Symbols look like variables, however, with a colon (:) prefixed. For example, :symbol_1. Symbols need not be predeclared and assigned a value. Ruby guarantees that the symbol has a particular value, no matter where it appears in a Ruby program.

Symbols are very useful because a given symbol name refers to the same object throughout a Ruby program. Two strings with the same content are two different objects; however, for a given name, there can only be one single symbol object. Let's examine the following example to illustrate this fact:

irb
2.1-head :001 > puts "string".object_id
70168328185680
 => nil
2.1-head :002 > puts "string".object_id
70168328173400
 => nil
2.1-head :003 > puts :symbol.object_id
394888
 => nil
2.1-head :004 > puts :symbol.object_id
394888
nil

As you can see, we started by creating a string object with the string value and sought its object ID using the object_id method. Next, we tried the same thing once more. In both the cases, we received...