Nil coalescing operator for dictionary

Question

Trying to access/ check the key in dictionary and add values.

myDict["Algebra"] initially returns nil. Why "nil coalescing" doesn't work here?

var myDict = [String : [Int]]()
myDict["Algebra"]?.append(contentsOf: [98,78,83,92]) ?? myDict["Algebra"] = [98,78,83,92]

Answer 1

Trying use like yours give you and error : Left side of mutating operator has immutable type '[Int]?'

By putting parentheses it will be no compile error and it will work

var myDict = [String : [Int]]()
myDict["Algebra"]?.append(contentsOf: [98,78,83,92]) ?? (myDict["Algebra"] = [98,78,83,92])
print(myDict) // ["Algebra": [98, 78, 83, 92]]

Swift Documentation is here for Infix Operators.

Answer 2

While this works with parentheses, the problem you're trying to solve is exactly what the default subscript does, without abusing the ?? operator into an implicit if statement with side-effects:

myDict["Algebra", default: []].append(contentsOf: [98,78,83,92])

You may also find this syntax a little clearer:

myDict["Algebra", default: []] += [98,78,83,92]