Map, FlatMap, Reduce & More

Simple Problem Redux

released Wed, 20 Feb 2019
Swift Version 5.0

We can now go back to our initial count & average problem and try to solve it with reduce.

InfoFromState, take two

   func infoFromState(state: String, persons: [[String: Any]]) 

       -> (count: Int, age: Float) {

       // The type alias in the function will keep the code cleaner

       typealias Acc = (count: Int, age: Float)

       // reduce into a temporary variable

       let u = persons.reduce((count: 0, age: 0.0)) {

           (ac: Acc, p) -> Acc in

           // Retrive the state and the age

           guard let personState = (p[\"city\"] as? String)?.componentsSeparatedByString(\", \").last,

                 personAge = p[\"age\"] as? Int

             // make sure the person is from the correct state

             where personState == state

             // if age or state are missing, or personState!=state, leave

             else { return ac }

           // Finally, accumulate the acount and the age

           return (count: ac.count + 1, age: ac.age + Float(personAge))


   // our result is the count and the age divided by count

   return (age: u.age / Float(u.count), count: u.count)


print(infoFromState(state: \"CA\", persons: persons))

// prints: (count: 3, age: 34.3333)

As in earlier examples above, we're once again using a tuple to share state in the accumulator. Apart from that, the code is easy to understand.

We also defined a typealias Acc within the func in order to simplify the type annotations a bit.