对于swift 5，你可以简单地这样做:

为字符串创建扩展:

extension String {
    var firstUppercased: String {
        let firstChar = self.first?.uppercased() ?? ""
        return firstChar + self.dropFirst()
    }
}

then

yourString.firstUppercased

样本:"abc" -> "abc"

包括与API准则一致的突变和非突变版本。

斯威夫特3:

extension String {
    func capitalizingFirstLetter() -> String {
        let first = String(characters.prefix(1)).capitalized
        let other = String(characters.dropFirst())
        return first + other
    }

    mutating func capitalizeFirstLetter() {
        self = self.capitalizingFirstLetter()
    }
}

斯威夫特4:

extension String {
    func capitalizingFirstLetter() -> String {
      return prefix(1).uppercased() + self.lowercased().dropFirst()
    }

    mutating func capitalizeFirstLetter() {
      self = self.capitalizingFirstLetter()
    }
}

Swift 3 (xcode 8.3.3)

大写字符串的所有首字符

let str = "your string"
let capStr = str.capitalized

//Your String

全部大写

let str = "your string"
let upStr = str.uppercased()

//YOUR STRING

字符串的第一个字符只能大写

 var str = "your string"
 let capStr = String(str.characters.prefix(1)).uppercased() + String(str.characters.dropFirst())

//Your string

extension String {
    func firstCharacterUpperCase() -> String? {
        let lowercaseString = self.lowercaseString

        return lowercaseString.stringByReplacingCharactersInRange(lowercaseString.startIndex...lowercaseString.startIndex, withString: String(lowercaseString[lowercaseString.startIndex]).uppercaseString)
    }
}

let x = "heLLo"
let m = x.firstCharacterUpperCase()

对于Swift 5:

extension String {
    func firstCharacterUpperCase() -> String? {
        guard !isEmpty else { return nil }
        let lowerCasedString = self.lowercased()
        return lowerCasedString.replacingCharacters(in: lowerCasedString.startIndex...lowerCasedString.startIndex, with: String(lowerCasedString[lowerCasedString.startIndex]).uppercased())
    }
}

对于单词使用中的第一个字符。在swift中大写，对于整个单词使用。大写()

我用柯尔斯泰因斯解得到了第一个复制的字符。这将大写第一个字符，不会看到双字符:

var s: String = "hello world"
s = prefix(s, 1).capitalizedString + suffix(s, countElements(s) - 1)

我不知道它的效率是高还是低，我只知道它给了我想要的结果。