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.

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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.

Answer 2

The accepted answer bypasses the following much simpler possibility, which also works for older Swift versions:

var dict = Dictionary<String, Array<Int>>() dict["key"] = [1, 2, 3]  print(dict)  dict["key", default: [Int]()].append(4)  print(dict) 

This will print:

["key": [1, 2, 3]] ["key": [1, 2, 3, 4]] 

And this:

var dict = Dictionary<String, Array<Int>>() dict["key", default: [Int]()].append(4) print(dict) 

will print:

["key": [4]]