Chapter 9: Strings

大綱

• Strings as collections

  • Grapheme clusters

  • Indexing strings

  • Equality with combining characters

• Strings as bi-directional collections

• Substrings

• Encoding

  • UTF-8

  • UTF-16

• Converting indexes between encoding views

• Key points

Strings as collections

• String是一種collections

let string = "Matt"
for char in string {
  print(char)
}

let stringLength = string.count
let fourthChar = string[3] // error: 'subscript' is unavailable: cannot subscript String with an Int, see the documentation comment for discussion

Grapheme clusters

• There are two ways to represent some characters. One example is the é in café.

  • The single character to represent this is code point 233.

  • The two-character case is an e on its own followed by an acute accent combining character

  

  • The combination of these two characters in the second diagram forms what is known as a grapheme cluster defined by the Unicode standard

    • Grapheme clusters are represented by the Swift type Character.

  // GRAPHEME CLUSTERS
  let cafeNormal = "café"
  let cafeCombining = "cafe\u{0301}"

  cafeNormal.count // 4
  cafeCombining.count // 4

  cafeNormal.unicodeScalars.count // 4
  cafeCombining.unicodeScalars.count // 5

  for codePoint in cafeCombining.unicodeScalars {
    print(codePoint.value)
  }
  99
  97
  102
  101
  769

Indexing strings

// 取得第一個字
let firstIndex = cafeCombining.startIndex // firstIndex的type是String.Index不是integer.
let firstChar = cafeCombining[firstIndex] // firstChar的type是Character也是grapheme cluster

// 取得最後一個字
let lastIndex = cafeCombining.endIndex // Fatal error: String index is out of bounds
let lastIndex = cafeCombining.index(before: cafeCombining.endIndex)
let lastChar = cafeCombining[lastIndex]

// 取得第四個字
let fourthIndex = cafeCombining.index(cafeCombining.startIndex, offsetBy: 3)
let fourthChar = cafeCombining[fourthIndex]

fourthChar.unicodeScalars.count
fourthChar.unicodeScalars.forEach { codePoint in
  print(codePoint.value)
}

Equality with combining characters

• String comparison in Swift uses a technique known as canonicalization

  • Before checking equality, Swift canonicalizes both strings, which means they’re converted to use the same special character representation.

  • café using the single é character and café using the e plus combining accent character had the same length.

let equal = cafeNormal == cafeCombining // true

Strings as bi-directional collections

• 如何進行string的反轉

let name = "Matt"
let backwardsName = name.reversed() // ReversedCollection<String>
let secondCharIndex = backwardsName.index(backwardsName.startIndex, offsetBy: 1)
let secondChar = backwardsName[secondCharIndex]

let backwardsNameString = String(backwardsName) // 透過init反轉回String格式

Substrings

• The reason for this extra Substring type is a cunning optimization.

• A Substring shares the storage with its parent String that it was sliced from. This means that when you’re in the process of slicing a string, you use no extra memory.

let fullName = "Matt Galloway"
let spaceIndex = fullName.index(of: " ")! // String.Index
let firstName = fullName[..<spaceIndex]  // Matt
let lastName = fullName[fullName.index(after: spaceIndex)...] // Galloway (String.SubSequence)
let lastNameString = String(lastName) // Galloway(String)

Encoding

UTF-8

UTF-16

Converting indexes between encoding views

Key points

• Strings are collections of Character types.

• A Character is grapheme cluster and is made up of one or more code points.

• A combining character is a character that alters the previous character in some way.

• You use special (non-integer) indexes to subscript into the string to a certain grapheme cluster.

• Swift’s use of canonicalization ensures that the comparison of strings accounts for combining characters.

• Slicing a string yields a substring with type Substring, which shares storage with its parent String.

• You can convert from a Substring to a String by initializing a new String and passing the Substring.

• Swift String has a view called unicodeScalars, which is itself a collection of the individual Unicode code points that make up the string.

• There are multiple ways to encode a string. UTF-8 and UTF-16 are the most popular.

• The individual parts of an encoding are called code units. UTF-8 uses 8-bit code units, and UTF-16 uses 16-bit code units.

• Swift’s String has views called utf8 and utf16 that are collections which allow you to obtain the individual code units in the given encoding