NSDictionary class represents an unordered collection of objects associate each value with a key.

Create NSDictionary

    NSDictionary *cars = @{
    @"Mercedes" : @"Benz SLK250"],
    @"BMW M3 Coupe" : @"M3 Coupe",
    @"Honda" : @"Civic",
};
NSLog(@"Dictionary: %@", cars);
NSLog(@"Manufacturers: %@", [cars allKeys]);
NSLog(@"Models: %@", [cars allValues]);

Enumerate NSDictionary

[cars enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
    NSLog(@"There are %@ %@'s in stock", obj, key);
}];

Compare NSDictionaries

 NSDictionary *warehouse = @{
    @"Mercedes-Benz SLK250" : [NSNumber numberWithInt:13],
    @"BMW X6" : [NSNumber numberWithInt:16],
};
NSDictionary *showroom = @{
    @"Mercedes-Benz SLK250" : [NSNumber numberWithInt:13],
    @"BMW X6" : [NSNumber numberWithInt:16],
};
if ([warehouseInventory isEqualToDictionary:showroomInventory]) {
    NSLog(@"Why are we storing same model cars ?");
}

Sorting Dictionary Keys

NSDictionary *cars = @{
    @"Mercedes" : @"Benz SLK250"],
    @"BMW M3 Coupe" : @"M3 Coupe",
    @"Honda" : @"Civic",
};
NSArray *sortedKeys = [cars keysSortedByValueUsingComparator:
                       ^NSComparisonResult(id obj1, id obj2) {
                           return [obj2 compare:obj1];
                       }];
NSLog(@"%@", sortedKeys);

Create a NSMutableDictionary

NSMutableDictionary *owners = [NSMutableDictionary
                             dictionaryWithDictionary:@{
                                 @"Audi TT" : @"John",
                                 @"Audi Quattro (Black)" : @"Mary",
                                 @"Audi Quattro (Silver)" : @"Bill",
                                 @"Audi A7" : @"Bill"
                             }];
NSLog(@"%@", owners);

Add/Remove from a NSMutableDictionary

NSMutableDictionary *carJobs = [NSMutableDictionary
                             dictionaryWithDictionary:@{
                                 @"Audi TT" : @"John",
                                 @"Audi Quattro (Black)" : @"Mary",
                                 @"Audi Quattro (Silver)" : @"Bill",
                                 @"Audi A7" : @"Bill"
                             }];
// Transfer an existing job to Mary
[carJobs setObject:@"Mary" forKey:@"Audi TT"];
// Finish a job
[carJobs removeObjectForKey:@"Audi A7"];
// Add a new job
carJobs[@"Audi R8 GT"] = @"Jack";

Combine NSMutableDictionary

NSMutableDictionary *carJobs = [NSMutableDictionary
                             dictionaryWithDictionary:@{
                                 @"Audi TT" : @"John",
                                 @"Audi Quattro (Black)" : @"Mary",
                                 @"Audi Quattro (Silver)" : @"Bill",
                                 @"Audi A7" : @"Bill"
                             }];

NSDictionary *carJobsSeekers = @{
    @"BMW 640i" : @"Dick",
    @"BMW X5" : @"Brad"
};
[carJobs addEntriesFromDictionary:carJobsSeekers];

*Happy Coding :)