Strings and Characters

// "Hello World!" is a String literal. 
let sentence = "Hello World!"

// Multiline
let story = """
There are some people in the room.
They are having a party.
Because today is the Christmas.
"""
// every line has the line breaks.
// the start and end sign(""") must take a single line.

// backlash(\), it means that the string is not broken. Then line breaks not to be part of the string's value.
let content = """
In a happy atmos\
phere. We start the conversation
"""
print(content) // result: In a happy atmosphere. We start the conversation.

// Alignment of quotation marks in the multiline string.
let desc = """
    Ha it's so funny.
"""
print(desc)  // result:     Ha its so funny.

let desc = """
    Ha it's so funny.
    """
print(desc)  // result: Ha its so funny.

let desc = """
        Ha it's so funny.
    """
print(desc)  // result:     Ha its so funny.

let dollarSign = "\u{24}"    // $

let blackHeart = "\u{2665}"  // ♥

let redHeart = "\u{1f496}"   // 💖

// Because multiline string literals use three double quotation marks, one double quotation can be used in multiline string litral without escaping it.
// And should escape the three double quotation which inside the multiline string literal.

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

// Abosultly string
// use the #""" and """# to include a absultly pure string.
let words = #"""
    It sounds very good."""
    """#
print(words)  // result: It sounds very good."""

// use the #" and "# to include a absultly pure string.
let words = #"It sounds """ very good."#
print(words)  // result: It sounds """ very good.

// So it has no need to add escaping character(\).

// If we want to cancel the effect of '#' in a string, use another '#' near the character.
let otherWords = #"hello \#rworld"#
print(otherWords)  // result: 

// If we want to cancel the effect of '#' in a multiline string, use another '#' near the character.
let otherWords = #"""
hello \###rworld
"""#
print(otherWords)  // result: hello     world 

// They are all empty, they are equivalent to each other.
var myName = ""
var hisName = String()

// Find out if the string is empty.
if myName.isEmpty {
    print("It's empty.")
}

var a = "Horse "
var b = "carriage"
b = a + b     // it willf create a new String and assign to b

let str = "he"
str += "ran"  // it will trigger the compile-time error.
// constant string cannot be modified.

for character in "dog!" {
    print(character)
}

let c : Character = "!"

let characters : [Character] = ["c", "a", "t", "s"]
let str = String(characters)

// concatenation 
str.append(c)

let pet = "dog " + "cat" 

var name = "Mike "
name += "Amy"

// multiline string
let start = """
one
two
"""
let end = """
three
"""
let goodStart = """
one
two

"""

print(start + end)
// result: 
// one
// twothree

print(goodStart + end)
// result:
// one 
// two
// three

let value = 3
let message = "The value is \(value). Four times is \(value * 4). \("It's " + "fine.")"
print(message)  // result: the value is 3. Four times is 12. It's fine.

print(#"It's a \(value)."#)  
// It will print the literal string without escaping the 'value'.
// It's a \(value).

print(#"It's a \#(value)."#)
// It will escape the value.
// It's a 3.

print("\u{E9}")      // é
print("\u{65}\u{301} // é  // two character compound into one character automatically.

print("\u{1f425}")   // 🐥

let c = "\u{1F1E8}\u{1F1F3}" 
print(c + String(c.count)  // 🇨🇳1  // two character compound into one. cn = c + n

let str = "chick\u{1f425}"
print(str + String(str.count))    // result: chick🐥6

var word = "cafe"
word += "\u{301}"
print(word + String(word.count))  // result: café4

var str = "peak"
str[str.startIndex] = "g" // not allowed, can not assign through subscript
print(str[str.startIndex] // subscript is get-only, the result is : p
print(str[str.index(before: str.endIndex)])      // result: k
print(str[str.index(after: str.startIndex)])     // result: e
print(str[str.index(str.startIndex, offsetBy:2)])// result: a

for c in str.indices {
	print("\(str[c]) ", terminator: "") // set terminator empty to cancel the line feed.
}
// result: p e a k

// Insert and remove single character
var word = "world"
word.insert("k", at: word.index(word.endIndex, offsetBy: -2)) 
word.remove(at: word.index(word.startIndex, offsetBy: 4))
word.remove(at: word.index(before: word.endIndex))
print(word)
// result: work

// Insert and remove a substring
var message = "we"
message.insert(contentsOf : "lcome to here", at : message.endIndex)
print(message)  //print: welcome to here 

message.removeSubrange(message.index(message.startIndex, offsetBy: 7)..<message.endIndex)

print(message)  //print: welcome

var str = "Hello, World!"
let index = str.firstIndex(of: ",") ?? str.endIndex
var sub = str[..<index]    // It doesn't has a whole copy of String, just create a new type 'Substring' to reference the part we cut.
print(sub)

sub.remove(at: sub.startIndex)
print(sub)     // print: ello
print(str)     // It doesn't affect the origin String, because 'sub' is a independent Substring instance.

let newString = String(sub)// Create an instance of String. Transform a subString to String for long term keep.

var a = String("word")
var b = String("word")
print(a == b)         // print: true

var str = String("Hello, world!\u{E9}")
var quote = String("Hello, world!\u{65}\u{301}")
if str == quote {
    print("Equal") 
    // The combine result of 'quote' is same as in str. So the these two Strings are equal.
    // e +  ́ = é
}
// print: equal

// Single character
var c = String("\u{41}")
var d = String("\u{041}")
print(c == d)            // print: true

var c = String("\u{41}")
var d = String("\u{0410}")
print(c == d)           // print: false

var sentences = [
    "Hello, world!",
    "Hello, friend!",
    "Hi,Hello, friends all!",
    "Welcome, friend!",
    "Welcome, friends all!"
]
for sentence in sentences {
    if sentence.hasPrefix("Hello") {
        print(sentence)
    }
}
// Hello, world!
// Hello, friend!

for sentence in sentences {
		if sentence.hasSuffix("friend!") {
        print("[" + sentence + "]")
    }
}
// [Hello, friend!]
// [Welcome, friend!]     

let word = "dog!!\u{1F436}"

print(word)
print("UTF-8:", terminator: " ")
for c in word.utf8 {
    print("\(c)", terminator : " ")
}
print("\nUTF-16",terminator: " ")
for c in word.utf16 {
    print("\(c)", terminator: " ")
}
print("\nUnicode:", terminator: " ")
for c in word.unicodeScalars {
    print("\(c.value)", terminator: " ")
}
print()
print(word.count)