100 doors in a row are all initially closed. You make 100 passes by the doors. The first time through, you visit every door and toggle the door.  The second time you only visit every 2nd door (door #2, #4, #6, ...).  The third time, every 3rd door (door #3, #6, #9, ...), etc, until you only visit the 100th door.
After all is done - how many doors are OPEN?

A great way to start coding in Swift on the iOS platform or Mac is to take an introductary course on one of the many learning sites.  If you do this and have a recommendation - please let us know - we'd love to include your recommendation.

I did a Udemy course with Angela  -  Complete iOS App Development Bootcamp - I can highly recommend this for the novice programmer.

From Beginner to iOS App Developer with Just One Course! Fully Updated with a Comprehensive Module Dedicated to SwiftUI! The Complete iOS App Development Bootcamp. With over 39,000 5 star ratings and a 4.8 average my iOS course is the HIGHEST RATED iOS Course in the history of Udemy! By the end of this course, you will be fluently programming in Swift 5.1 and be ready to make your own apps or start a freelancing job as an iOS 13 developer.

---

I will also recommend Getting Started with macOS Programming by Gabriel Theodoropoulos

100 days of OSX Development video tutorials - Learn to build Mac App using Swift by Harry Ng

Any writing or video by Paul Hudson - Hacking with Swift site:

 Building a SwiftUI app for iOS  - YouTube 

SwiftUI by Example is the world's largest collection of SwiftUI examples, tips, and techniques giving you over 400 pages of hands-on code to help you build apps, solve problems, and understand how SwiftUI really works.

100 Days of Swift This is a free collection of videos, tutorials, tests, and more, all drawn from around my work here on Hacking with Swift, and all designed to help you learn SwiftUI effectively.  It’s aimed squarely at beginners who want to learn to build real iOS apps, but struggle to find a good, free course that can help them achieve their goals. If that’s you, welcome – I hope you’re ready to show the iOS world what you’re made of!

“Programming is an art. Don't spend all your time sharpening your pencil when you should be drawing.” -- Paul Hudson

A really nice book - almost like a coffee table book is SwiftUI Views Quick Start by Mark Moeykens.

You may want to create a mind-map of SwiftUI and figure out what interest you most and start there.  I forsee one BIG problem with this... creating a reasonable mind-map of SwiftUI will be very difficult if you don't know it very well yet... so start with someone else's map:  Karin Prater has a nice one: On the road to learn SwiftUI

The classic Kata that I learned many moons ago is the Fibonacci Kata.  First described to me in Kent Beck's TDD book, appendix II p211.

Start with this peusudo code.

Fib(0) should return 0

Fib(1) should return 1

Fib(1) should return 2

Fib(2) should return 3

Fib(3) should return 5

Fib(5) should return 8

... continue until you have a well factored and concise Fib() function... Red-Green-Refactor.  Here is a Starter Project on gitHub.

My last attempt at this Kata the implementation code started like this:

step 0:

static func fib(_ number: Int) -> Int {

        return 0

    }

step 1:     

static func fib(_ number: Int) -> Int {

        return number

    }

my next implementation was an infinite recursive function - it failed....