Ruby - Introduction Constants

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Introduction

The unchanging values are called constants, and can be represented in Ruby by a variable name beginning with a capital letter: 

Pi = 3.141592

If you enter the preceding line and try to change the value of Pi, but you'll get a warning:

Demo

Pi = 3.141592 
Pi = 3.141592 
Pi = 500

Result

Constants are objects whose values never change.

For example, PI in Ruby's Math module is a constant.

Constants in Ruby begin with a capital letter.

Class names are also constants. You can obtain a list of all defined constants using the constants method:

Demo

puts Object.constants

Result

Ruby provides the const_get and const_set methods to get and set the value of named constants specified as symbols.

symbols identifiers are preceded by a colon such as :RUBY_VERSION.

Ruby's constants may be assigned new values: 

RUBY_VERSION = "1.8.7" 
RUBY_VERSION = "2.5.6"

The previous reassignment of the RUBY_VERSION constant produces an "already initialized constant" warning but not an error!

You can even reassign constants declared in Ruby's standard class library.

For example, here I reassign the value of PI.

Although this displays a warning, the assignment succeeds nonetheless:

Demo

puts Math::PI    #=> 3.141592653589793 
Math::PI = 100   #=> warning: already initialized constant PI 
puts Math::PI    #=> 100
# from   w  w w  .jav a 2 s  .com

Result