swift optionals － 1

学习是个循序渐进的过程，万事开头难. Swift说起来简单易上手，但有一些知识要反复琢磨，多加练习，才会深入理解。

Need to practise a lot and test what is Optiontal Type and how to use it very well

Optional 介绍： 

An optional value either contains a value or contains nil (no value) to indicate that a value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

If a var or constant could be a nil, the var must be declared as optional

Optional is an enumeration

Optional 例子：

1. pNumber is a string constant

2. when cast it to a Int, it might be a Int or nil since some strings can not be casted, liked "good"

3. Optional binding statement is if let a = b {}

If you are sure the optional does contain a value, it can be accessed by pNumber! (Implicitly optional)

Sample code:

let pNumber = "123"

//let converterNumber = Int(possibleNumber!)

if let actualNumber = Int(pNumber) { //casting and force it to a constant. A constan

    print("\'\(pNumber)\' has an integer valuel of \(actualNumber)")

} else {

    print("\'\(pNumber)\' could not be converted to an integer")

}