Ruby (programming language)

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File:Ruby logo.svg
Usual file extensions .rb, .rbw
Paradigm multi-paradigm
Appeared in 1995
Designed by Yukihiro Matsumoto
Developer Yukihiro Matsumoto, et al.
Stable release 1.9.1-p376 (December 7, 2009; 367976383 ago)
Preview release 1.9.2 (July 20, 2009; 380072383 ago)
Typing discipline duck, dynamic, strong
Major implementations Ruby MRI, JRuby, YARV
Influenced by Smalltalk, Perl, Lisp, Scheme, Python, CLU, Eiffel, Ada, Dylan, T-RAY
Influenced Groovy, Nu, Falcon, Ioke
OS Cross-platform
License Ruby License
GNU General Public License

Ruby is a dynamic, reflective, general purpose object-oriented programming language that combines syntax inspired by Perl with Smalltalk-like features. Ruby originated in Japan during the mid-1990s and was initially developed and designed by Yukihiro "Matz" Matsumoto. It is based on Perl, Smalltalk, Eiffel, Ada,[citation needed] and Lisp.

Ruby supports multiple programming paradigms, including functional, object oriented, imperative and reflective. It also has a dynamic type system and automatic memory management; it is therefore similar in varying respects to Python, Perl, Lisp, Dylan, and CLU.

The standard 1.8.7 implementation is written in C, as a single-pass interpreted language. There is currently no specification of the Ruby language, so the original implementation is considered to be the de facto reference. As of 2010, there are a number of complete or upcoming alternative implementations of the Ruby language, including YARV, JRuby, Rubinius, IronRuby, MacRuby and HotRuby, each of which takes a different approach, with IronRuby, JRuby and MacRuby providing just-in-time compilation and MacRuby also providing ahead-of-time compilation. The official 1.9 branch uses YARV, as will 2.0 (development), and will eventually supersede the slower Ruby MRI.



File:Yukihiro Matsumoto.JPG
Yukihiro Matsumoto, the creator of Ruby.

Ruby was conceived on February 24, 1993 by Yukihiro Matsumoto who wished to create a new language that balanced functional programming with imperative programming.[1] Matsumoto has stated, "I wanted a scripting language that was more powerful than Perl, and more object-oriented than Python. That's why I decided to design my own language".[2]

Etymology of the name "Ruby"

The name "Ruby" was decided on during an online chat session between Matsumoto and Keiju Ishitsuka on February 24, 1993, before any code had been written for the language.[3] Initially two names were proposed: "Coral" and "Ruby", with the latter being chosen by Matsumoto in a later email to Ishitsuka.[4] Coral already had a long history as the name of a programming language originating in 1964 at the Royal Radar Establishment in the UK, so it was fortunate that the other name was chosen to eliminate confusion. Matsumoto has later stated that a factor in choosing the name "Ruby" was because it was the birthstone of one of his colleagues.[5]

First publication

The first public release of Ruby 0.95 was announced on Japanese domestic newsgroups on December 21, 1995.[6][7] Subsequently three more versions of Ruby were released in two days.[3] The release coincided with the launch of the Japanese language ruby-list mailing list which was the first mailing list for the new language.

Already present at this stage of development were many of the features familiar in later releases of Ruby, including object-oriented design, classes with inheritance, mixins, iterators, closures, exception handling, and garbage collection.[8]

Ruby 1.0

Ruby reached version 1.0 on December 25, 1996.[3]

Following the release of Ruby 1.3 in 1999 the first English language mailing list ruby-talk began,[2] which signalled a growing interest in the language outside of Japan. In September 2000, the first English language book Programming Ruby was printed, which was later freely released to the public further widening the adoption of Ruby amongst English speakers.

Ruby 1.9.1

The latest stable version of the reference implementation is 1.9.1.

Ruby 1.9.1 introduces many significant changes over version 1.8.6. Some examples are:

  • Block local variables (variables that are local to the block in which they are declared)
  • An additional lambda syntax (fun = ->(a,b) { puts a + b })
  • Per-string character encodings are supported


Matsumoto has said that Ruby is designed for programmer productivity and fun, following the principles of good user interface design.[9] He stresses that systems design needs to emphasize human, rather than computer, needs:[10]

Often people, especially computer engineers, focus on the machines. They think, "By doing this, the machine will run faster. By doing this, the machine will run more effectively. By doing this, the machine will something something something." They are focusing on machines. But in fact we need to focus on humans, on how humans care about doing programming or operating the application of the machines. We are the masters. They are the slaves.

