ROT13. The ROT13 algorithm obscures text by changing the letters in words to be shifted 13 places forward or backward. It is not encryption.

In Swift 5.8, we implement ROT13 in a func. We use the UnicodeScalar type to manipulate Characters by their numeric codes. We create a rotated string.

Func implementation. We introduce the rot13 func. This method receives a String and returns a String. It begins by creating an empty Character array.

Convert Int, Character

func rot13(value: String) -> String {
    // Empty character array.
    var result = [Character]()
    // Some ASCII constants.
    // A = 65
    // M = 77
    // Z = 90
    // a = 97
    // m = 109
    // z = 122
    let upperA = 65
    let upperM = 77
    let upperZ = 90
    let lowerA = 97
    let lowerM = 109
    let lowerZ = 122

    // Loop over utf8 values in string.
    for u in value.utf8 {

        let s = Int(u)
        var resultCharacter = Character(UnicodeScalar(s)!)
        if s >= lowerA && s <= lowerZ {
            if s >= lowerM {
                resultCharacter = Character(UnicodeScalar(s - 13)!)
            } else {
                resultCharacter = Character(UnicodeScalar(s + 13)!)
            }
        } else if s >= upperA && s <= upperZ {
            if s >= upperM {
                resultCharacter = Character(UnicodeScalar(s - 13)!)
            } else {
                resultCharacter = Character(UnicodeScalar(s + 13)!)
            }
        }
        // Append to Character array.
        result.append(resultCharacter)

    }
    // Return String.
    return String(result)
}

// Test the method.
let input = "Do you have any cat pictures?"
let result = rot13(value: input)
let roundTrip = rot13(value: result)

print(input)
print(result)
print(roundTrip)
Do you have any cat pictures?
Qb lbh unir nal png cvpgherf?
Do you have any cat pictures?

Some notes. ROT13 is a terrible way to encrypt text. But this example helps us learn a programming language. We must access UnicodeScalars and manipulate character values in Swift.

A short review. A trick to manipulating strings in Swift is the use of UnicodeScalar. With this type we can shift characters based on their numeric values. The utf8 property also helps.