Collections in Objective C

Collections in Objective-C are things such as arrays and sets. They are used to contain objects (usually of the same nature) inside a container. There are many advantages to using collections.

Collections in Objective C: Arrays

Above all, the primary advantage of an array is that it has an index that orders objects. As a result, an array index begins at 0 and counts upwards. To use NSArray, we need to both allocate and initialize it. Here is an example of NSArray:

// NSArray Sample

NSArray* myArr;
myArr = [[NSArray alloc] initWithObjects:   @"Monday", 
                                            @"Tuesday", 
                                            @"Wednesday", nil];
                                            
                                            

We can use count to get the number of elements in an array. We can access the different elements of an array using either objectAtIndex or using a subscript ( [] ).

However, NSArrays are immutable so you can not add new objects to them. For adding or removing objects into an array, we use mutable arrays.

Collections in Objective C: Sets

On the other hand, we have sets. Sets are very similar to an array except that they don’t have an order. As a result the objects in a set can not be duplicates and must unique. Here is an example:

// NSSet Sample

NSSet* mySet = [[NSSet alloc] initWithObjects:@"one", @"two", nil];

Because of the nature of the sets, to find objects in a set we need to iterate through them. You could also use the C arrays. These are arrays made of any kind of object such as int such as this:

// C Style Arrays

int intArray[6] = {1, 2, 3, 4, 5, 6};

Objects in an array do NOT have to be of the same type.

Collections in Objective C: Dictionaries

Dictionaries are a type of collection in Objective-C that have a key-value pairing between them. As a result each value is paired to a certain key and can be searched and found using that key. You can use the NSDictionary to initialize a dictionary such as:

// NSDictionary Sample

NSDictionary *myDict;
myDict =  [NSDictionary dictionaryWithObject:@"value" forKey:@"key"];

As a result, you would most likely be using a NSMutableDictionary such as:

// NSMutableDictionary Sample

NSMutableDictionary *myMutDict = [NSMutableDictionary new];
[myMutDict setObject:@"Amir" forKey:@"Name"];

In Addition, to access a value for a certain key, you would be doing:

// Accessing Dictionary Objects

[myMutDict objectForKey:@"Name"];

Similarly, you could also initialize a dictionary with multiple keys and values at once, subsequently you would write code like the following:

// Initializing a Dictionary

NSDictionary *multipleDict;
multipleDict = [NSDictionary dictionaryWithObjectsAndKeys:	@"Value 1", 
                                                            @"Key1", 
                                                            @"Value 2", 
                                                            @"Key2", nil];

Finally, it’s worth noticing that the keys are arbitrary values and you can use whatever you like. For instance, can get all the keys:

// All Dictionary Keys

[myMutDict allKeys];

Or in other words, we could get all the values:

// All Dictionary Values

[myMutDict allvalues];

Dictionaries can be initialized in many different ways.

Summary

In conclusion, we user arrays, dictionaries and sets as collections in objective-C. You can nest arrays and dictionaries within one another.

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