Swift

 // This program demonstrates string functions.
 
 import Foundation
 
 func main() {
     let string:String = "Hello"
 
     print("string: " + string)
     print("string.lowercased(): " + string.lowercased())
     print("string.uppercased(): " + string.uppercased())
     print("find(string, \"e\"): " + String(find(string:string, character:"e")))
     print("string.count: " + String(string.count))
     print("string.replacingOccurrences(of:\"H\", with:\"j\"): " + string.replacingOccurrences(of:"H", with:"j"))
     print("string.reversed(): " + String(string.reversed()))
     print("substring(2, 2): " + substring(string:string, start:2, length:2))
     print("string.trimmingCharacters(\"H\"): " + string.trimmingCharacters(in:CharacterSet.init(charactersIn: "H")))
 
     let name:String = "Bob"
     let value:Double = 123.456
     print("\(name) earned $" + String(format:"%.2f", value))
 }
 
 func find(string:String, character:Character) -> Int {
     var result: Int
 
     if let index = string.firstIndex(of:character) {
         result = string.distance(from: string.startIndex, to: index)
     } else {
         result = -1
     }
     return result
 }
 
 func substring(string:String, start:Int, length:Int) -> String {
     let startIndex = string.index(string.startIndex, offsetBy: start)
     let endIndex = string.index(string.startIndex, offsetBy: start + length - 1)
     return String(string[startIndex...endIndex])
 }
 
 main()

 // This program creates a file, adds data to the file, displays the file,
 // appends more data to the file, displays the file, and then deletes the file.
 // It will not run if the file already exists.
 //
 // References:
 //     https://www.mathsisfun.com/temperature-conversion.html
 //     https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html
 
 import Foundation
 
 func fileExists(filename:String) -> Bool {
     let fileManager = FileManager.default
     return fileManager.fileExists(atPath:filename)
 }
 
 func calculateFahrenheit(celsius:Double) -> Double {
     var fahrenheit: Double
     fahrenheit = celsius * 9 / 5 + 32    
     return fahrenheit
 }
 
 func createFile(filename:String) {
     var text: String
     var fahrenheit: Double
 
     text = "Celsius,Fahrenheit\n"
     for celsius in stride(from: 0.0, through: 50.0, by: 1.0) {
         fahrenheit = calculateFahrenheit(celsius:celsius)
         text += String(celsius) + "," + String(fahrenheit) + "\n"
     }
 
     do {
         try text.write(toFile: filename, atomically: true, encoding: .utf8)
     } catch {
         print("Error creating ", filename)
         print(error.localizedDescription)
     }
 }
 
 func readFile(filename:String) {
     var text = ""
 
     do {
         text = try String(contentsOfFile: filename, encoding: .utf8)
 
         let lines = text.components(separatedBy:"\n")
         for line in lines {
             print(line)
         }
     } catch {
         print("Error reading " + filename)
         print(error.localizedDescription)
     }
 }
 
 func appendFile(filename:String) {
     var text: String
     var fahrenheit: Double
 
     do {
         text = try String(contentsOfFile: filename, encoding: .utf8)
 
         for celsius in stride(from: 51.0, through: 100.0, by: 1.0) {
             fahrenheit = calculateFahrenheit(celsius:celsius)
             text += String(celsius) + "," + String(fahrenheit) + "\n"
         }
 
         try text.write(toFile: filename, atomically: true, encoding: .utf8)
     } catch {
         print("Error appending to ", filename)
         print(error.localizedDescription)
     }
 }
 
 func deleteFile(filename:String) {
     do {
         let fileManager = FileManager.default
         try fileManager.removeItem(atPath:filename)
     } catch {
         print("Error deleting", filename)
         print(error.localizedDescription)
     }
 }
 
 func main() {
     let filename:String = "~file.txt"
 
     if (fileExists(filename:filename)) {
         print("File already exists.")
     }
     else {
         createFile(filename:filename)
         readFile(filename:filename)
         appendFile(filename:filename)
         readFile(filename:filename)
         deleteFile(filename:filename)
     }
 }
         
 main()