The Ruby Programming Language

Variable References

A variable is simply a name for a value. Variables are created and values assigned to them by assignment expressions, which are covered later in this chapter. When the name of a variable appears in a program anywhere other than the lefthand side of an assignment, it is a variable reference expression and evaluates to the value of the variable:

one = 1.0     # This is an assignment expression
one           # This variable reference expression evaluates to 1.0

As explained in Chapter 2, there are four kinds of variables in Ruby, and lexical rules govern their names. Variables that begin with $ are global variables, visible throughout a Ruby program. Variables that begin with @ and @@ are instance variables and class variables, used in object-oriented programming and explained in Chapter 7. And variables whose names begin with an underscore or a lowercase letter are local variables, defined only within the current method or block. (See Blocks and Variable Scope for more about the scope of local variables.)

Variables always have simple, unqualified names. If a . or :: appears in an expression, then that expression is either a reference to a constant or a method invocation. For example, Math::PI is a reference to a constant, and the expression item.price is an invocation of the method named price on the value held by the variable item.

The Ruby interpreter predefines a number of global variables when it starts up. See Chapter 10 for a list of these variables.

Uninitialized Variables ...