iOS 6 Programming Cookbook

1.24. Allocating and Making Use of Dictionaries

Problem

You want to store key-value data in an object, or you would like to retrieve objects from an array using a key into the array, but arrays won’t quite suffice for this purpose, as they do not facilitate finding objects inside the array using a key or a marker for that object.

Solution

Use NSDictionary and its mutable counterpart, NSMutableDictionary.

Discussion

A dictionary is a special container for objects in which each object is given a key, which itself is an object. That is one of the key differences between dictionaries and arrays. An array has a numeric index into each item/object that it holds, whereas a dictionary holds a key to each item. I’ll show you what I mean.

Let’s say we want to store a person’s first name, last name, and age into an array and then into a dictionary. This is how we would store those values in an array:

NSArray *person = @[
@"Anthony",
@"Robbins",
@51];

NSLog(@"First Name = %@", person[0]);
NSLog(@"Last Name = %@", person[1]);
NSLog(@"Age = %@", person[2]);

You can see that we are using an index into the array to access each one of these values. With dictionaries, we give each value a key, which is an object, and then use that key to access those values. Let’s look at the same example but this time using dictionaries. We have a "First Name" key with the value "Anthony" and so on:

NSDictionary *person = @{
@"First Name" : @"Anthony",
@"Last Name" : @"Robbins",
@"Age" : @51
};

NSLog(@"First Name = %@", person[@"First Name"]);
NSLog(@"Last Name = %@", person[@"Last Name"]);
NSLog(@"Age = %@", person[@"Age"]);

The results will then be printed out as shown here:

First Name = Anthony
Last Name = Robbins
Age = 51

As you can see, we initialized the dictionary with values and keys. We give a value followed by the key for that value. When we used NSLog, we printed out each value by handing the key to the dictionary’s objectForKey: method.

The mutable version of NSDictionary, NSMutableDictionary, can be modified after it has been allocated and initialized. For instance, if we want to remove the object associated with the key Age from our dictionary after its initialization, we would use a mutable dictionary like so:

NSMutableDictionary *person = [@{
                                @"First Name" : @"Anthony",
                                @"Last Name" : @"Robbins",
                                @"Age" : @51
                               } mutableCopy];

[person removeObjectForKey:@"Age"];

NSLog(@"First Name = %@", person[@"First Name"]);
NSLog(@"Last Name = %@", person[@"Last Name"]);
NSLog(@"Age = %@", person[@"Age"]);

We have simply removed the object associated with the key Age. The results printed to the console window will be similar to this:

First Name = Anthony
Last Name = Robbins
Age = (null)

Note

"Age" is not just empty, but totally missing.

If you want to enumerate all keys with their objects inside a dictionary, you can simply use the enumerateKeysAndObjectsUsingBlock: method of the dictionary. In the previous example, the method would print the "First Name" and "Last Name" elements, but not "Age", because we removed it. The parameter to this method is a block object with no return value and three parameters:

Key

An id that tells you which key is being enumerated at the moment.

Object

An id that gives you the object associated with the key being currently enumerated.

A pointer to a value of type BOOL

At any point during the enumeration, if you want to stop the process, you can simply put the value YES into this pointer’s memory address. Keep it untouched if you want to enumerate through all the keys in the dictionary.

Let’s see an example:

NSDictionary *person = @{
@"First Name" : @"Anthony",
@"Last Name" : @"Robbins",
@"Age" : @51
};

[person enumerateKeysAndObjectsUsingBlock:
 ^(__strong id key, __strong id obj, BOOL *stop) {

     NSLog(@"Key = %@, Object For Key = %@", key, obj);

 }];

And the results, which get printed to the console window, are shown here:

Key = Last Name, Object For Key = Robbins
Key = First Name, Object For Key = Anthony
Key = Age, Object For Key = 51

If you want to do a manual fast enumeration without block objects, you can use the allKeys method of the dictionary to go through all methods and, once you enumerate the keys, use the keys to find the objects associated with the keys using the objectForKey: method, like so:

for (id keyInDictionary in [person allKeys]){

  id objectForKey = [person objectForKey:keyInDictionary];
  NSLog(@"Key = %@, Object For Key = %@", keyInDictionary, objectForKey);

}

Bear in mind that you can traverse the keys in a dictionary in various ways. We’ve just seen two ways of doing this. There is another method that we can use: calling the keyEnumerator method of the dictionary to get an object of type NSEnumerator. Here is an example:

NSEnumerator *keys = [person keyEnumerator];
id keyInDictionary = nil;

while ((keyInDictionary = [keys nextObject]) != nil){

  id objectForKey = [person objectForKey:keyInDictionary];
  NSLog(@"Key = %@, Object For Key = %@", keyInDictionary, objectForKey);

}

Warning

When using the keyEnumerator method of a mutable dictionary, you are not allowed to change the values inside the dictionary while going through the keys. The same rule, if you remember, applies to mutable arrays as well.