Cannot use mutating member on immutable value inside a closure

Question

I try to capitalize the first letter of each word of a string, and I have the following code:

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
    }

However, I get the following error:

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

I tried adding var before word (var word), but I get the error:

Parameters may not have the 'var' specifier.

How can I solve this?

@LeoDabus, thank you, but I wanted to implement this function by myself because of a TDD course I am following. Definitely your solution avoids reinventing the wheel. — lmiguelvargasf, Nov 18 '16 at 02:10
I would suggest using https://developer.apple.com/reference/foundation/nsstring/1416774-enumeratesubstrings byWords options — Leo Dabus, Nov 18 '16 at 02:12

score 7 · Accepted Answer · answered Nov 18 '16 at 01:20

7

Make a local mutable copy:

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
}

answered Nov 18 '16 at 01:20

Alexander

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your code works, but what is the difference between var word insider the closure and (word)? It is like confusing for me. Btw, thank you so much for your response. – lmiguelvargasf Nov 18 '16 at 01:24
I don't understand your question, if that was a question – Alexander Nov 18 '16 at 01:24
when you say `var word = word` how the compiler differentiates the closure argument, and the local mutable copy? – lmiguelvargasf Nov 18 '16 at 01:29
1

The mutable copy shadows the immutable parameter – Alexander Nov 18 '16 at 01:29

Cannot use mutating member on immutable value inside a closure

1 Answers1