Reverse a String in Swift

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Welcome back to yet another installment of the Reverse a String in Every Language series. Today, we’re honored to have a submission from Marty who has brought us Reverse a String in Swift.

If this is your first time playing around in Swift, we recommend you check out Hello World in Swift first.

Table of Contents

Reverse A String In Swift

At long last, let’s take a look at the solution to Reverse a String in Swift:

import Foundation

guard CommandLine.argc > 1 else {
  exit(0)
}

let usersString = CommandLine.arguments[1]
let reversedCollection = usersString.reversed()
let reversedString = String(reversedCollection)

print(reversedString)

As we can see, reversing a string in Swift is pretty short and sweet. At this point, let’s dig into the code a bit.

Foundation

On line 1 of our sample code, we import Foundation:

import Foundation

Foundation is a framework that contains many basic data types. This framework also bridges between Swift types, such as String, and Objective-C types, such as NSString. When you import almost any of Apple’s Swift frameworks, you are implicitly importing Foundation along with them.

Guard

The guard statement on line 3 is one of Swift’s many syntax features which favor safety and human readability:

guard CommandLine.argc > 1 else {
  exit(0)
}

A guard statement is a conditional that only fires off if something isn’t true. This is good for highlighting error conditions, and prevents code from becoming littered with nested if-else statements. It is especially powerful when dealing with Swift’s Optionals.

Commandline

Commandline.arguments is a built-in way to access user input:

let usersString = CommandLine.arguments[1]

Because Commandline.arguments is a String array, not an Optional, we check its length, rather than look for nil, in order to see if the user has passed us any input. In Swift, only Optionals can contain nil.

Furthermore, Commandline.arguments is always initialized with at least one thing inside it: the path of the particular file the code is stored in. Therefore, to access user input, we check if the array contains more than one String. Then we call the String at the second index; the first index will always contain the path.

Finally, Commandline has a special property, .argc, which allows us to know how many Strings are inside Commandline.arguments. We could also call .length on Commandline.arguments to access this information. This is the usual way we access the number of things in an array, but here we use .argc to highlight a unique feature of working with Commandline.

String

When we reverse a String in many languages, we may opt for the manual way, by looping over each character. Instead, we can reverse the string directly through a builtin call:

let reversedCollection = usersString.reversed()

Swift has an unusual approach to Strings. Swift stores all characters in Strings as Unicode, even if they are within the ASCII character set. Therefore, we don’t need to do anything funky to cover all our edge cases: Swift can innately handle accented characters, Cyrillic, kanji, and so on.

While this allows Swift to seamlessly handle emoji and languages that use non-English characters, it can make working with Strings a bit more complicated.

Swift’s String API has changed frequently from version to version, as Apple continually tries to balance universal language support with ease of use. In early versions, Swift Strings were more like Javascript’s String implementation, where Strings were not quite arrays, but you could still call a specific character by indexing into it with an integer value.

As of Swift 4, Strings are composed of a special kind of character collection, quite distinct from arrays. Indexing into a String is done via special methods and indexing objects specific to the task. This makes looping over all of the characters in a String a more difficult task than in other languages.

Thankfully, we do not have to resort to this to reverse a String — we can simply call the .reversed() method (or .reverse(), in earlier Swift versions).

Reversed()

The .reversed() method does not mutate the original String, nor does it return a new String. Instead, it works over the character collection making up our String, and we get back an instance of ReversedCollection, an incompatible type. This forces us to be more precise about what we intend to do with our return values. Later, we use our ReversedCollection to make a new String object:

let reversedString = String(reversedCollection)

Fortunately, we can perform a quick cast to get our reversed string.

Print

Finally, on line 11, we come to a nice, straight-forward print statement:

print(reversedString)

Unlike other languages, we don’t need to specify the console. The compiler implicitly knows. Apple also offers a logging system, for more robust results when debugging.

How To Run The Solution

The most common way for programmers to run Swift code is via Apple’s XCode IDE. It’s free and available on the Mac App Store. You can paste this code snippet directly into an XCode file and run it.

You should use an online Swift compiler if your machine can’t handle XCode, or you just don’t want to download it (it’s quite big). This web compiler is up-to-date with Swift 4. Note that Swift updates yearly, so many web compilers are out of date and unable to run this code.

Sample Programs in Every Language

That’s it for reversing a string in Swift. If you like this article, feel free to share. It’s part of the Sample Programs in Every Language project. If you feel inspired, check out the repo and start contributing.

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