Working With Files

Part of the Object Oriented Programming section of Coddy's Swift journey. Lesson 1 of 57.

Real programs are split into several files. In Swift every .swift file of an app is compiled together into one module, so a function or type declared in one file can be used in any other file without an import:

// Greeting.swift
func greeting(for name: String) -> String {
    "Hello, \(name)!"
}

// main.swift
print(greeting(for: "Ada"))

Running this program prints Hello, Ada!: main.swift calls the function that lives in Greeting.swift.

Only one file may contain top-level statements, code that runs on its own outside any function or type. That file is main.swift, and it is where the program starts. The other files hold declarations only:

// Greeting.swift
func greeting(for name: String) -> String { "Hello, \(name)!" }
// print("hi")      // error: statements are not allowed at the top level here

// main.swift
let name = readLine() ?? "friend"   // top-level code is fine in main.swift
print(greeting(for: name))

The usual layout is one type per file, named after the type: Dog.swift holds Dog, Account.swift holds Account. In this section's challenges, your code goes in those files and main.swift holds the supplied input/output code, which you usually cannot edit. The editor shows each file as a tab.

Dog.swift        // class Dog { ... }
Account.swift    // struct Account { ... }
main.swift       // reads the input and prints the results

Access control follows files too. private and fileprivate hide a declaration from other files, while the default, internal, makes it visible in every file of the module:

// Secret.swift
fileprivate let salt = "x7"          // only this file can see it
func hashed(_ s: String) -> String { s + salt }

// main.swift
print(hashed("pw"))                  // fine: hashed is internal
// print(salt)                       // error: salt is fileprivate

Challenge

Easy

The program has two files. main.swift reads a name and prints the result of greeting(for:). Complete greeting(for:) in Greeting.swift so it returns Hello, Ada! for the name Ada, or Hello, friend! when the name is empty.

Your code goes in Greeting.swift. main.swift holds the supplied input/output code and cannot be edited.

Try it yourself

// Supplied input/output code: keep it as it is
var input: [String] = []
while let line = readLine() { input.append(line) }
for name in input { print(greeting(for: name)) }