Create empty dictionary - swift

Question

I have a dictionary initialized
var dictionary = [String: [Double]]()
And I want to append
dictionary["Hello"].append(0.0)
but this gives me error "nil".
I tried to solve this by
extension Dictionary {
func appendish(key: String, value: Double) {
if self[key] == nil {
this give me error "Ambiguous reference to member 'subscript'"
}
}
}
How do I solve this? Been stuck at this for hours.

Answer 1

Subscripting a Dictionary with a key returns an optional of type Value?. In your case, dictionary["Hello"] returns a [Double]?. This optionality models the possibility that the dictionary doesn't contain a value for the given key.
If you're only dealing with static data, it's best to just use a literal expression:
let dictionary = [
"Hello": [0.0]
]
If you're using dynamic data, then there are several ways to do what you're trying to achieve, depending on how you would like to handle the nil case:
Use optional chaining
dictionary["Hello"]?.append(0.0)
This appends to the array stored for the key "Hello", but does nothing if there's no such value for that key.
This has the downside of making bugs harder to catch, because the consequence of the silent nil case might not be observed until long after this part of the code has run.
Use force unwrapping
dictionary["Hello"]!.append(0.0)
This appends to the array stored for the key "Hello", but crashes the program if there's no such value for that key.
Unlike optional chaining, this makes it easy to catch the point of failure at runtime. Of course, it comes with the drawback of crashing your program.
Handle the nil case in your own way
if var array = dictionary["Hello"] {
dictionary["Hello"] = nil // This line is a performance optimisation that removes the need for array to be copied
array.append(0.0)
dictionary["Hello"] = array
}
else {
print("No array for the key \"Hello\"") // Handle this as you wish
}

Answer 2

A dictionary look up returns an Optional value because the key might not exist, in which case it returns nil.
If your intention is to append to the array if it exists or create one if there isn't one yet, then the nil coalescing operator ?? comes in handy:
var dict = [String: [Double]]()
dict["hello"] = (dict["hello"] ?? []) + [1]
print(dict) // ["hello": [1.0]]
dict["hello"] = (dict["hello"] ?? []) + [2]
print(dict) // ["hello": [1.0, 2.0]]
This method does create a new array instead of mutating the existing one.

Answer 3

There are a few ways you can do this. Firstly, this is incorrect code:
dictionary["Hello"].append(0.0)
There might not be an array associated with the key "Hello", in which case nil will be returned by the subscript of the dictionary. So you need to unwrap it, either forced or un-forced:
dictionary["Hello"]?.append(0.0)
// or
dictionary["Hello"]!.append(0.0)
But I think what you really want to do is
if dictionary["Hello"] != nil {
dictionary["Hello"]!.append(0.0)
} else {
dictionary["Hello"] = [0.0]
}
After a long time of fiddling around with extensions and stuff (I am not familiar with this area of swift), I finally wrote the method appendish method that you were intended to write:
extension Dictionary where Value : RangeReplaceableCollection & ExpressibleByArrayLiteral, Value.Iterator.Element == Value.Element {
mutating func appendish(key: Key, value: Value.Element) {
if self[key] != nil {
self[key]!.append(value)
} else {
self[key] = [value]
}
}
}
// test
var dict = [String: [Double]]()
dict.appendish(key: "Hello", value: 0.0)