Basics Of Objective-C And Dictionaries (NSDictionary)

July 9, 2012

Dictionaries are very common in iOS application development. A dictionary is nothing more than a collection of key-value pairs. The keys are represented as strings and the values are objects. The keys in a dictionary must be unique. As with other collections, dictionaries have two variants, mutable and non-mutable.

Let’s look at a few examples. The code below creates a mutable dictionary and adds a series of objects, including a string, a number and an array (with various embedded object types):

NSArray *array = [NSArray arrayWithObjects:
  @"String 1",
  [NSNumber numberWithInteger:99],
  [NSArray arrayWithObjects:
             @"array value 1",
             @"array value 2",
             nil],
  nil];
 
NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];
[dictionary setObject:@"String value" forKey:@"stringKey"];
[dictionary setObject:[NSNumber numberWithInt:25] forKey:@"numberKey"];
[dictionary setObject:array forKey:@"arrayKey"];
 
NSLog(@"dictionary: %@", dictionary);

The output for this dictionary follows:

dictionary: {
    arrayKey =     (
        "String 1",
        99,
                (
            "array value 1",
            "array value 2"
        )
    );
    numberKey = 25;
    stringKey = "String value";
}

To get the values from a dictionary, use the method objectForKey:

NSLog(@"Value for arrayKey: %@", [dictionary objectForKey:@"arrayKey"]);
NSLog(@"Value for stringKey: %@", [dictionary objectForKey:@"stringKey"]);

The output for the two keys are as follows:

Value for arrayKey: (
    "String 1",
    99,
        (
        "array value 1",
        "array value 2"
    )
)
Value for stringKey: String value

Here is a more involved example, where I create an array of dictionaries. I define the keys as strings – also, notice that I call dictionaryWithObjectsAndKeys to create a dictionary from a series of key-value pairs, versus the approach above where key-value pairs are added one by one. Each dictionary is added to a mutable array:

// Define keys
NSString *style = @"Style";
NSString *appearance = @"Appearance";
NSString *hopProfile = @"Bitterness";
 
// A dictionary object
NSDictionary *dict;
 
// Create array to hold dictionaries 
NSMutableArray *arrayOfDictionaries = [NSMutableArray array];
 
// Create three dictionaries
dict = [NSDictionary dictionaryWithObjectsAndKeys:
                    // Object and key pairs
                     @"Bock", style,    
                     @"Deep Gold", appearance, 
                     [NSNumber numberWithInt:25], hopProfile, 
                     nil];
[arrayOfDictionaries addObject:dict];
 
 
dict = [NSDictionary dictionaryWithObjectsAndKeys:
                    // Object and key pairs
                     @"India Pale Ale (IPA)", style, 
                     @"Copper", appearance, 
                     [NSNumber numberWithInt:50], hopProfile, 
                     nil];
[arrayOfDictionaries addObject:dict];
 
 
dict = [NSDictionary dictionaryWithObjectsAndKeys:
                   @"Stout", style, 
                   @"Jet Black", appearance, 
                   [NSNumber numberWithInt:35], hopProfile, 
                   nil];
[arrayOfDictionaries addObject:dict];
 
NSLog(@"array of dictionaries: %@", arrayOfDictionaries);

I find it helpful to see the output of a dictionary to get a visual of the contents. The NSLog output is shown below:

array of dictionaries: (
        {
        Appearance = "Deep Gold";
        Bitterness = 25;
        Style = Bock;
    },
        {
        Appearance = Copper;
        Bitterness = 50;
        Style = "India Pale Ale (IPA)";
    },
        {
        Appearance = "Jet Black";
        Bitterness = 35;
        Style = Stout;
    }
)