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

Cheat sheet

subtracting returns elements in the first set not in the second:

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

symmetricDifference returns elements in exactly one of the two sets:

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

Use a set as an O(1) filter with contains:

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"]

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)