Subtracting and Symmetric

Part of the Logic & Flow section of Coddy's Swift journey. Lesson 20 of 56.

Two more set operations finish the toolkit.

subtracting returns the elements of the first set that are not in the second:

let a: Set = [1, 2, 3, 4]
let b: Set = [3, 4, 5]
a.subtracting(b)             // {1, 2}

symmetricDifference returns the elements that are in exactly one of the two sets, never both:

a.symmetricDifference(b)     // {1, 2, 5}

Together with union and intersection, these four cover every set algebra question you'll meet day to day.

Set membership is also useful as a filter: ask whether a value lives in a set in O(1):

let stopwords: Set = ["the", "a", "an"]
let words = ["the", "cat", "sat", "on", "a", "mat"]
let real = words.filter { !stopwords.contains($0) }
// ["cat", "sat", "on", "mat"]

Challenge

Easy

Read two lines of input, each a comma-separated list of strings. Treat them as currentMembers and renewedMembers respectively.

Print three lines, each sorted alphabetically and joined with , (or empty when there are none):

Lapsed: <...> > current minus renewed
New: <...> > renewed minus current
Stayed: <...> > intersection of the two

For input alice,bob,cara on line 1 and bob,cara,dan on line 2, the output is:

Lapsed: alice
New: dan
Stayed: bob,cara

Try it yourself

let current = Set(readLine()!.components(separatedBy: ","))
let renewed = Set(readLine()!.components(separatedBy: ","))

// TODO: print Lapsed (current - renewed), New (renewed - current), Stayed (intersection)