Swift dictionary member compile error

Question

When I make a dictionary a member, assignment doesn't compile:

struct MyClass {
var lists = [String:Int]();
init() {}

func add() {

    // this compiles
    var x = [String:Int]();
    x["y"] = 3;

    // this gets the compiler error 'cannot assign to the result of this expression'
    self.lists["y"] = 3;
}

What is it about membership that breaks the compilation? I don't get this error if I put that line in init() FWIW.

If you want your struct to be able to modify its `self` you have to do this: http://stackoverflow.com/a/30477167/2227743 — Eric Aya, Jun 01 '15 at 17:23
You are right. I added "mutating" before "func add" and that fixed it. — Yusuf X, Jun 01 '15 at 17:25

Lucas Huang · Accepted Answer · 2015-06-01T18:35:29.153

2

You need to add mutating from the function declaration like this because properties are readonly if you don't specify that keyword in struct:

mutating func add()

edited Jun 01 '15 at 18:35

answered Jun 01 '15 at 17:26

Lucas Huang

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Swift dictionary member compile error

1 Answers1