Programming Challenge: Are You a Swift Ninja? Part 2

Marin Todorov
Are you a Swift Ninja?

Are you a Swift Ninja?

Update 8/5/14: Series updated for Xcode6-beta 5.

Welcome back to our “Are you a Swift Ninja?” Programming Challenge!

In the first part of this series, you got some practice with default values in functions, variadic parameters, map/reduce, advanced switch statement features, and more.

Hopefully, you earned plenty of shurikens along the way!

In this second and final part of the series, you will get 4 more challenges to test your ninja skills.

In addition, this tutorial has a special final challenge, where you will get a chance to compete against other developers for fame and fortune!

The best solution to the final challenge will be featured in this post, and will also get a free copy of our upcoming Swift by Tutorials Bundle, which includes three books about programming in Swift.

Ninja mode activate – the challenge continues!

Challenge #5

Stretch those fingers and assume the position. It’s time to do another problem that involves recursion and function syntax.

Write a single function that reverses the text in a string. For example, when passed the string “Marin Todorov” will return the string “vorodoT niraM”.

  • You can’t use any loop operators nor subscripts (i.e. no square brackets in the code).
  • You can’t use any built-in Array functions.
  • Don’t use variables.

Here’s an example of a function call and its output:

reverseString("Marin Todorov") //--> "vorodoT niraM"


Solution Inside: Hints SelectShow
Solution Inside: Tutorial SelectShow

Challenge #6

Your next challenge has to do with operator overloading — one of the most powerful features of Swift. I hope you’ve had a chance to look into how to do that :]

Your challenge is to overload the “*” operator so it takes a Character and an Int and produces a String with the character repeated Int times.

Here’s an example usage and output:

"-" * 10 //output is: "----------"

Make usage of everything you learned so far and don’t use any variables, loops, inout parameters, or subscripts.

You might need to define an extra auxiliary function. At the time of writing, Xcode crashes when you try to define a nested function inside an operator overload. Hopefully this is corrected at some point.


Solution Inside: Hints SelectShow
Solution Inside: Tutorial SelectShow

Challenge #7

This challenge, while not necessarily pushing you to write beautiful and optimized code, will lead you to discover (or exercise) another very powerful feature of Swift.

"What's that?" you might ask. Well, you'll just have to work through it and figure that out for yourself!

For this challenge you'll need to use this function:

import Foundation

func doWork() -> Bool {
    return arc4random() % 10 > 5

This function, for the purpose of writing and testing your solution, randomly succeeds or fails (eg. returns true or false).

Write code (and/or additional functions) that will output the message "success!" when doWork() returns true, and will output "error" when doWork() returns false. Your solution should meet the following requirements:

  • You can't modify the source of doWork().
  • You can't use if, switch, while, or let.
  • You can't use the ternary operator ?:.


Solution Inside: Hints SelectShow
Solution Inside: Tutorial SelectShow

Challenge #8

Currying is a relatively unexplored area outside of functional languages like ML, SML, and Haskel. But as you may have noticed from the presentations at WWDC, Swift has this feature as well -- and the Apple engineers seem to be pretty excited about it.

Are you up to the challenge of using currying and partial function application?

Extend the Array structure and add 3 new functions that you could call like this on an array of any type:

  1. list.swapElementAtIndex(index: Int)(withIndex: Int): Returns a copy of the original array with the elements at indexes index and withIndex exchanged.
  2. list.arrayWithElementAtIndexToFront(index: Int): Returns a copy of the original array with the element at index index exchanged with the first element .
  3. list.arrayWithElementAtIndexToBack(index: Int): Returns a copy of the original array with the element at index index exchanged with the last element.

(The examples above use an array called list).


  • You can use the keyword func only one time - to declare swapElementAtIndex.

Here is an example of usage and its output:

let list = [1, 4, 5, 6, 20, 50] //--> [1, 4, 5, 6, 20, 50]

list.arrayWithElementAtIndexToBack(2) //--> [1, 4, 50, 6, 20, 5]

list.arrayWithElementAtIndexToFront(4) //--> [20, 4, 5, 6, 1, 50]


Solution Inside: Hints SelectShow
Solution Inside: Tutorial SelectShow

The Final Challenge


Time for the last challenge. This time there will be no hints and no tutorial. It's all on you, dear Ninja.

Approach this challenge carefully and design a beautiful solution. Then post your solution in the comments on this post. Note it's ideal if you post your solution as a gist so it has nice syntax highlighting.

I will select one solution as the winner. The winner will be immortalized in this post as the correct solution for this challenge! Remember to leave your name along the code, so you can live in infamy, forever known as a true Swift Ninja :]

In addition, the winner will will receive a a free copy of our upcoming Swift by Tutorials Bundle, which includes three books about programming in Swift!

In choosing a winner, I will consider correctness, brevity and use of Swift's language features. Embrace all the techniques you've explored in this post. Should you and another developer post the same winning solution, I'll choose the one that was posted first.

You have 2 weeks from the time this post goes live. Better get to it!

Let's get to coding! Here's the enumerations and a struct to get started:

enum Suit {
    case Clubs, Diamonds, Hearts, Spades

enum Rank {
    case Jack, Queen, King, Ace
    case Num(Int)

struct Card {
    let suit: Suit
    let rank: Rank

Write a function called countHand that takes in an array of Card instances and counts the total value of the cards given.

The requirements for your solution are as follows:

  • The function returns the value of the cards in the hand as an Int.
  • Does not use loops or nested functions.
  • The card values are as follows:
    1. Any Ace preceded by 5 of Diamonds is worth 100 points.
    2. Any odd numeric card (3, 5, 7, 9) of any suit's worth the double of its rank value in points when immediately preceded in the hand by any card of the Hearts. Examples:
      1. The hand 3♥, 7♣ has total value of 14.
      2. The hand 3♣, 7♥ has total value of 0.
  • Since brevity is one of the points on which your code will be assessed, consider using one statement functions, closures and case statements.

Here's an example of usage and its result. Use this to check your solution:

  Card(suit:Suit.Hearts, rank:Rank.Num(10)),
  Card(suit:Suit.Hearts, rank:Rank.Num(6)),
  Card(suit:Suit.Diamonds, rank:Rank.Num(5)),
  Card(suit:Suit.Clubs, rank:Rank.Ace),
  Card(suit:Suit.Diamonds, rank:Rank.Jack)
]) //--> 110


Where to Go From Here

First, check your challenge result!

Calculate how many shurikens you earned -- for the final challenge, give yourself 3 shurikens 3shurikens if you solved the problem and none if you didn't.
How ninja are you?

  • 23 or more shurikens: Congratulations! You're a Swift ninja! Fantastic!
  • 16 to 22 shurikens: You're doing well, but you'll benefit from delving into the nitty gritty details of Swift. Luckily, this website is a great resource to learn more about Swift.
  • 15 or less shurikens: Though you may not have been able to beat the challenges without help, you certainly learned tons of new Swift techniques and are on your way to being a certifiable ninja. Props!

You can download the complete Playground solution to the first 8 challenges here: NinjaChallenge-Completed-beta6.playground

Even if you mastered the challenges, you can always learn more! Check out some additional Swift resources:

Remember to post your solution to the final challenge and your name, and good luck. Thanks for reading this tutorial, and if you have comments or questions, please join in the forum discussion below!

Credit: All images in this post are from the public domain, and are available at:

Marin Todorov

Part of: Realm and Author of books and apps. More: <a href=""></a>

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