Swift cannot append to subscript? - swift

Question

I can't seem to use .append() on a subscript.
For example, here's the array:
var arrayTest = [
"test": 8,
"test2": 4,
"anotherarry": [
"test4": 9
]
]
I am able to do this:
arrayTest.append(["test3": 3])
But I can't append to the array inside arrayTest. This is what I'm trying:
arrayTest["anotherarray"].append(["finaltest": 2])

Answer 1

First note: your variable arrayTest is a dictionary of type [String: NSObject], not an array. Similarly the value for the key anotherarray is also a dictionary.
Second note: you are setting the key anotherarry and retrieving the key anotherarray which would be nil in this example.
I'm also not sure how you are able to call append() on arrayTest since it is a dictionary and doesn't have that method.
But the key issue with what you are trying to do is that dictionaries and arrays are value types and are copied when passed around, rather than referenced. When you subscript arrayTest to get anotherarray, you are getting a copy of the value, not a reference to the value inside the dictionary.
If you want to modify something directly inside an array or dictionary (as opposed to replacing it), that something must be a reference type (a class). Here's an example of how your code could be accomplished:
var arrayTest = [
"test": 8,
"test2": 4,
"anotherarray": ([
"test4": 9
] as NSMutableDictionary)
]
(arrayTest["anotherarray"] as? NSMutableDictionary)?["test5"] = 10
Note that this code forces "anotherarray" to explicitly be an NSMutableDictionary (a class type from Objective-C) instead of defaulting to a Swift dictionary (a value type). That's what makes it possible to modify it from outside the dictionary, since it is now being passed as a reference and not copied.
Further Note:
As pointed out in the comments, using NSMutableDictionary is not something I personally recommend and isn't a pure Swift solution, it's just the way to arrive at a working example with the fewest changes to your code.
Your other options would include replacing the anotherarray value entirely with a modified copy instead of trying to subscript it directly, or if it's important for you to be able to chain your subscripts, you could create a class wrapper around a Swift dictionary like this:
class DictionaryReference<Key:Hashable, Value> : DictionaryLiteralConvertible, CustomStringConvertible {
private var dictionary = [Key : Value]()
var description: String {
return String(dictionary)
}
subscript (key:Key) -> Value? {
get {
return dictionary[key]
}
set {
dictionary[key] = newValue
}
}
required init(dictionaryLiteral elements: (Key, Value)...) {
for (key, value) in elements {
dictionary[key] = value
}
}
}
Then you would use it similarly to the NSMutableDictionary example:
var arrayTest = [
"test": 8,
"test2": 4,
"anotherarray": ([
"test4": 9
] as DictionaryReference<String, Int>)
]
(arrayTest["anotherarray"] as? DictionaryReference<String, Int>)?["test5"] = 10

Answer 2

For example, here's the array:
Nope, arrayTest is NOT an array. It's a dictionary.
I am able to do this...
No you're not. There is no such append method into a dictionary.
The problem
So it looks like you have a dictionary like this
var dict: [String:Any] = [
"test": 8,
"test2": 4,
"anotherdict": ["test4": 9]
]
You want to change the array inside the key anotherdict (yes I renamed your key) in order to add the following key/value pair
"finaltest": 2
Here's the code
if var anotherdict = dict["anotherdict"] as? [String:Int] {
anotherdict["finaltest"] = 2
dict["anotherdict"] = anotherdict
}
Result
[
"test2": 4,
"test": 8,
"anotherdict": ["test4": 9, "finaltest": 2]
]