swift Swift String Playground示例

Posted 2021-05-19

// Swift Standard Librray - String
// Keith Harrison http://useyourloaf.com

// Import Foundation if you want to bridge to NSString
import Foundation

// ====
// Initializing a String
// ====

var emptyString = ""        // Empty String
var stillEmpty = String()   // Another empty String
let helloWorld = "Hello World!" // String inferred

let a = String(true)        // from boolean: "true"
let b: Character = "A"      // Explicit type required to create a Character
let c = String(b)           // from character "A"
let d = String(3.14)        // from Double "3.14"
let e = String(1000)        // from Int "1000"
let f = "Result = \(d)"     // Interpolation "Result = 3.14"
let g = "\u{2126}"          // Unicode Ohm sign Ω
let h = String(count:3, repeatedValue:b) // Repeated character "AAA"

// ===
// Strings are Value Types
// ===

// Strings are value types that are copied when assigned
// or passed to a function. The copy is performed lazily
// on mutation.

var aString = "Hello"
var bString = aString
bString += " World!"
print("\(aString)")   // "Hello\n"

// ===
// Testing for empty String
// ===

emptyString.isEmpty         // true

// ===
// Testing for equality
// ===

// Swift is Unicode correct so the equality operator
// (“==”) checks for Unicode canonical equivalence.
// This means that two Strings that are composed from
// different Unicode scalars will be considered equal
// if they have the same linguistic meaning and appearance:

let spain = "España"
let tilde = "\u{303}"
let country = "Espan" + "\(tilde)" + "a"
if country == spain {
  print("Matched!")       // "Matched!\n"
}

// Order
if "aaa" < "bbb" {
  print("aaa is less than bbb")
}

// ===
// Testing for suffix/prefix
// ===

let line = "0001 Some test data here %%%%"
line.hasPrefix("0001")    // true
line.hasSuffix("%%%%")    // true

// ===
// Converting to upper/lower case
// ===

let mixedCase = "AbcDef"
let upper = mixedCase.uppercaseString // "ABCDEF"
let lower = mixedCase.lowercaseString // "abcdef"

// ===
// Views
// ===

// Strings are not collections of anything but do provide
// collection views for different representations accessed
// through the appropriate property:

country.characters       // characters
country.unicodeScalars   // Unicode scalar 21-bit codes
country.utf16            // UTF-16 encoding
country.utf8             // UTF-8 encoding

// ===
// Counting
// ==

// String does not directly have a property to return a count
// as it only has meaning for a particular representation.
// So count is implemented on each of the collection views:

// spain = "España"
print("\(spain.characters.count)")      // 6
print("\(spain.unicodeScalars.count)")  // 6
print("\(spain.utf16.count)")           // 6
print("\(spain.utf8.count)")            // 7

// ===
// Using Index to traverse a collection
// ===

// Each of the collection views has an Index
// that you use to traverse the collection.
// This is maybe one of the big causes of pain
// when getting to grips with String. You cannot
// randomly access an element in a string using
// a subscript (e.g. string[5]).

// To iterate over all items in a collection
// with a for..in loop:

var sentence = "Never odd or even"
for character in sentence.characters {
  print(character)
}

// Each collection has two instance properties you
// can use as subscripts to index into the collection:
//   startIndex: the position of the first element if
//               non-empty, else identical to endIndex.
//   endIndex: the position just “past the end” of the string.
//
// Note the choice for endIndex means you cannot use it
// directly as a subscript as it is out of range.

let cafe = "café"
cafe.startIndex // 0
cafe.endIndex   // 4 - after last char

// The startIndex and endIndex properties become more
// useful when modified with the following methods:
//   successor() to get the next element
//   predecessor() to get the previous element
//   advancedBy(n) to jump forward/backward by n elements
//
// Some examples, note that you can chain operations if required:

cafe[cafe.startIndex] // "c"
cafe[cafe.startIndex.successor()] // "a"
cafe[cafe.startIndex.successor().successor()] // "f"
// Note that cafe[endIndex] is a run time error
cafe[cafe.endIndex.predecessor()] // "é"
cafe[cafe.startIndex.advancedBy(2)] // "f"

