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 that can be used to send and received typed messages over a distance. It was one of the first devices that 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 one of the most useful. It allows us to easily output data from our program, most often to the console. As such, it is invaluable for both a completed programming and for helping 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.

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

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Mutliple Items

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

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print(12, 13.0, true, "Water supply")

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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:

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

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Mutliple Items on a Single Line

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

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print("First") print("Second") print("Third") print("Fourth")

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

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

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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:

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print("Hello\nWorld")

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Consider the carriage return:

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print("Hello Dear\rWorld")

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Consider the tab:

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print("Hello\tDear\tWorld")

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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 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.

Exercises

Exercises

Exercises previously deferred:

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1.  M1037-10  Complete  Merlin Mission Manager  Mission M1037-10.
2.  M1039-10  Complete  Merlin Mission Manager  Mission M1039-10.
3.  M1039-11  Complete  Merlin Mission Manager  Mission M1039-11.

References

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