A variable is a named “shoebox” whose contained value must be of a single well-defined type. Every variable must be explicitly and formally declared. To put a value into the shoebox, thus causing the variable name to refer to that value, you assign the value to the variable. The variable name becomes a reference to that value.
This chapter goes into detail about declaration and initialization of variables. It then discusses all the primary built-in Swift simple types. (I mean “simple” as opposed to collections; the primary built-in collection types are discussed at the end of Chapter 4.)
A variable not only gives its referent a name; it also, by virtue of where it is declared, endows its referent with a particular scope (visibility) and lifetime. Assigning a value to a variable is a way of ensuring that this value can be seen by code that needs to see it and that it persists long enough to serve its purpose.
In the structure of a Swift file (see Example 1-1), a variable can be declared just about anywhere. It will be useful to distinguish several levels of variable scope and lifetime:
A global variable, or simply a global, is a variable declared at the top level of a Swift file. A global variable lives as long as the file lives, which is as long as the program runs. A global variable is visible everywhere (that’s what “global” means). It is visible to all code within the same file, because it ...