dictionary - swift: modifying arrays inside dictionaries

Question

How can I easily add elements to an array inside a dictionary? It's always complaining with could not find member 'append' or could not find an overload for '+='

var dict = Dictionary<String, Array<Int>>()
dict["key"] = [1, 2, 3]

// all of these fail
dict["key"] += 4
dict["key"].append(4) // xcode suggests dict["key"].?.append(4) which also fails
dict["key"]!.append(4)
dict["key"]?.append(4)

// however, I can do this:
var arr = dict["key"]!
arr.append(4) // this alone doesn't affect dict because it's a value type (and was copied)
dict["key"] = arr

if I just assign the array to a var, modify it and then reassign it to the dict, won't I be copying everything? that wouldn't be efficient nor elegant.

Answer 1

Swift beta 5 has added this functionality, and you've nailed the new method in a couple of your attempts. The unwrapping operators ! and ? now pass through the value to either operators or method calls. That is to say, you can add to that array in any of these ways:

dict["key"]! += [4]
dict["key"]!.append(4)
dict["key"]?.append(4)

As always, be careful about which operator you use -- force unwrapping a value that isn't in your dictionary will give you a runtime error:

dict["no-key"]! += [5]        // CRASH!

Whereas using optional chaining will fail silently:

dict["no-key"]?.append(5)     // Did it work? Swift won't tell you...

Ideally you'd be able to use the new null coalescing operator ?? to address this second case, but right now that's not working.

---

Answer from pre-Swift beta 5:

It's a quirk of Swift that it's not possible to do what you're trying to do. The issue is that the value of any Optional variable is in fact a constant -- even when forcibly unwrapping. If we just define an Optional array, here's what we can and can't do:

var arr: Array<Int>? = [1, 2, 3]
arr[0] = 5
// doesn't work: you can't subscript an optional variable
arr![0] = 5
// doesn't work: constant arrays don't allow changing contents
arr += 4
// doesn't work: you can't append to an optional variable
arr! += 4
arr!.append(4)
// these don't work: constant arrays can't have their length changed

The reason you're having trouble with the dictionary is that subscripting a dictionary returns an Optional value, since there's no guarantee that the dictionary will have that key. Therefore, an array in a dictionary has the same behavior as the Optional array, above:

var dict = Dictionary<String, Array<Int>>()
dict["key"] = [1, 2, 3]
dict["key"][0] = 5         // doesn't work
dict["key"]![0] = 5        // doesn't work
dict["key"] += 4           // uh uh
dict["key"]! += 4          // still no
dict["key"]!.append(4)     // nope

If you need to change something in an array in the dictionary you'll need to get a copy of the array, change it, and reassign, like this:

if var arr = dict["key"] {
    arr.append(4)
    dict["key"] = arr
}

ETA: Same technique works in Swift beta 3, though constant arrays no longer allow changes to contents.