Strings and Characters

String Literals

Multiline String Literals

a single line String can be initialized by a pair of double quotation marks(" ")

But a multiline String should be initialized by a pair of three double quotation marks(""" """)。

let singleLineString = "These are the same."



let multilineString = """

The White Rabbit put on his spectacles.  "Where shall I begin,

please your Majesty?" he asked.



"Begin at the beginning," the King said gravely, "and go on

till you come to the end; then stop."

"""

and for multiline String, we can use a backslash() to make it easier to read.

let softWrappedQuotation = """

The White Rabbit put on his spectacles.  "Where shall I begin, \

please your Majesty?" he asked.



"Begin at the beginning," the King said gravely, "and go on \

till you come to the end; then stop."

"""

Special Characters in String Literals

An arbitrary Unicode scalar value, written as \u{n}, where n is a 1–8 digit hexadecimal number.

let dollarSign = "\u{24}"    // $, Unicode scalar U+0024

let blackHeart = "\u{2665}"   // ♥, Unicode scalar U+2665

let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496

but swift can support the Unicode as String directly, so this rarely to use.

Extended String Delimiters

if you use “#” this sign to surround the String, you can print a string without any effect.

let string = #"Line1 \n Line2"# // print "Line1 \n Line2" will not being wrapped

but if you want to make effect anyway, you can try this:

let string = #"Line1 \n Line2 \#n Line3"# 
/*print "Line1 \n Line2
Line3" 
will being wrapped before Line3
*/

Initializing an Empty String

An empty String means that the string dosen’t have length.

var emptyString = ""               // empty string literal
var anotherEmptyString = String()  // initializer syntax
// these two strings are both empty, and are equivalent to each other

print(emptyString.isEmpty) // is true
print(anotherEmptyString.isEmpty) // is true

Strings Are Value Types

String will alway be copied to another one when it’s assigned or passed to a function or method.

Concatenating Strings and Characters

For multiline string, interesting, just for example:

let badStart = """
one
two
"""
let end = """
three
"""
print(badStart + end)
// Prints two lines:
// one
// twothree

let goodStart = """
one
two

"""
print(goodStart + end)
// Prints three lines:
// one
// two
// three

Accessing and Modifying a String

String Indices

Get indices of String:

First, we can get the first and last index with those properties:

1. startIndex
2. endIndex

Then, we can use those function to get each other indices:

1. index(before:)
2. index(after:)
3. index(_:offsetBy:)

For example:

let greeting = "Guten Tag!"
greeting[greeting.startIndex]
// G
greeting[greeting.index(before: greeting.endIndex)]
// !
greeting[greeting.index(after: greeting.startIndex)]
// u
let index = greeting.index(greeting.startIndex, offsetBy: 7)
greeting[index]
// a

If the index outside of a string’s range, we will get a runtime error:

greeting[greeting.endIndex] // Error
greeting.index(after: greeting.endIndex) // Error

we can for-each indices:

for index in greeting.indices {
    print("\(greeting[index]) ", terminator: "")
}
// Prints "G u t e n   T a g ! "

Inserting and Removing

1. insert(_:at:) //insert a charactor into a string at a specific index
2. insert(contentsOf:at:) //insert a string into a string from a specific index
3. remove(at:) //remove a charactor from a string at a specific index
4. removeSubrange(_:) //remove a string from a string at a specific range

For example:

var welcome = "hello"
welcome.insert("!", at: welcome.endIndex)
// welcome now equals "hello!"

welcome.insert(contentsOf: " there", at: welcome.index(before: welcome.endIndex))
// welcome now equals "hello there!"

welcome.remove(at: welcome.index(before: welcome.endIndex))
// welcome now equals "hello there"

let range = welcome.index(welcome.endIndex, offsetBy: -6)..<welcome.endIndex
welcome.removeSubrange(range)
// welcome now equals "hello"

Substrings

Substrings aren’t suitable for long-term storage—because they reuse the storage of the original string.

When you’re ready to store the result for a longer time, you convert the substring to an instance of String.

let greeting = "Hello, world!"
let index = greeting.firstIndex(of: ",") ?? greeting.endIndex
let beginning = greeting[..<index]
// beginning is "Hello"

// Convert the result to a String for long-term storage.
let newString = String(beginning)

Let’s think!