無法在一成不變的值使用可變成員的封閉

Question

我試圖利用一個串的每個單詞的第一個字母裏面，我有以下代碼：

func makeHeadline(string: String) -> String { 
     let headline = words.map { (word) -> String in 
     var word = word 
     let firstCharacter = word.remove(at: word.startIndex) 
     return "\(String(firstCharacter).uppercased())\(word)" 
     }.joined(separator: " ") 

     return headline 
    } 

不過，我得到以下錯誤：

Cannot use mutating member on immutable value: 'word' is a 'let' constant.

我試圖字(var word)之前添加變種，但我得到的錯誤：

Parameters may not have the 'var' specifier.

我該如何解決這個問題？

Answer 1

做一個本地副本可變：

func makeHeadline(string: String) -> String { 
    let words = string.components(separatedBy: " ") 

    let headline = words.map { (word) -> String in 
     var word = word 
     let firstCharacter = word.removeAtIndex(word.startIndex) 
     return String(firstCharacter).uppercaseString + word 
    }.joined(separator: " ") 

    return headline 
}