For those who are familiar with Objective-C, you will know that there are different classes for mutable and immutable collections. For example, to define a mutable array, we use the NSMutableArray class, and to define an immutable array, we use the NSArray class. Swift is a little different because it does not contain separate classes for mutable and immutable collections. Instead, we define whether a collection is a constant (immutable) or a variable (mutable) using the let and var keywords. This should seem familiar since, in Swift, we define constants with the let keyword and variables with the var keyword.
Note
It is good practice to create immutable collections unless there is a specific need to change the objects within the collection. This allows the compiler to optimize performance.
Let's begin our tour of collections by looking at the most common collection type-the array type.