Changing value in nested dictionary in swift

Question

I am wondering why when setting the value of a nested dictionary, the containing dictionary does not reflect those changes? On line3, is a copy of the dictionary being retur

Answer 1

It's because dictionaries are value types and not reference types in Swift.

When you call this line...

var dic2 = dic["key"] as Dictionary<String, String>

... you are creating an entirely new dictionary, not a reference to the value in dic. Changes in dic2 will not be reflected in dic, because dic2 is now a second entirely separate dictionary. If you want dic to reflect the changes that you made, you need to reassign the value to the appropriate key in dic, like this:

dic["key"] = dic2

Hope that helps...

Answer 2

Yes, in swift structs are passed by value not reference. In other words, you are getting a copy, with the same values. As I understand it, it actually does some optimization under the hood, whereby it doesn't actually make the copy until you change something. Regardless, what you get is a separate instance of the dictionary.

Same thing goes for arrays.

Answer 3

Roman is correct that when you created dic2, because dictionaries pass by value, you're creating a new dictionary.

You could either copy the dictionary back in, as outlined Access a dictionary in dictionary or you can modify the dictionary in place:

var dic = [String: [String: String]]() // Dictionary<String, Dictionary<String, String>>()  // line1
dic["key"] = ["a": "a"] // line2
dic["key"]?["a"] = "b"  // line4