Unable to append string to mutable array in Swift

Question

Context - I'm currently learning Swift Struct. So I decided to create my own Phone structure in Playground. (see below)

Problem - The downloadApp method on the phone struct is throwing the following error.. Cannot use mutating member on immutable value: 'self' is immutable.

Expect outcome - To peacefully append new strings to my apps property, which is an array of strings.

Swift code

struct Phone {
    let capacity : Int
    let Brand : Sting
    var name: String
    let model: String
    var apps: [String]
    var contacts: [String]

    var formatCapacity: String {
        return "\(capacity)GB"
    }

    func downloadApp(name: String){
        apps.append(name) // ERROR 
    }
}

David Pasztor · Accepted Answer · 2018-07-25 20:29:32Z

1

You simply need to mark downloadApp as mutating. The issue is caused by the fact that downloadApp is declared in a struct type, which is a value type and hence if you mutate any properties (namely, the apps array here) of the struct, you actually mutate the struct itself. By marking the function as mutating, the compiler allows such modification of the struct through a member function of the type.

mutating func downloadApp(name: String){
    apps.append(name)
}

answered Jul 25, 2018 at 20:29

David Pasztor

55.1k9 gold badges97 silver badges130 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Samuel Over a year ago

Thank you very much! Apples Intro to app development book didn't show me that! Baby steps I guess

Collectives™ on Stack Overflow

Unable to append string to mutable array in Swift

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related