W1040 Printing and String Interpolation

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

Teletype

Prerequisites

• W1037 Expressions

Background

A teletype machine (or teleprinter) is an electromechanical device which can be used to send and received typed messages over a distance. It was one of the first devices which enabled the layman, not trained in Morse Code, to communicate easily with another.

In addition to processing commands to print printable characters the teletype processed non-printable characters. The most common of these are:

TAB

A control character (ASCII code 0x09) used to move a device's position to the next horizontal tab stop.

LINE FEED

A control character (ASCII code 0x0A) used to move a device's position to the next line. Abbreviated as LF.

CARRIAGE RETURN

A control character (ASCII code 0x0D) used to reset a device's position to the beginning of a line of text. Abbreviated as CR.

Introduction

The print function is one of the most basic functions available and is also one of the most useful. It provides us with the ability to easily output data from our program, most often to the console. As such, it is invaluable for both a completed programming and for assisting us to debug a program during development.

Basic Usage

In its most basic form, the print function takes a single argument. It's able to print integers, floating points, Booleans, and strings.

print(12)    // An Int
print(13.0)  // A Double
print(true)  // A Boolean
print("Water supply") // A String

Mutliple Items

The print function can also print multiple items of various types.

print(12, 13.0, true, "Water supply") 
// Prints: 12 13.0 true Water supply

By default, each of the items is separated by a space character. Swift enables us to easily change this separator to any character (or characters) that we'd prefer:

print(12, 13.0, true, "Water supply", separator:"|") 
// Prints: 12|13.0|true|Water supply

Mutliple Items on a Single Line

By default, Swift will insert a new line at the end of each print statement. Consider:

print("First")
print("Second")
print("Third")
print("Fourth")
// Prints:
// First
// Second
// Third
// Fourth

Swift enables us to easily change this terminator to any character (or characters) that we'd prefer:

print("First", terminator:"-")
print("Second", terminator:"-")
print("Third", terminator:"-")
print("Fourth")
// Prints:
// First-Second-Third-Fourth

Special Characters

We can use special characters to alter what we're printing:

• \t represents a Tab
• \n represents a Newline
• \r represents a Carriage return

Consider the newline:

print("Hello\nWorld")
// Prints:
// Hello
// World

Consider the carriage return:

print("Hello Dear\rWorld")
// Prints:
// World Dear

Consider the tab:

print("Hello\tDear\tWorld")
// Prints:
// Hello   Dear    World

Concatenation

Strings can be combined by using the + concatenation operator:

let s1 = "The"
let s2 = " Inner "
let s3 = "Light"
print(s1 + s2 + s3)
// Prints: 
// The Inner Light

String Interpolation

String interpolation enables us to easily form complex strings by interpolating other values (literals, constants, variables, and expressions) into the string. The values to be interpolated are enclosed within parentheses and preceded by a backslash. For example:

let a = 7 * 7
let b = 4.0 * 4.0
let c = false
print("This is a string.  7 * 7 is \(a).  4.0 * 4.0 is \(b). The opposite of true is \(c).")
// Prints: 
// This is a string.  7 * 7 is 49.  4.0 * 4.0 is 16.0. The opposite of true is false.

	Coming Soon
	More examples of string interpolation, including expressions and dealing with optional nils.

Standard Output

Standard Streams

By default, a user's console output (nowadays to a screen) is fed from a program through the standard output stream. This stream of data is established by the operating system and connected to the running program automatically.

References

• Teleprinter (Wikipedia)
• Print (Swift Documentation)
• Strings (Swift Documentation)