File

A file contains string data. With the NSString init method, we can access this data as a string in Swift 5.9. We must specify arguments to load the file.

With contentsOfFile, we create a string based on the specified path. We must specify an encoding—String.encoding.ascii works for a simple text file.

Example code

First we must import Foundation to access NSString. Then we use a constant string for the path. Please change the path to a file that exists on your computer.

import Foundation

// Read data from this file.
let path = "programs/example.txt"

// Use contentsOfFile overload.
// ... Specify ASCII encoding.
// ... Ignore errors.
let data = try? NSString(contentsOfFile: path, encoding: String.Encoding.ascii.rawValue)

// If a value was returned, print it.
print(data!)
Some example text.

Loop over file lines

This program reads in a file that has three lines of text. It uses the contentsOfFile method. It then uses enumerateLines to iterate over the lines.

import Foundation

// File path (change this).
let path = "programs/example2.txt"

// Read an entire text file into an NSString.
let contents = try? NSString(contentsOfFile: path, encoding: String.Encoding.ascii.rawValue)

// Print all lines.
contents!.enumerateLines({ (line, stop) -> () in
    print("Line = \(line)")
})
Line = Hello
Line = friend
Line = how
Line = are
Line = you

Write, toFile

We can write a string to a text file with the write() toFile function. This will create a new file if one does not already exist.

import Foundation

// Target path.
let path = "example.txt"

// Write this text.
let text = "Dante, The Divine Comedy"

// Write the text to the path.
try text.write(toFile: path,
    atomically: true,
    encoding: String.Encoding.ascii)
Dante, The Divine Comedy

Try keyword

The NSString init method shown may cause an error. The file may not exist, or may not be available to the program.

For many important tasks in Swift, we must access the Foundation library. Swift provides helpful types like String. But with NSString we load strings with contentsOfFile.