+ dictionaryWithCapacity:

Creates and returns a mutable dictionary, initially giving it enough allocated memory to hold a given number of entries.

NSMutableDictionary *dict =  [NSMutableDictionary dictionaryWithCapacity:1];
NSLog(@"%@",dict);

- init

Initializes a newly allocated mutable dictionary.

NSMutableDictionary *dict =  [[NSMutableDictionary alloc] init];        
NSLog(@"%@",dict);

+ dictionaryWithSharedKeySet:

Creates a mutable dictionary which is optimized for dealing with a known set of keys.

id sharedKeySet = [NSDictionary sharedKeySetForKeys:@[@"key1", @"key2"]]; // returns NSSharedKeySet
NSMutableDictionary *dict = [NSMutableDictionary dictionaryWithSharedKeySet:sharedKeySet];
dict[@"key1"] = @"Easy";
dict[@"key2"] = @"Tutorial";
//We can an object thats not in the shared keyset
dict[@"key3"] = @"Website";
NSLog(@"%@",dict);  

OUTPUT

{
    key1 = Eezy;
    key2 = Tutorials;
    key3 = Website;
}

Adding Entries to a Mutable Dictionary

- setObject:forKey:

Adds a given key-value pair to the dictionary.

NSMutableDictionary *dict =  [NSMutableDictionary dictionary];
[dict setObject:@"Easy" forKey:@"Key1"];
NSLog(@"%@",dict);

OUTPUT

{
    Key1 = Eezy;
}

- setObject:forKeyedSubscript:

Adds a given key-value pair to the dictionary.

NSMutableDictionary *dict =  [NSMutableDictionary dictionary];
[dict setObject:@"Easy" forKeyedSubscript:@"Key1"];
NSLog(@"%@",dict);  

OUTPUT { Key1 = Easy; }

    

- removeObjectForKey:

Removes a given key and its associated value from the dictionary.

NSMutableDictionary *dict =  [NSMutableDictionary dictionaryWithDictionary:@{@"key1":@"Easy",@"key2": @"Tutorials"}];
[dict removeObjectForKey:@"key1"];
NSLog(@"%@",dict);

OUTPUT

{
    key2 = Tutorials;
} 

- removeAllObjects

Empties the dictionary of its entries.

NSMutableDictionary *dict =  [NSMutableDictionary dictionaryWithDictionary:@{@"key1":@"Eezy",@"key2": @"Tutorials"}];
[dict removeAllObjects];
NSLog(@"%@",dict);

OUTPUT

{
}

- removeObjectsForKeys:

Removes from the dictionary entries specified by elements in a given array.

NSMutableDictionary *dict =  [NSMutableDictionary dictionaryWithDictionary:@{@"key1":@"Easy",@"key2": @"Tutorials"}];
[dict removeObjectsForKeys:@[@"key1"]];
NSLog(@"%@",dict);

OUTPUT

{
    key2 = Tutorials;
}