Swift学习笔记-字符串和字符(StringsandCharacters)-比较字符串(ComparingStrings) ...

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Swift 提供了三种方式来比较文本值：字符串字符相等、前缀相等和后缀相等。

字符串/字符相等 (String and Character Equality)

字符串/字符可以用等于操作符(==)和不等于操作符(!=)，详细描述在比较运算符：

let quotation = "We're a lot alike, you and I."
let sameQuotation = "We're a lot alike, you and I."
if quotation == sameQuotation {
    print("These two strings are considered equal")
}
// 打印输出 "These two strings are considered equal"

如果两个字符串（或者两个字符）的可扩展的字形群集是标准相等的，那就认为它们是相等的。在这个情况下，即使可扩展的字形群集是有不同的 Unicode 标量构成的，只要它们有同样的语言意义和外观，就认为它们标准相等。

例如，LATIN SMALL LETTER E WITH ACUTE(U+00E9)就是标准相等于LATIN SMALL LETTER E(U+0065)后面加上COMBINING ACUTE ACCENT(U+0301)。这两个字符群集都是表示字符é的有效方式，所以它们被认为是标准相等的：

// "Voulez-vous un café?" 使用 LATIN SMALL LETTER E WITH ACUTE
let eAcuteQuestion = "Voulez-vous un caf\u{E9}?"

// "Voulez-vous un café?" 使用 LATIN SMALL LETTER E and COMBINING ACUTE ACCENT
let combinedEAcuteQuestion = "Voulez-vous un caf\u{65}\u{301}?"

if eAcuteQuestion == combinedEAcuteQuestion {
    print("These two strings are considered equal")
}
// 打印输出 "These two strings are considered equal"

相反，英语中的LATIN CAPITAL LETTER A(U+0041，或者A)不等于俄语中的CYRILLIC CAPITAL LETTER A(U+0410，或者A)。两个字符看着是一样的，但却有不同的语言意义：

let latinCapitalLetterA: Character = "\u{41}"

let cyrillicCapitalLetterA: Character = "\u{0410}"

if latinCapitalLetterA != cyrillicCapitalLetterA {
    print("These two characters are not equivalent")
}
// 打印 "These two characters are not equivalent"

注意：在 Swift 中，字符串和字符并不区分区域。

前缀/后缀相等 (Prefix and Suffix Equality)

通过调用字符串的hasPrefix(_:)/hasSuffix(_:)方法来检查字符串是否拥有特定前缀/后缀，两个方法均接收一个String类型的参数，并返回一个布尔值。

下面的例子以一个字符串数组表示莎士比亚话剧《罗密欧与朱丽叶》中前两场的场景位置：

let romeoAndJuliet = [
    "Act 1 Scene 1: Verona, A public place",
    "Act 1 Scene 2: Capulet's mansion",
    "Act 1 Scene 3: A room in Capulet's mansion",
    "Act 1 Scene 4: A street outside Capulet's mansion",
    "Act 1 Scene 5: The Great Hall in Capulet's mansion",
    "Act 2 Scene 1: Outside Capulet's mansion",
    "Act 2 Scene 2: Capulet's orchard",
    "Act 2 Scene 3: Outside Friar Lawrence's cell",
    "Act 2 Scene 4: A street in Verona",
    "Act 2 Scene 5: Capulet's mansion",
    "Act 2 Scene 6: Friar Lawrence's cell"
]

您可以调用hasPrefix(_:)方法来计算话剧中第一幕的场景数：

var act1SceneCount = 0
for scene in romeoAndJuliet {
    if scene.hasPrefix("Act 1 ") {
        ++act1SceneCount
    }
}
print("There are \(act1SceneCount) scenes in Act 1")
// 打印输出 "There are 5 scenes in Act 1"

相似地，您可以用hasSuffix(_:)方法来计算发生在不同地方的场景数：

var mansionCount = 0
var cellCount = 0
for scene in romeoAndJuliet {
    if scene.hasSuffix("Capulet's mansion") {
        ++mansionCount
    } else if scene.hasSuffix("Friar Lawrence's cell") {
        ++cellCount
    }
}
print("\(mansionCount) mansion scenes; \(cellCount) cell scenes")
// 打印输出 "6 mansion scenes; 2 cell scenes"