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<title>Ruby syntax</title>

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Ruby syntax

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The syntax of the <a href="Ruby_programming_language" class="mw-redirect" title="Ruby programming language">Ruby programming language</a> is broadly similar to that of <a href="Perl" title="Perl">Perl</a> and <a href="Python_(programming_language)" title="Python (programming language)">Python</a>. Class and method definitions are signaled by keywords, whereas code blocks can be defined by either keywords or braces. In contrast to Perl, variables are not obligatorily prefixed with a <a href="Sigil_(computer_programming)" title="Sigil (computer programming)">sigil</a>. When used, the sigil changes the semantics of scope of the variable. For practical purposes there is no distinction between <a href="Expression_(programming)" class="mw-redirect" title="Expression (programming)">expressions</a> and <a href="Statement_(programming)" class="mw-redirect" title="Statement (programming)">statements</a>.<a href="#cite_note-while-1">[1]</a><a href="#cite_note-precedence-2">[2]</a> Line breaks are significant and taken as the end of a statement; a semicolon may be equivalently used. Unlike Python, indentation is not significant.

One of the differences from Python and Perl is that Ruby keeps all of its instance variables completely private to the class and only exposes them through accessor methods (<code>attr_writer</code>, <code>attr_reader</code>, etc.). Unlike the "getter" and "setter" methods of other languages like <a href="C%2B%2B" title="C++">C++</a> or <a href="Java_(programming_language)" title="Java (programming language)">Java</a>, accessor methods in Ruby can be created with a single line of code via <a href="Metaprogramming" title="Metaprogramming">metaprogramming</a>; however, accessor methods can also be created in the traditional fashion of C++ and Java. As invocation of these methods does not require the use of parentheses, it is trivial to change an instance variable into a full function without modifying a single line of calling code or having to do any refactoring achieving similar functionality to <a href="C_Sharp_(programming_language)" title="C Sharp (programming language)">C#</a> and <a href="VB.NET" class="mw-redirect" title="VB.NET">VB.NET</a> property members.

Python's property descriptors are similar, but come with a trade-off in the development process. If one begins in Python by using a publicly exposed instance variable, and later changes the implementation to use a private instance variable exposed through a property descriptor, code internal to the class may need to be adjusted to use the private variable rather than the public property. Ruby's design forces all instance variables to be private, but also provides a simple way to declare <code>set</code> and <code>get</code> methods. This is in keeping with the idea that in Ruby one never directly accesses the internal members of a class from outside the class; rather, one passes a message to the class and receives a response.

<div class="mw-heading mw-heading2"><h2 id="Interactive_sessions">Interactive sessions</h2></div>

The following examples can be run in a Ruby shell such as <a href="Interactive_Ruby_Shell" class="mw-redirect" title="Interactive Ruby Shell">Interactive Ruby Shell</a>, or saved in a file and run from the command line by typing <code>ruby <filename></code>.

Classic <a href="%22Hello%2C_World!%22_program" title=""Hello, World!" program">Hello world</a> example:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>puts 'Hello World!'

</pre></div>

Some basic Ruby code:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre># Everything, including a literal, is an object, so this works:

-199.abs # => 199

'ice is nice'.length # => 11

'ruby is cool.'.index('u') # => 1

"Nice Day Isn't It?".downcase.split('').uniq.sort.join

# => " '?acdeinsty"

</pre></div>

Input:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>print 'Please type name >'

name = gets.chomp

puts "Hello #{name}."

</pre></div>

Conversions:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>puts 'Give me a number'

number = gets.chomp

puts number.to_i

output_number = number.to_i + 1

puts output_number.to_s + ' is a bigger number.'

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Strings">Strings</h2></div>

There are a variety of ways to define strings in Ruby.

