In Swift, you can declare a dict where the value is an array type, eg:
var dict: [Int: [Int]] = [:]
However, if you assign an array for a given key:
dict[1] = []
then it appears that Swift treats the array as immutable. For example, if we try:
(dict[1] as [Int]).append(0) // explicit cast to avoid DictionaryIndex
then we get the error 'immutable value of type [Int] has only mutating members named 'append''.
If we explicitly make the array mutable, then the append works, but doesn't modify the original array:
var arr = dict[1]!
arr.append(0) // OK, but dict[1] is unmodified
How can you append to an array which is a dict value?
More generally, how can you treat the values of a dictionary as mutable?
One workaround is to reassign the value afterwards, but this does not seem like good practice at all:
var arr = dict[1]!
arr.append(0)
dict[1] = arr
