One rather important discussion that we've left out is how to subtract, edit, or add to arrays, sets, and dictionaries. However, before we do this, you should understand the concept of mutable and immutable data/collections.
A mutable collection is simple data that can be changed, added to, or subtracted from, whereas an immutable collection cannot be changed, added to, or subtracted from.
To work with mutable and immutable collections efficiently in Objective-C, we had to explicitly state the mutability of the collection beforehand. For example, an array of the type
NSArray in Objective-C is always immutable. There are methods we can call on
NSArray that would edit the collection, but behind the scenes, this would ...