The following assignments are equivalent:

a = %Q{\nThis is a double-quoted string\n}

a = %{\nThis is a double-quoted string\n}

a = %/\nThis is a double-quoted string\n/

a = <<-BLOCK

This is a double-quoted string

BLOCK

</pre></div>

Strings support <a href="Variable_interpolation" class="mw-redirect" title="Variable interpolation">variable interpolation</a>:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>var = 3.14159

"pi is #{var}"

=> "pi is 3.14159"

</pre></div>

The following assignments are equivalent and produce <a href="Raw_string" class="mw-redirect" title="Raw string">raw strings</a>:

a = %q{This is a single-quoted string}

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Collections">Collections</h2></div>

Constructing and using an <a href="Array_data_type" class="mw-redirect" title="Array data type">array</a>:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>a = [3, 'hello', 14.5, 1, 2, [6, 15]]

a[2] # => 14.5

a.[](2) # => 14.5

a.reverse # => [[6, 15], 2, 1, 14.5, 'hello', 3]

a.flatten.uniq # => [3, 'hello', 14.5, 1, 2, 6, 15]

</pre></div>

Constructing and using an <a href="Associative_array" title="Associative array">associative array</a> (in Ruby, called a hash):

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>hash = Hash.new # equivalent to hash = {}

hash = { water: 'wet', fire: 'hot' } # makes the previous line redundant as we are now

# assigning hash to a new, separate hash object

puts hash[:fire] # prints "hot"

hash.each_pair do |key, value| # or: hash.each do |key, value|

puts "#{key} is #{value}"

end

# returns {:water=>"wet", :fire=>"hot"} and prints:

# water is wet

# fire is hot

hash.delete :water # deletes the pair :water => 'wet' and returns "wet"

hash.delete_if {|key,value| value == 'hot'} # deletes the pair :fire => 'hot' and returns {}

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Control_structures">Control structures</h2></div>

If statement:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre># Generate a random number and print whether it's even or odd.

if rand(100).even?

puts "It's even"

else

puts "It's odd"

end

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Blocks_and_iterators">Blocks and iterators</h2></div>

The two syntaxes for creating a code block:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>{ puts 'Hello, World!' } # note the braces

# or:

do

puts 'Hello, World!'

end

</pre></div>

A code block can be passed to a method as an optional block argument. Many built-in methods have such arguments:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>File.open('file.txt', 'w') do |file| # 'w' denotes "write mode"

file.puts 'Wrote some text.'

end # file is automatically closed here

File.readlines('file.txt').each do |line|

puts line

end

# => Wrote some text.

</pre></div>

Parameter-passing a block to be a <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closure</a>:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre># In an object instance variable (denoted with '@'), remember a block.

def remember(&a_block)

@block = a_block

end

# Invoke the preceding method, giving it a block that takes a name.

remember {|name| puts "Hello, #{name}!"}

# Call the closure (note that this happens not to close over any free variables):

@block.call('Jon') # => "Hello, Jon!"

</pre></div>

Creating an <a href="Anonymous_function" title="Anonymous function">anonymous function</a>:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>proc {|arg| puts arg}

Proc.new {|arg| puts arg}

lambda {|arg| puts arg}

->(arg) {puts arg} # introduced in Ruby 1.9

</pre></div>

Returning <a href="Closure_(computer_science)" class="mw-redirect" title="Closure (computer science)">closures</a> from a method:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>def create_set_and_get(initial_value=0) # note the default value of 0

closure_value = initial_value

[ Proc.new {|x| closure_value = x}, Proc.new { closure_value } ]

end

setter, getter = create_set_and_get # returns two values

setter.call(21)

getter.call # => 21

# Parameter variables can also be used as a binding for the closure,

# so the preceding can be rewritten as:

def create_set_and_get(closure_value=0)

[ proc {|x| closure_value = x } , proc { closure_value } ]

end

</pre></div>

Yielding the flow of program control to a block that was provided at calling time:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>def use_hello

yield "hello"

end

# Invoke the preceding method, passing it a block.

use_hello {|string| puts string} # => 'hello'

