Mark closure element mutable Swift - swift

Question

I have 2 structs, first is:
struct LineData {
init (name: String,
colorValue: String,
values: [Int]){
self.name = name
self.colorValue = colorValue
self.values = values
}
private var cachedMaxValue: Int? = nil
let name: String
let colorValue: String
let values: [Int]
// describe max value for Y axis for specific Line
mutating func maxValue() -> Int{
if let cached = cachedMaxValue {
return cached
}
self.cachedMaxValue = values.max()
return cachedMaxValue ?? 0
}
}
Second have array of LineData structs:
struct CharData {
init(xAxis: XAxis,
lines: [LineData]){
self.xAxis = xAxis
self.lines = lines
}
private var cachedMaxValue: Int? = nil
var xAxis: XAxis
var lines: [LineData]
// describe max value for Y axis among lines
func maxValue() -> Int{
var maxValues: [Int] = []
lines.forEach{it in
maxValues.append(it.maxValue())
}
return 0
}
}
Code above not compile, because, of error on method maxValues for struct CharData. It says Cannot use mutating member on immutable value: 'it' is a 'let' constant
What i want is, iterate through an array of lines and among it max values find greater value.

Answer 1

Since lines is an ordinary array, how about simply:
for i in 0..<lines.count {
maxValues.append(lines[i].maxValue())
}
perhaps not quite as Swifty, but nothing gets copied. The optimizer ought to give you pretty much the same performance as forEach.

Answer 2

It's the it parameter/object in the forEach that's immutable. Just like the error says: "it is a let". You could probably do something like this:
lines.forEach { it in
var mutableIt = it
maxValues.append(mutableIt.maxValue())
}
It should be noted that this will create a mutable copy of the "it" struct instance.