[swift-evolution] Arrays Returning Optionals instead of Index Out of Bounds
Jeremy Pereira
jeremy.j.pereira at googlemail.com
Tue Jun 7 11:06:28 CDT 2016
> IMO dictionary [Type1:Type2?] is a special case where you need to use myDict.keys.contains(keyValue) first to determinate if you have a value for key and after this you can get the value itself(which is Optional).
I don’t understand why you think it is a special case. The return type of subscripting [ T1 : T2?] is T2?? or Optional<Optional<T2>>. You can do all the normal unwrapping you would expect except that one unwrapping results in another optional e.g.
---
var sqrt: [Int : Int?] = [ 1: 1, 4 : 2, 2 : nil]
func tellMeTheSquareRoot(n : Int) -> String
{
if let lookupResult = sqrt[n]
{
if let theSquareRoot = lookupResult
{
return "\(n) has a square root and it is \(theSquareRoot)"
}
else
{
return "\(n) has a square root but it is not an integer"
}
}
else
{
return "I can't imagine what the square root of \(n) might be"
}
}
print(tellMeTheSquareRoot(4))
print(tellMeTheSquareRoot(2))
print(tellMeTheSquareRoot(-1))
—
gives the results
4 has a square root and it is 2
2 has a square root but it is not an integer
I can't imagine what the square root of -1 might be
Comparing to nil is ambiguous to the human eye but the compiler makes a decision
sqrt[2] == nil // false (huh?)
sqrt[-1] == nil // true
The reason the first one is false is because the return result is not nil but
Optional<Optional<Int>>.Some(Optional<Int>.None>)
A similar rule applies to assignment to a subscript, the compiler assumes the “top level” of optional but you can force the addition of an element with a nil value by being explicit.
sqrt[2] = nil // [4: Optional(2), 1: Optional(1)] - 2 has been zapped
sqrt[2] = Optional<Int>.None // [2: nil, 4: Optional(2), 1: Optional(1)] - 2 has been re-added
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