</pre></div>

Iterating over enumerations and arrays using blocks:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>array = [1, 'hi', 3.14]

array.each {|item| puts item }

# prints:

# 1

# 'hi'

# 3.14

array.each_index {|index| puts "#{index}: #{array[index]}" }

# prints:

# 0: 1

# 1: 'hi'

# 2: 3.14

# The following uses a (a..b) Range

(3..6).each {|num| puts num }

# prints:

# 3

# 4

# 5

# 6

# The following uses a (a...b) Range

(3...6).each {|num| puts num }

# prints:

# 3

# 4

# 5

</pre></div>

A method such as <code>inject</code> can accept both a parameter and a block. The <code>inject</code> method iterates over each member of a list, performing some function on it while retaining an aggregate. This is analogous to the <code><a href="Foldl" class="mw-redirect" title="Foldl">foldl</a></code> function in <a href="Functional_programming_languages" class="mw-redirect" title="Functional programming languages">functional programming languages</a>. For example:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>[1,3,5].inject(10) {|sum, element| sum + element} # => 19

</pre></div>

On the first pass, the block receives 10 (the argument to inject) as <code>sum</code>, and 1 (the first element of the array) as <code>element</code>. This returns 11, which then becomes <code>sum</code> on the next pass. It is added to 3 to get 14, which is then added to 5 on the third pass, to finally return 19.

Using an enumeration and a block to square the numbers 1 to 10 (using a range):

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>(1..10).collect {|x| x*x} # => [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

</pre></div>

Or invoke a method on each item (<code>map</code> is a synonym for <code>collect</code>):

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>(1..5).map(&:to_f) # => [1.0, 2.0, 3.0, 4.0, 5.0]

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Classes">Classes</h2></div>

The following code defines a class named <code>Person</code>. In addition to <code>initialize</code>, the usual constructor to create new objects, it has two methods: one to override the <code><=></code> comparison operator (so <code>Array#sort</code> can sort by age) and the other to override the <code>to_s</code> method (so <code>Kernel#puts</code> can format its output). Here, <code>attr_reader</code> is an example of metaprogramming in Ruby: <code>attr_accessor</code> defines getter and setter methods of instance variables, but <code>attr_reader</code> only getter methods. The last evaluated statement in a method is its return value, allowing the omission of an explicit <code>return</code> statement.

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>class Person

attr_reader :name, :age

def initialize(name, age)

@name, @age = name, age

end

def <=>(person) # the comparison operator for sorting

@age <=> person.age

end

def to_s

"#{@name} (#{@age})"

end

end

group = [

Person.new("Bob", 33),

Person.new("Chris", 16),

Person.new("Ash", 23)

]

puts group.sort.reverse

</pre></div>

The preceding code prints three names in reverse age order:

<div class="mw-highlight mw-highlight-lang-text mw-content-ltr" dir="ltr"><pre>Bob (33)

Ash (23)

Chris (16)

</pre></div>

<code>Person</code> is a constant and is a reference to a <code>Class</code> object.

<div class="mw-heading mw-heading3"><h3 id="Open_classes">Open classes</h3></div>

In Ruby, classes are never closed: methods can always be added to an existing class. This applies to all classes, including the standard, built-in classes. All that is needed to do is open up a class definition for an existing class, and the new contents specified will be added to the existing contents. A simple example of adding a new method to the standard library's <code>Time</code> class:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre># re-open Ruby's Time class

class Time

def yesterday

self - 86400

end

end

today = Time.now # => 2013-09-03 16:09:37 +0300

yesterday = today.yesterday # => 2013-09-02 16:09:37 +0300

</pre></div>

Adding methods to previously defined classes is often called <a href="Monkey_patch" title="Monkey patch">monkey-patching</a>. If performed recklessly, the practice can lead to both behavior collisions with subsequent unexpected results and code scalability problems.

Since Ruby 2.0 it has been possible to use <a rel="nofollow" class="external text" href="http://yehudakatz.com/2010/11/30/ruby-2-0-refinements-in-practice/">refinements</a> to reduce the potentially negative consequences of monkey-patching, by limiting the scope of the patch to particular areas of the code base.

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre># re-open Ruby's Time class

module RelativeTimeExtensions

refine Time do

def half_a_day_ago

self - 43200

end

end

module MyModule

class MyClass

# Allow the refinement to be used

using RelativeTimeExtensions

def window

Time.now.half_a_day_ago

end

end

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Exceptions">Exceptions</h2></div>

An exception is raised with a <code>raise</code> call:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>raise

</pre></div>

An optional message can be added to the exception:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>raise "This is a message"

</pre></div>

Exceptions can also be specified by the programmer:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>raise ArgumentError, "Illegal arguments!"

