无法在一成不变的值使用可变成员的封闭

Question

我试图利用一个串的每个单词的第一个字母里面，我有以下代码：

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 
    } 

不过，我得到以下错误：

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

我试图字(var word)之前添加变种，但我得到的错误：

Parameters may not have the 'var' specifier.

我该如何解决这个问题？

Answer 1

做一个本地副本可变：

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 
}