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The Ruby Programming Language by David Flanagan, Yukihiro Matsumoto

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Variables and Constants

Kernel, Object, and Moduledefine reflective methods for listing the names (as strings) of all defined global variables, currently defined local variables, all instance variables of an object, all class variables of a class or module, and all constants of a class or module:

global_variables   # => ["$DEBUG", "$SAFE", ...]
x = 1              # Define a local variable
local_variables    # => ["x"]

# Define a simple class
class Point
  def initialize(x,y); @x,@y = x,y; end # Define instance variables
  @@classvar = 1                        # Define a class variable
  ORIGIN = Point.new(0,0)               # Define a constant
end

Point::ORIGIN.instance_variables # => ["@y", "@x"]
Point.class_variables            # => ["@@classvar"]
Point.constants                  # => ["ORIGIN"]

The global_variables, local_variables, instance_variables, class_variables, and constants methods return arrays of strings in Ruby 1.8 and arrays of symbols in Ruby 1.9.

Querying, Setting, and Testing Variables

In addition to listing defined variables and constants, Ruby Object and Module also define reflective methods for querying, setting, and removing instance variables, class variables, and constants. There are no special purpose methods for querying or setting local variables or global variables, but you can use the eval method for this purpose:

x = 1
varname = "x"
eval(varname)           # => 1
eval("varname = '$g'")  # Set varname to "$g"
eval("#{varname} = x")  # Set $g to 1
eval(varname)           # => 1

Note that eval evaluates its code in a temporary scope. eval can alter the value of local ...

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