functional1.swift

import UIKit

let words = ["Cat", "Chicken", "Fish", "Dog", "Mouse", "Pig", "Monkey"]

typealias Entry = (Character, [String])

func distinct<T: Equatable>(source: [T]) -> [T] {
    var unique = [T]()
    for item in source {
        if !contains(unique, item) {
            unique.append(item)
        }
    }
    return unique
}

func buildIndex(words: [String]) -> [Entry] {
    let letters = words.map { (word) -> Character in
        Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString)
    }

    let distinctLetters = distinct(letters)

    return distinctLetters.map { (letter) -> Entry in
        return (letter, words.filter { (word) -> Bool in
            Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString) == letter
        })
    }
}

println(buildIndex(words))

functional2.swift

import UIKit

let words = ["Cat", "Chicken", "Fish", "Dog", "Mouse", "Pig", "Monkey"]

typealias Entry = (Character, [String])

func distinct<T: Equatable>(source: [T]) -> [T] {
    var unique = [T]()
    for item in source {
        if !contains(unique, item) {
            unique.append(item)
        }
    }
    return unique
}

func buildIndex(words: [String]) -> [Entry] {

    func firstLetter(str: String) -> Character {
        return Character(str.substringToIndex(advance(str.startIndex, 1)).uppercaseString)
    }

    return distinct(words.map(firstLetter)).map { (letter) -> Entry in
        return (letter, words.filter { (word) -> Bool in
            firstLetter(word) == letter
        })
    }
}

println(buildIndex(words))

imperative.swift

import UIKit

let words = ["Cat", "Chicken", "Fish", "Dog", "Mouse", "Pig", "Monkey"]

typealias Entry = (Character, [String])

func buildIndex(words: [String]) -> [Entry] {
    var result = [Entry]()
    var letters = [Character]()

    for word in words {
        let firstLetter = Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString)

        if !contains(letters, firstLetter) {
            letters.append(firstLetter)
        }
    }

    for letter in letters {
        var wordsForLetter = [String]()
        for word in words {
            let firstLetter = Character(word.substringToIndex(advance(word.startIndex, 1)).uppercaseString)

            if firstLetter == letter {
                wordsForLetter.append(word)
            }
        }
        result.append((letter, wordsForLetter))
    }
    return result
}

println(buildIndex(words))