// The indices property returns a range for all elements
// in a String that can be useful for iterating through
// the collection:

for index in cafe.characters.indices {
  print(cafe[index])
}

// You cannot use an index from one String to access a
// different string. You can convert an index to an integer
// value with the distanceTo method:

let word1 = "ABCDEF"
let word2 = "012345"
let indexC = word1.startIndex.advancedBy(2)
let distance = word1.startIndex.distanceTo(indexC) // 2
let digit = word2[word2.startIndex.advancedBy(distance)] // "2"

// ===
// Using a range
// ===

// To identify a range of elements in a String collection
// use a range. A range is just a start and end index:

let fqdn = "useyourloaf.com"
let rangeOfTLD = Range(start: fqdn.endIndex.advancedBy(-3), end: fqdn.endIndex)
let tld = fqdn[rangeOfTLD] // "com"

// Alternate creation of range (... or ..< syntax)
let rangeOfDomain = fqdn.startIndex..<fqdn.endIndex.advancedBy(-4)
let domain = fqdn[rangeOfDomain] // "useyourloaf"

// ===
// Getting a substring using index or range
// ===

// Various methods to get a substring identified by an
// index or range:

var template = "<<<Hello>>>"
let indexStartOfText = template.startIndex.advancedBy(3)
let indexEndOfText = template.endIndex.advancedBy(-3)
let subString1 = template.substringFromIndex(indexStartOfText)
let subString2 = template.substringToIndex(indexEndOfText)

let rangeOfHello = indexStartOfText..<indexEndOfText
let subString3 = template.substringWithRange(rangeOfHello)
let subString4 = template.substringWithRange(indexStartOfText..<indexEndOfText)

// ===
// Getting a Prefix or Suffix
// ===

// If you just need to drop/retrieve elements at the beginning 
// or end of a String:

let digits = "0123456789"
let tail = String(digits.characters.dropFirst()) // "123456789"
let less = String(digits.characters.dropFirst(3)) // "3456789"
let head = String(digits.characters.dropLast(3)) // "0123456"
let prefix = String(digits.characters.prefix(2)) // "01"
let suffix = String(digits.characters.suffix(2)) // "89"

let index4 = digits.startIndex.advancedBy(4)
let thru4 = String(digits.characters.prefixThrough(index4)) // "01234"
let upTo4 = String(digits.characters.prefixUpTo(index4)) // "0123"
let from4 = String(digits.characters.suffixFrom(index4)) // "456789"

// ===
// Inserting/removing
// ===

// Insert a character at index
var stars = "******"
stars.insert("X", atIndex: stars.startIndex.advancedBy(3)) // "***X***"

// Insert a string at index (converting to characters)
stars.insertContentsOf("YZ".characters, at: stars.endIndex.advancedBy(-3)) // "***XYZ***"

// ===
// Replace with Range
// ===

let xyzRange = stars.startIndex.advancedBy(3)..<stars.endIndex.advancedBy(-3)
stars.replaceRange(xyzRange, with: "ABC") // "***ABC***"

// ===
// Append
// ===

var message = "Welcome"
message += " Tim" // "Welcome Tim"
message.appendContentsOf("!!!") // "Welcome Tim!!!

// ===
// Remove and return element at index
// ===

// This invalidates any indices you may have on the string

var grades = "ABCDEF"
let ch = grades.removeAtIndex(grades.startIndex) // "A"
print(grades) // "BCDEF"

// ===
// Remove Range
// ===

// Invalidates all indices
var sequences = "ABA BBA ABC"
let midRange = sequences.startIndex.advancedBy(4)...sequences.endIndex.advancedBy(-4)
sequences.removeRange(midRange)  // "ABA ABC"

//
// Bridging to NSString (import Foundation)
// 

// String is bridged to Objective-C as NSString.
// If the Swift Standard Library does not have what
// you need import the Foundation framework to get
// access to methods defined by NSString.
  
// Be aware that this is not a toll-free bridge so
// stick to the Swift Standard Library where possible.

// Don't forget to import Foundation
let welcome = "hello world!"
welcome.capitalizedString // "Hello World!"

// ===
// Searching for a substring
// ===

// An example of using NSString methods to perform a
// search for a substring:

let text = "123045780984"
if let rangeOfZero = text.rangeOfString("0",options: NSStringCompareOptions.BackwardsSearch) {
  // Found a zero
  let suffix = String(text.characters.suffixFrom(rangeOfZero.endIndex)) // "984"
}