How to Mutate an Array in a Dictionary?

问题

I've tried the following in a Playground:

var d1 = [String: [String]]()
d1["a"] = [String]()

var a1 = d1["a"]!
a1.append("s1")

println(d1)

The output is: [a: []]
I was hoping for: [a: ["s1"]]

What would be the right way to mutate an array in a dictionary?

回答1:

In swift, structures are copied by value when they get assigned to a new variable. So, when you assign a1 to the value in the dictionary it actually creates a copy. Here's the line I'm talking about:

var a1 = d1["a"]!

Once that line gets called, there are actually two lists: the list referred to by d1["a"] and the list referred to by a1. So, only the second list gets modified when you call the following line:

a1.append("s1")

When you do a print, you're printing the first list (stored as the key "a" in dictionary d1). Here are two solutions that you could use to get the expected result.

Option1: Append directly to the array inside d1.

var d1 = [String : [String]]()
d1["a"] = [String]()
d1["a"]?.append("s1")
println(d1)

Option 2: Append to a copied array and assign that value to "a" in d1.

var d1 = [String : [String]]()
d1["a"] = [String]()
var a1 = d1["a"]!
a1.append("s1")
d1["a"] = a1
println(d1)

The first solution is more performant, since it doesn't create a temporary copy of the list.

回答2:

Extending the excellent answer of @jfocht here is

Option 3: Append within function where array is passed by reference

func myAdd(s : String, inout arr : [String]?) {
    if arr == nil
    {
        arr = [String]()
    }
    arr?.append(s)
}

var d1 = [String : [String]]()

myAdd("s1", &d1["a"])
myAdd("s2", &d1["b"])

println(d1) // [ b: [s2], a: [s1]]

Added some sugar in the sense that it actually creates a new array if given a dictionary key with no arrays attached currently.

回答3:

Swift Array and Dictionary are not an objects like NSArray and NSDictionary. They are struct. So by doing var a1 = d1["a"] would be like doing var a = point.x and then change a but of course the point.x will not change.

So by doing this instead:

d1["a"] = []
d1["a"]!.append("")

Would be like doing point.x = 10

回答4:

you need to set a new value for key "a" inside your dictionary. Which means:

d1["a"] = a1

来源：https://stackoverflow.com/questions/29189955/how-to-mutate-an-array-in-a-dictionary