swift2 字符与字符串

字符串是值类型

与NSString不同，创建了一个NSString实例，并将其传递给一个函数/方法，或者赋值给一个变量。

传递或赋值的是该NSString实例的一个引用，除非您特别要求进行值拷贝，否则字符串不会生成新的副本来进行赋值操作。

Swift 默认字符串拷贝的方式保证了在函数/方法中传递的是字符串的值。

很明显无论该值来自于哪里，都是独自拥有的，字符串本身不会被更改。

在实际编译时，Swift 编译器会优化字符串的使用，使实际的复制只发生在绝对必要的情况下，这意味着将字符串作为值类型的同时可以获得极高的性能。

计算字符数量

let str = "hello world,hello swift"
print(str.characters.count)

比较字符串

let quotation = "我们是一样一样滴."
let sameQuotation = "我们是一样一样滴."
if quotation == sameQuotation {
    print("这两个字符串被认为是相同的")
}

字符串字面值相同则认为字符串相等。

前缀或后缀相等

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"
]
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"

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"

大小写字符串

let normal = "Could you help me,please?"
let shouty = normal.uppercaseString
// shouty 值为 "COULD YOU HELP ME,PLEASE?"
let whispered = normal.lowercaseString
// whispered 值为 "could you help me,please?"