ruby--ruby/doc/syntax/miscellaneous.rdoc

= Miscellaneous Syntax

== Ending an Expression

Ruby uses a newline as the end of an expression.  When ending a line with an
operator, open parentheses, comma, etc. the expression will continue.

You can end an expression with a <code>;</code> (semicolon).  Semicolons are
most frequently used with <code>ruby -e</code>.

== Indentation

Ruby does not require any indentation.  Typically, ruby programs are indented
two spaces.

If you run ruby with warnings enabled and have an indentation mismatch, you
will receive a warning.

== +alias+

The +alias+ keyword is most frequently used to alias methods.  When aliasing a
method, you can use either its name or a symbol:

  alias new_name old_name
  alias :new_name :old_name

For methods, Module#alias_method can often be used instead of +alias+.

You can also use +alias+ to alias global variables:

  $old = 0

  alias $new $old

  p $new # prints 0

You may use +alias+ in any scope.

== +undef+

The +undef+ keyword prevents the current class from responding to calls to the
named methods.

  undef my_method

You may use symbols instead of method names:

  undef :my_method

You may undef multiple methods:

  undef method1, method2

You may use +undef+ in any scope.  See also Module#undef_method

== +defined?+

+defined?+ is a keyword that returns a string describing its argument:

  p defined?(UNDEFINED_CONSTANT) # prints nil
  p defined?(RUBY_VERSION)       # prints "constant"
  p defined?(1 + 1)              # prints "method"

You don't need to use parenthesis with +defined?+, but they are recommended due
to the {low precedence}[rdoc-ref:syntax/precedence.rdoc] of +defined?+.

For example, if you wish to check if an instance variable exists and that the
instance variable is zero:

  defined? @instance_variable && @instance_variable.zero?

This returns <code>"expression"</code>, which is not what you want if the
instance variable is not defined.

  @instance_variable = 1
  defined?(@instance_variable) && @instance_variable.zero?

Adding parentheses when checking if the instance variable is defined is a
better check.  This correctly returns +nil+ when the instance variable is not
defined and +false+ when the instance variable is not zero.

Using the specific reflection methods such as instance_variable_defined? for
instance variables or const_defined? for constants is less error prone than
using +defined?+.

+defined?+ handles some regexp global variables specially based on whether
there is an active regexp match and how many capture groups there are:

  /b/ =~ 'a'
  defined?($~) # => "global-variable"
  defined?($&) # => nil
  defined?($`) # => nil
  defined?($') # => nil
  defined?($+) # => nil
  defined?($1) # => nil
  defined?($2) # => nil

  /./ =~ 'a'
  defined?($~) # => "global-variable"
  defined?($&) # => "global-variable"
  defined?($`) # => "global-variable"
  defined?($') # => "global-variable"
  defined?($+) # => nil
  defined?($1) # => nil
  defined?($2) # => nil

  /(.)/ =~ 'a'
  defined?($~) # => "global-variable"
  defined?($&) # => "global-variable"
  defined?($`) # => "global-variable"
  defined?($') # => "global-variable"
  defined?($+) # => "global-variable"
  defined?($1) # => "global-variable"
  defined?($2) # => nil

== +BEGIN+ and +END+

+BEGIN+ defines a block that is run before any other code in the current file.
It is typically used in one-liners with <code>ruby -e</code>.  Similarly +END+
defines a block that is run after any other code.

+BEGIN+ must appear at top-level and +END+ will issue a warning when you use it
inside a method.

Here is an example:

  BEGIN {
    count = 0
  }

You must use <code>{</code> and <code>}</code> you may not use +do+ and +end+.

Here is an example one-liner that adds numbers from standard input or any files
in the argument list:

  ruby -ne 'BEGIN { count = 0 }; END { puts count }; count += gets.to_i'