Swift5-6 on blog.iankulin.com

Using Swift's map

Thu, 13 Oct 2022 00:00:00 +0000

In Day 39’s Moonshot tutorial app, Paul uses .map on an array without much comment about what’s going on. I assume this might be a common concept in modern languages, but it was new to me.

First, here’s Paul’s code

 init(mission: Mission, astronauts: [String: Astronaut]) {
 self.mission = mission

 self.crew = mission.crew.map { member in
 if let astronaut = astronauts[member.name] {
 return CrewMember(role: member.role, astronaut: astronaut)
 } else {
 fatalError("Missing \(member.name)")
 }
 }
 }

Mission here contains an array of crew which is a struct with two strings, one of them being name, but self.crew (which belongs to the view we’re in) is an array of CrewMember which is a struct with a role: String and another struct astronaut. That sounds confusing, but essentially, an array of one type is being processed to return an array of another type where there’s a 1:1 relationship between the elements of each array.

That’s what map does - it works through a collection, and uses the code in a closure to act on each element to transform it, then adds it to the array to be returned.

Let me try to make it clearer with a simpler example. Time to open a Playground. Let’s map an array of integers into an array of strings:

let intArray = [0, 55, 21, 13]
var stringArray = [String]()

stringArray = intArray.map{ "\($0)"}

print(stringArray)
// ["0", "55", "21", "13"]

$0 is a stand in for each element. This is a common Swift feature but this variable can be named for clarity if desired:

let intArray = [0, 55, 21, 13]
var stringArray = [String]()

stringArray = intArray.map{ number in "\(number)"}

print(stringArray)
// ["0", "55", "21", "13"]

If we didn’t have map, we could do it the long way, something like this.

let intArray = [0, 55, 21, 13]
var stringArray = [String]()

intArray.forEach {
 stringArray.append("\($0)")
}

print(stringArray)
// ["0", "55", "21", "13"]

or, going back even further in time:

let intArray = [0, 55, 21, 13]
var stringArray = [String]()

for number in intArray {
 stringArray.append("\(number)")
}

print(stringArray)
// ["0", "55", "21", "13"]

So the .map method on a sequence returns an array of values built by applying to the passed closure to each element of the sequence.

Paul’s code at the top steps through the mission.crew array which contains the crews names as strings, then attempts to look them up in the astronauts dictionary. If it finds them, it adds them (as a struct) to the crew array.

Ranges

Sun, 25 Sep 2022 00:00:00 +0000

I wondered aloud, in a previous post, about the differences in writing a range as

 ForEach(1..<21) {
 Text(String($0))
 }

versus

 ForEach(1...20) {
 Text(String($0))
 }

And that’s been answered in in one of the Day 34 articles. It sounds like older versions of Swift might not have allowed the second version.

How to upgrade XCode and Swift

Sun, 18 Sep 2022 00:00:00 +0000

With the September release of XCode 14 and Swift 5.7 it was time for my first update - I looked in “About” for an update link but there wasn’t one - so if you’re as dense as me, the tip is to head to the Mac App Store and have a look at the Updates page.

Your current XCode version can, of course be found in the XCode | About dialogue. Mine was on 13.4.1. There’s a couple of ways of finding the Swift version - If you’ve got an XCode project open, click on the .xcodeproj in the explorer,and have a look in Build Settings for Swift Compiler - Language for the major version.

Or for a bit more accuracy, try xcrun swift -version at the command line. I got a few errors, then the version 5.6.1

I won’t bang on about the changes for XCode or Swift since that’s already been done well elsewhere, and the Swift changes are mostly beyond my expertise level.