How to count objects in a set using NSCountedSet

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

One of my favorite little gems in Cocoa Touch is called NSCountedSet, and it's the most efficient way to count objects in a set. Here's the code:

let set = NSCountedSet()
set.add("Bob")
set.add("Charlotte")
set.add("John")
set.add("Bob")
set.add("James")
set.add("Sophie")
set.add("Bob")

print(set.count(for: "Bob"))

Now to how it works: a set is like an array, except each item can appear only once. If we used an array in the above code, it would contain seven objects: three Bobs, one Charlotte, one John, one James and one Sophie. If we used a Swift set in the above code, it would contain four objects: one Bob, one Charlotte, one John, one James and one Sophie – the set ensures each item appears only once.

Now for the twist: NSCountedSet works similar to a set insofar as each object can appear only once, but it keeps track of how many times each item was added and removed. So, our counted set will have four objects in (like it would if it were a regular set), but NSCountedSet has a count(for:) method that will report back that Bob was added three times.