An NSSet object represents a static, unordered collection of distinct objects. NSSet/NSMutableSet, NSArray/NSMutableArray, and NSDictionary/NSMutableDictionary are the three core collection classes .

NSSet is immutable and NSMutableSet is mutable or we can say that a NSMutableSet can be modified(Add/Remove) where NSSet can't be.

Initialize a NSSet and NSMutableSet

NSSet *carManufacturers = [NSSet setWithObjects:@"Chrysler", @"Ford",
                                             @"General Motors", nil];
NSLog(@"%@", carManufacturers);

NSMutableSet *carManufacturers = [NSMutableSet setWithObjects:
                            @"Honda Civic", @"Nissan Versa", nil];
NSMutableSet *topCars = [NSMutableSet setWithCapacity:5];

Compare NSSet

NSSet *cars = [NSSet setWithObjects:@"Honda", @"Nissan",
                              @"Mitsubishi", @"Toyota", nil];
NSSet *myFav = [NSSet setWithObjects:@"Honda", nil];
NSSet *allTimeFav = [NSSet setWithObjects:@"Toyota",
                                                  @"Alfa Romeo", nil];

if ([myFav isEqualToSet:cars]) {
    NSLog(@"I like all the Japanese auto makers and no others");
}
if ([myFav intersectsSet:cars]) {
    // You'll see this message
    NSLog(@"I like at least one Japanese auto maker");
}
if ([myFav isSubsetOfSet:cars]) {
    // And this one, too
    NSLog(@"All of the auto makers that I like are Japanese");
}
if ([allTimeFav isSubsetOfSet:cars]) {
    NSLog(@"All of the auto makers that people likes are Japanese");
}

Find an Object in NSSet check if an object is in a particular NSSet.

NSSet *carMakers = [NSSet setWithObjects:@"Maserati",
                                             @"Porsche", nil];
// BOOL checking
if ([carMakers containsObject:@"Maserati"]) {
    NSLog(@"The user seems to like expensive cars");
}
// nil checking
NSString *result = [carMakers member:@"Maserati"];
if (result != nil) {
    NSLog(@"%@ is one of the selected makes", result);
}