language guide

[Bridging Between String and NSString](https://developer.apple.com/documentation/swift/string#2919514)

> Any `String`instance can be bridged to `NSString` using the type-cast operator (`as`)
> 

> if you import Foundation, you can access those `NSString` methods on `String` without casting.
> 

String Literals

let someString = "Some string literal value"

let quotation = """
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."
"""

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."
"""

let wiseWords = "\"Imagination is more important than knowledge\" - Einstein"
// "Imagination is more important than knowledge" - Einstein
let dollarSign = "\u{24}"        // $,  Unicode scalar U+0024
let blackHeart = "\u{2665}"      // ♥,  Unicode scalar U+2665
let sparklingHeart = "\u{1F496}" // 💖, Unicode scalar U+1F496

let threeDoubleQuotationMarks = """
Escaping the first quotation mark \"""
Escaping all three quotation marks \"\"\"
"""

Initializing an Empty String

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

String Mutability

var variableString = "Horse"
variableString += " and carriage"
// variableString is now "Horse and carriage"

let constantString = "Highlander"
constantString += " and another Highlander"
// this reports a compile-time error - a constant string cannot be modified

Strings Are Value Types

Working with Characters

for character in "Dog!🐶" {
    print(character)
}
// D
// o
// g
// !
// 🐶

let catCharacters: [Character] = ["C", "a", "t", "!", "🐱"]
let catString = String(catCharacters)
print(catString)
// Prints "Cat!🐱"

Concatenating Strings and Characters

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome now equals "hello there"

let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome now equals "hello there!"

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

String Interpolation

let multiplier = 3
let message = "\(multiplier) times 2.5 is \(Double(multiplier) * 2.5)"
// message is "3 times 2.5 is 7.5"

print(#"Write an interpolated string in Swift using \(multiplier)."#)
// Prints "Write an interpolated string in Swift using \(multiplier)."

print(#"6 times 7 is \#(6 * 7)."#)
// Prints "6 times 7 is 42."

Unicode

A Unicode scalar value is a unique 21-bit number for a character or modifier, such as U+0061 for LATIN SMALL LETTER A ("a"), or U+1F425 for FRONT-FACING BABY CHICK ("🐥")

Counting Characters

Accessing and Modifying a String

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

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

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

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"