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 ;
(semicolon). Semicolons
are most frequently used with ruby -e
.
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 mis-match 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 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 "expression"
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?
.
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 ruby -e
.
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 {
and }
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'