在swift 5中

https://www.hackingwithswift.com/example-code/strings/how-to-capitalize-the-first-letter-of-a-string

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

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

和你的绳子一起使用

let test = "the rain in Spain"
print(test.capitalizingFirstLetter())

斯威夫特4.0

string.capitalized(with: nil)

or

string.capitalized

但是，每个单词的首字母都要大写

苹果公司的文档:

A capitalized string is a string with the first character in each word changed to its corresponding uppercase value, and all remaining characters set to their corresponding lowercase values. A “word” is any sequence of characters delimited by spaces, tabs, or line terminators. Some common word delimiting punctuation isn’t considered, so this property may not generally produce the desired results for multiword strings. See the getLineStart(_:end:contentsEnd:for:) method for additional information.

下面是我一小步一小步完成的方法，类似于@Kirsteins。

func capitalizedPhrase(phrase:String) -> String {
    let firstCharIndex = advance(phrase.startIndex, 1)
    let firstChar = phrase.substringToIndex(firstCharIndex).uppercaseString
    let firstCharRange = phrase.startIndex..<firstCharIndex
    return phrase.stringByReplacingCharactersInRange(firstCharRange, withString: firstChar)
}

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()

extension String {
    var lowercased:String {
        var result = Array<Character>(self.characters);
        if let first = result.first { result[0] = Character(String(first).uppercaseString) }
        return String(result)
    }
}

Swift 5.1或更高版本

extension StringProtocol {
    var firstUppercased: String { prefix(1).uppercased() + dropFirst() }
    var firstCapitalized: String { prefix(1).capitalized + dropFirst() }
}

斯威夫特5

extension StringProtocol {
    var firstUppercased: String { return prefix(1).uppercased() + dropFirst() }
    var firstCapitalized: String { return prefix(1).capitalized + dropFirst() }
}

"Swift".first  // "S"
"Swift".last   // "t"
"hello world!!!".firstUppercased  // "Hello world!!!"

"DŽ".firstCapitalized   // "Dž"
"Dž".firstCapitalized   // "Dž"
"dž".firstCapitalized   // "Dž"