I'm trying to understand why casting in the first case works while in the second it fails. What is the difference between the two?
var myVariable = 2
var myDoubleTest1 = Double(myVariable)
var myDoubleTest2 = myVariable as? Double
print(myDoubleTest1) // 2.0
print(myDoubleTest2) // nil
var myDoubleTest1 = Double(myVariable)
This line is just an initializer on Double. It takes an int, and initializes a new double. The documentation is here.
var myDoubleTest2 = myVariable as? Double
The as? keyword in Swift performs a cast. This means, in English, "create a variable named myDoubleTest2. If myVariable is of type Double, then use that. Otherwise, make it nil. We know, from the first line in your code (var myVariable = 2), that myVariable is not a Double (it is an Int), so myDoubleTest2 will be nil. You can read more about type casting here.
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