Maybe It’s There, Maybe It Isn’t: Optionals

A few times so far we’ve seen our log messages include the term “optional,” a behavior we’ve put off explaining until now. But it’s time to deal with it, because optionals are one of Swift’s defining features. Create a new playground called OptionalsPlayground and delete the "Hello, playground" line, as usual.

We’ll start by adding the sizeInMm dictionary from a few sections back, since that’s something that started giving us this “optional” stuff.

 let​ sizeInMm = [
 "iPhone 6s"​: 138.1,
 "iPhone 6s Plus"​ : 158.1,
 "iPad Air 2"​ : 240.0]

Looking at this, we can see that sizeInMm["iPhone 6s"] should evaluate to 138.1, which is a

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