</pre></div>

Alternatively, an exception instance can be passed to the <code>raise</code> method:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>raise ArgumentError.new("Illegal arguments!")

</pre></div>

This last construct is useful when raising an instance of a custom exception class featuring a constructor that takes more than one argument:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>class ParseError < Exception

def initialize(input, line, pos)

super "Could not parse '#{input}' at line #{line}, position #{pos}"

end

end

raise ParseError.new("Foo", 3, 9)

</pre></div>

Exceptions are handled by the <code>rescue</code> clause. Such a clause can catch exceptions that inherit from <code>StandardError</code>. Other flow control keywords that can be used when handling exceptions are <code>else</code> and <code>ensure</code>:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>begin

# do something

rescue

# handle exception

else

# do this if no exception was raised

ensure

# do this whether or not an exception was raised

end

</pre></div>

It is a common mistake to attempt to catch all exceptions with a simple rescue clause. To catch all exceptions one must write:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>begin

# do something

rescue Exception

# Exception handling code here.

# Don't write only "rescue"; that only catches StandardError, a subclass of Exception.

end

</pre></div>

Or catch particular exceptions:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>begin

# do something

rescue RuntimeError

# handle only RuntimeError and its subclasses

end

</pre></div>

It is also possible to specify that the exception object be made available to the handler clause:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>begin

# do something

rescue RuntimeError => e

# handling, possibly involving e, such as "puts e.to_s"

end

</pre></div>

Alternatively, the most recent exception is stored in the magic global <code>$!</code>.

Several exceptions can also be caught:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>begin

# do something

rescue RuntimeError, Timeout::Error => e

# handling, possibly involving e

end

</pre></div>

<div class="mw-heading mw-heading2"><h2 id="Metaprogramming">Metaprogramming</h2></div>

Ruby code can programmatically modify, at <a href="Run_time_(program_lifecycle_phase)" class="mw-redirect" title="Run time (program lifecycle phase)">runtime</a>, aspects of its own structure that would be fixed in more rigid languages, such as class and method definitions. This sort of <a href="Metaprogramming" title="Metaprogramming">metaprogramming</a> can be used to write more concise code and effectively extend the language.

For example, the following Ruby code generates new methods for the built-in <code>String</code> class, based on a list of colors. The methods wrap the contents of the string with an HTML tag styled with the respective color.

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>COLORS = { black: "000",

red: "f00",

green: "0f0",

yellow: "ff0",

blue: "00f",

magenta: "f0f",

cyan: "0ff",

white: "fff" }

class String

COLORS.each do |color,code|

define_method "in_#{color}" do

"\"color: ##{code}\">#{self}"

end

end

</pre></div>

The generated methods could then be used like this:

<div class="mw-highlight mw-highlight-lang-ruby mw-content-ltr" dir="ltr"><pre>"Hello, World!".in_blue

=> "\"color: #00f\">Hello, World!"

</pre></div>

To implement the equivalent in many other languages, the programmer would have to write each method (<code>in_black</code>, <code>in_red</code>, <code>in_green</code>, etc.) separately.

Some other possible uses for Ruby metaprogramming include:

<ul><li>intercepting and modifying method calls</li>

<li>implementing new inheritance models</li>

<li>dynamically generating classes from parameters</li>

<li>automatic object serialization</li>

<li>interactive help and debugging</li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/1120">"[ruby-talk:01120] Re: The value of while..."</a> <q>In Ruby's syntax, statement is just a special case of an expression that cannot appear as an argument (e.g. multiple assignment).</q></cite>

</li>

<li id="cite_note-precedence-2"><a href="#cite_ref-precedence_2-0">^</a> <cite class="citation web cs1"><a rel="nofollow" class="external text" href="http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/2460">"[ruby-talk:02460] Re: Precedence question"</a>. <q>statement [...] can not be part of expression unless grouped within parentheses.</q></cite>

</li>

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