As we discussed in Chapter 2, Building Blocks – Variables, Collections, and Flow Control, all variables and constants must always have a value before they are used. This is a great safety feature because it prevents you from creating a scenario where you forget to give a variable an initial value. It may make sense for some number variables, such as the number of sandwiches ordered to start at zero, but it doesn't make sense for all variables. For example, the number of bowling pins standing should start at 10, not zero. In Swift, the compiler forces you to decide what the variable should start at, instead of providing a default value that could be incorrect.
However, there are other scenarios where you will have to represent the complete absence of a value. A great example is if you have a dictionary of word definitions and you try to lookup a word that isn't in the dictionary. Normally, this will return a String, so you could potentially return...