137 lines
3.8 KiB
Plaintext
137 lines
3.8 KiB
Plaintext
= Miscellaneous Syntax
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== Ending an Expression
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Ruby uses a newline as the end of an expression. When ending a line with an
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operator, open parentheses, comma, etc. the expression will continue.
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You can end an expression with a <code>;</code> (semicolon). Semicolons are
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most frequently used with <code>ruby -e</code>.
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== Indentation
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Ruby does not require any indentation. Typically, ruby programs are indented
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two spaces.
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If you run ruby with warnings enabled and have an indentation mismatch, you
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will receive a warning.
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== +alias+
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The +alias+ keyword is most frequently used to alias methods. When aliasing a
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method, you can use either its name or a symbol:
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alias new_name old_name
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alias :new_name :old_name
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For methods, Module#alias_method can often be used instead of +alias+.
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You can also use +alias+ to alias global variables:
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$old = 0
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alias $new $old
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p $new # prints 0
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You may use +alias+ in any scope.
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== +undef+
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The +undef+ keyword prevents the current class from responding to calls to the
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named methods.
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undef my_method
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You may use symbols instead of method names:
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undef :my_method
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You may undef multiple methods:
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undef method1, method2
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You may use +undef+ in any scope. See also Module#undef_method
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== +defined?+
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+defined?+ is a keyword that returns a string describing its argument:
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p defined?(UNDEFINED_CONSTANT) # prints nil
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p defined?(RUBY_VERSION) # prints "constant"
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p defined?(1 + 1) # prints "method"
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You don't need to use parenthesis with +defined?+, but they are recommended due
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to the {low precedence}[rdoc-ref:syntax/precedence.rdoc] of +defined?+.
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For example, if you wish to check if an instance variable exists and that the
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instance variable is zero:
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defined? @instance_variable && @instance_variable.zero?
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This returns <code>"expression"</code>, which is not what you want if the
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instance variable is not defined.
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@instance_variable = 1
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defined?(@instance_variable) && @instance_variable.zero?
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Adding parentheses when checking if the instance variable is defined is a
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better check. This correctly returns +nil+ when the instance variable is not
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defined and +false+ when the instance variable is not zero.
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Using the specific reflection methods such as instance_variable_defined? for
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instance variables or const_defined? for constants is less error prone than
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using +defined?+.
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+defined?+ handles some regexp global variables specially based on whether
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there is an active regexp match and how many capture groups there are:
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/b/ =~ 'a'
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defined?($~) # => "global-variable"
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defined?($&) # => nil
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defined?($`) # => nil
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defined?($') # => nil
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defined?($+) # => nil
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defined?($1) # => nil
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defined?($2) # => nil
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/./ =~ 'a'
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defined?($~) # => "global-variable"
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defined?($&) # => "global-variable"
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defined?($`) # => "global-variable"
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defined?($') # => "global-variable"
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defined?($+) # => nil
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defined?($1) # => nil
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defined?($2) # => nil
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/(.)/ =~ 'a'
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defined?($~) # => "global-variable"
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defined?($&) # => "global-variable"
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defined?($`) # => "global-variable"
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defined?($') # => "global-variable"
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defined?($+) # => "global-variable"
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defined?($1) # => "global-variable"
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defined?($2) # => nil
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== +BEGIN+ and +END+
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+BEGIN+ defines a block that is run before any other code in the current file.
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It is typically used in one-liners with <code>ruby -e</code>. Similarly +END+
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defines a block that is run after any other code.
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+BEGIN+ must appear at top-level and +END+ will issue a warning when you use it
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inside a method.
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Here is an example:
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BEGIN {
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count = 0
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}
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You must use <code>{</code> and <code>}</code> you may not use +do+ and +end+.
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Here is an example one-liner that adds numbers from standard input or any files
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in the argument list:
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ruby -ne 'BEGIN { count = 0 }; END { puts count }; count += gets.to_i'
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