"Test-Driven iOS Development with Swift 4"

vowelSample.swift

func numberOfVowels(in string: String) -> Int {
        let vowels: [Character] = ["a", "e", "i", "o", "u"]
        
        /*
        `characters` is deprecated
  
        return string.characters.reduce(0) {
            $0 + (vowels.contains($1) ? 1 : 0)
        }
        */
        return string.lowercased().reduce(0) { // instead of adding capitalization in the `vowels` array
            $0 + (vowels.contains($1) ? 1 : 0)
        }
    }