArray = [[unsortedDatesArray sortedArrayUsingComparator: ^(id a, id b) {
    NSDate *d1 = [NSDate dateWithString: s1];
    NSDate *d2 = [NSDate dateWithString: s2];
    return [d1 compare: d2];
}];

I suggest you convert all your strings to dates before sorting not to do the conversion more times than there are date items. Any sorting algorithm will give you more string to date conversions than the number of items in the array (sometimes substantially more)

a bit more on blocks sorting: http://sokol8.blogspot.com/2011/04/sorting-nsarray-with-blocks.html

Solution 5

What it worked in my case was the following:

    NSArray *aUnsorted = [dataToDb allKeys];
    NSArray *arrKeys = [aUnsorted sortedArrayUsingComparator:^NSComparisonResult(id obj1, id obj2) {
        NSDateFormatter *df = [[NSDateFormatter alloc] init];
        [df setDateFormat:@"dd-MM-yyyy"];
        NSDate *d1 = [df dateFromString:(NSString*) obj1];
        NSDate *d2 = [df dateFromString:(NSString*) obj2];
        return [d1 compare: d2];
    }];

I had a dictionary, where all keys where dates in format dd-MM-yyyy. And allKeys returns the dictionary keys unsorted, and I wanted to present the data in chronological order.

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