Modifying an array passed as an argument to a function in Swift

Question

Sorry for the newbie question; I'm still learning. I'm running into some odd behavior and couldn't find any documentation on this. I was wondering if you can help point out what I'm doing wrong here.

Error:

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

class mySingleton {
    static let sharedInstance = mySingleton()
    private init() {}

    var men = ["bob", "doug"]
    var women = ["alice", "lisa"]

    func removeFirst() {
        self.arr.removeFirst()
    }

    func removeFirstByGender(gender: String) {
        if gender == "men" {
              self.modify(arr: self.men) // <-- error occurs here.
        } else {
              self.modify(arr: self.women) // <-- error occurs here.
        }
    }

    func modify(arr: [String]) {
        arr.removeFirst()
    }
}

Answer 1

You need to change the definition of modify to accept an inout parameter. By default, function arguments are immutable, but by using the inout keyword, you can make them mutable. You also need to pass the argument by reference.

func modify( arr: inout [String]) {
    arr.removeFirst()
}
var stringArr = ["a","b"]
modify(arr: &stringArr) //stringArr = ["b"] after the function call