Lab 1: Swift Playground

Getting Started

  1. If you've installed Xcode and its command line tools properly, Git should already be installed on your computer. If it isn't, you can install it following the instructions on the Git website or using Homebrew.

  2. Open Terminal, and navigate to whichever directory you want to store assignments for this class in using the cd command.

  3. Clone the repository with this command:

git clone

Note: In the future, you can get new assignments when we update the repository by cd-ing to the repo's directory and using git pull

For this lab (and all subsequent labs), you'll be working with a partner from your group.

When you and your partner are done, ask your TA to check you off. If you haven't finished the lab in the allotted time (or are otherwise unable to get checked off), you can get checked off with your partner on next Tuesday's office hours, but we will not be checking off labs any later than that.

Good luck, and don't be afraid to ask other students in your group or the TAs for help if you're stuck!

Problem 1: Swift Warmup

Implement the following functions:

  1. A function tossCoin, which takes in no arguments and randomly returns the strings Heads or Tails. Recall that the function arc4random() will give you a random integer.

  2. A function simulateTosses, which takes in an integer n as function parameter numTosses. It should use tossCoin to toss a coin n times and return the number of heads shown as an integer.

Hint 1 The modulo operator (%) returns true if a number is divisible by another number and false otherwise. For example, n % 5 will tell you whether or not n is divisible by 5. You might be able to use this in tossCoin.

Hint 2 The range operator (...) allows you to iterate through a range of values (inclusive) if you use it inside a for loop.

Problem 2: Digits

Implement a function digits that takes an integer n and returns an array containing its digits in order.

Here's some sample output:

digits(123)  // [1, 2, 3]
digits(0)    // returns [0]

Again, recall that you can use _ to ignore the external parameter name.

Hint 1 Give recursion a try! What is the base case, and what should the recursive case do?

Hint 2 You can append an element onto the end of an array either using the + operator or the append function.

let a = [1, 2]
a += [3]    // a is now [1, 2, 3]
a.append(4) // a is now [1, 2, 3, 4] 

Problem 3: Number Frequency

Implement a function frequency that takes in an unsorted array of integers arr as function parameter ofNumbers. It should return a string containing the frequency of each one in this format:

"num1:freq1, num2:freq2..."

The frequency of a number is the number of times it appears in the array. The numbers should in ascending order followed by their frequency, and the last entry must not be followed by a comma.

Here's some sample output:

ns = [3, 1, 2, 3, 2, 1, 2, 3, 2, 2]
frequency(ofNumbers: ns)  // returns "1:2, 2:5, 3:3"

Hint 1 There are multiple ways to solve this problem, but the easiest way is probably to use a dictionary. How might a dictionary's hashing property benefit us in this problem?

If you do end up using a dictionary, you'll want to take a closer look at optionals - you may need the if let __ notation if you plan on accessing elements in the dictionary.

Hint 2 You can use the sort(by: ) function to sort an array by increasing or decreasing elements. Check out this documentation article.


Congrats on completing the first lab! Flag down your group's TA (or another TA, if they aren't available), and ask to get checked off. In general, check-offs consist of two parts:

  1. A quick run-through of your lab just to make sure everything is working properly.
  2. A conceptual question or two to make sure you've understood what happened.

After you've checked off, you're free to leave. But if you want a bit more practice thinking in Swift (or you're done early), give this next question a try :)

Problem 4 (Optional): Roman Numerals to Integer

Adapted from this LeetCode question

This is a pretty common whiteboard interview question, but for the purposes of learning we're more concerned with your code working than it being fully optimized.

First, some background: roman numerals are represented by seven different symbols: I, V, X, L, C, D and M.

Symbol Value
I 1
V 5
X 10
L 50
C 100
D 500
M 1000

For example, 2 is written as II: just two one's added together. 12 is written as XII: X + II.

Roman numerals are generally written largest to smallest from left to right. However, the numeral for four is not IIII. Instead, 4 is written as IV. Because the one is before the five we subtract it making four. The same principle applies to 9, which is written as IX. There are six instances where subtraction is used:

Implement a function romanToInt that takes a roman numeral as a string num and converts it to an integer. Input is guaranteed to be within the range from 1 to 3999.

Here's some sample output:

romanToInt("III")      // returns 3

romanToInt("LVIII")    // returns 58
// Explanation: L = 50, V= 5, III = 3

romanToInt("MCMXCIV")  // returns 1994
// Explanation: M = 1000, CM = 900, XC = 90 and IV = 4

We'll leave this one as a challenge to you for the most part, so it may be worth sketching out the solution on scratch paper before starting and trawling the Apple Developer Documentation and StackOverflow if you're confused as to how to implement something. Please ask a TA if you're stuck; that's what we're here for!

Hint If you need a nudge in the right direction, it may be worth considering the conditional ternary operator. It takes the form question ? answer1 : answer2, and is essentially a shortcut for evaluating one of two expressions based on whether the question is true or false.

Here's the relevant Swift documentation.