How do I get unique values from an array

Question

Please note this is for OSX, not for iOS.

I have looked and tried some solutions in other SO questions, but none seem to work for me, hence this:

I want to get a unique set of years out of an array.

My code is this:

NSMutableArray * unique = [NSMutableArray array];
        NSMutableSet * processed = [NSMutableSet set];
        for (yearString in yearStringArray) {
            if (![processed containsObject:yearString] == NO) {
                [unique addObject:yearString];
            }
            [processed addObject:yearString];
        }

        NSLog(@"unique: %@",unique );
        NSLog(@"processed: %@",processed );

    }];

The console returns:

unique: (
        (
        1941,
        1942,
        1943,
        1945,
        1948,
        1948,
        1948,
        1949
    )

processed: {(
        (
        1941,
        1942,
        1943,
        1945,
        1948,
        1948,
        1948,
        1949
    )
)}

I only want one 1948. I would appreciate any help.

UPDATES: Thanks for the answers all. Keep em coming please!!!

These are the code snips I've tried now:

1.

for (yearString in yearStringArray) {
            if ([processed containsObject:yearString] == NO) {
                [unique addObject:yearString];
            }
            [processed addObject:yearString];
        }

2.

   NSString *yearString = [[NSString alloc] init];
    NSArray *objectArray = [[NSArray alloc] init];
    [objectArray = objects copy];

    for (int j = 0; j <= objectArray.count; j++) {
        NSString *yearString = [objectArray valueForKey:@"year"];
        [yearStringArray addObject:yearString];
    }


    NSMutableArray * unique = [NSMutableArray array];
    NSMutableSet * processed = [NSMutableSet set];


    for (yearString in yearStringArray) {

        [unique addObject:yearString];
        [processed addObject:yearString];
    }

    NSArray *processed2 = [[NSSet setWithArray:unique] allObjects];
    NSLog(@"unique: %@",unique );
    NSLog(@"processed: %@",processed );
    NSLog(@"processed2: %@",processed2 );
    //[self setYears];
}];

3.

NSString *yearString = [[NSString alloc] init];
    NSArray *objectArray = [[NSArray alloc] init];
    [objectArray = objects copy];

    for (int j = 0; j <= objectArray.count; j++) {
        yearString = [objectArray valueForKey:@"year"];
        [yearStringArray addObject:yearString];
    }



    NSArray *uniqueYears = [yearStringArray valueForKeyPath:@"@distinctUnionOfObjects.self"];
    NSLog(@"uniqueYears: %@", uniqueYears);


}];



}

Still not getting unique years..

Answer 1

An simple way is to create an NSSet from the array with duplicates. The result is a set which by definition stores only the unique objects. Then using NSSet's -allObjects method to convert the NSSet back to an NSArray. The only downside is you lose the original ordering by converting to an NSSet.

NSArray *uniqueArray = [[NSSet setWithArray:duplicateArray] allObjects];

If you need to preserve the ordering and can require 10.7+, you can use an NSOrderedSet.

NSArray *uniqueArray = [[NSOrderedSet orderedSetWithArray:duplicateArray] array];

Edit:

Thanks to Mattt Thompson in the WWDC 2013 Session 228 - Hidden Gems in Cocoa and Cocoa Touch, there is another way without creating an intermediate set.

NSArray *uniqueArray = [duplicateArray valueForKeyPath:@"@distinctUnionOfObjects.self"]