Ruby is said to follow the principle of least surprise (POLS), meaning that the language should behave in such a way as to minimize confusion for experienced users. Matsumoto has said his primary design goal was to make a language which he himself enjoyed using, by minimizing programmer work and possible confusion. He has said that he had not applied the principle of least surprise to the design of Ruby,[10] but nevertheless the phrase has come to be closely associated with the Ruby programming language. The phrase has itself been a source of surprise, as novice users may take it to mean that Ruby's behaviors try to closely match behaviors familiar from other languages. In a May 2005 discussion on the comp.lang.ruby newsgroup, Matsumoto attempted to distance Ruby from POLS, explaining that because any design choice will be surprising to someone, he uses a personal standard in evaluating surprise. If that personal standard remains consistent there will be few surprises for those familiar with the standard.[11]

Matsumoto defined it this way in an interview[10]:

Everyone has an individual background. Someone may come from Python, someone else may come from Perl, and they may be surprised by different aspects of the language. Then they come up to me and say, 'I was surprised by this feature of the language, so Ruby violates the principle of least surprise.' Wait. Wait. The principle of least surprise is not for you only. The principle of least surprise means principle of least my surprise. And it means the principle of least surprise after you learn Ruby very well. For example, I was a C++ programmer before I started designing Ruby. I programmed in C++ exclusively for two or three years. And after two years of C++ programming, it still surprises me.



Ruby is object-oriented: every data type is an object, including classes and types which many other languages designate as primitives (such as integers, booleans, and "nil"). Every function is a method. Named values (variables) always designate references to objects, not the objects themselves. Ruby supports inheritance with dynamic dispatch, mixins and singleton methods (belonging to, and defined for, a single instance rather than being defined on the class). Though Ruby does not support multiple inheritance, classes can import modules as mixins. Procedural syntax is supported, but all methods defined outside of the scope of a particular object are actually methods of the Object class. Since this class is parent to every other class, the changes become visible to all classes and objects.

Ruby has been described as a multi-paradigm programming language: it allows procedural programming (defining functions/variables outside classes makes them part of the root, 'self' Object), with object orientation (everything is an object) or functional programming (it has anonymous functions, closures, and continuations; statements all have values, and functions return the last evaluation). It has support for introspection, reflection and metaprogramming, as well as support for interpreter-based[17] threads. Ruby features dynamic typing, and supports parametric polymorphism.

According to the Ruby FAQ,[18] "If you like Perl, you will like Ruby and be right at home with its syntax. If you like Smalltalk, you will like Ruby and be right at home with its semantics. If you like Python, you may or may not be put off by the huge difference in design philosophy between Python and Ruby/Perl."


The syntax of Ruby is broadly similar to Perl and Python. Class and method definitions are signaled by keywords. In contrast to Perl, variables are not obligatorily prefixed with a sigil. When used, the sigil changes the semantics of scope of the variable. The most striking difference from C and Perl is that keywords are typically used to define logical code blocks, without braces (i.e., pair of { and }). For practical purposes there is no distinction between expressions and statements[19]. 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 of Ruby compared to Python and Perl is that Ruby keeps all of its instance variables completely private to the class and only exposes them through accessor methods (attr_writer, attr_reader, etc). Unlike the "getter" and "setter" methods of other languages like C++ or Java, accessor methods in Ruby are created with a single line of code via metaprogramming. 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 code or having to do any refactoring achieving similar functionality to C# and VB.NET property members. Python's property descriptors are similar, but come with a tradeoff 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 removes this design decision by forcing all instance variables to be private, but also provides a simple way to declare set and get methods. This is in keeping with the idea that in Ruby, one never directly accesses the internal members of a class from outside of it. Rather one passes a message to the class and receives a response.

See the examples section for samples of code demonstrating Ruby syntax.

Deviations from behaviour elsewhere

Some features which differ notably from languages such as C or Perl:

  • The language syntax is sensitive to the capitalization of identifiers, in most cases treating capitalized variables as constants.
  • The sigils $ and @ do not indicate variable data type as in Perl, but rather function as scope resolution operators.
  • To denote a floating point without a decimal component, one must follow with a zero digit (99.0) or an explicit conversion (99.to_f). It is insufficient to append a dot (99.) since numbers are susceptible to method syntax.
  • Conversion of floating point to integer is performed by the round method (round to nearest integer, halfway cases away from zero): 2.5.round gives 3, 3.5.round gives 4, -3.5.round gives −4. The BigDecimal class offers additional rounding options.
  • Boolean evaluation of non-boolean data is strict: 0, "" and [] are all evaluated to true. In C, the expression 0 ? 1 : 0 evaluates to 0 (i.e. false). In Ruby, however, it yields 1, as all numbers evaluate to true; only nil and false evaluate to false. A corollary to this rule is that Ruby methods by convention — for example, regular-expression searches — return numbers, strings, lists, or other non-false values on success, but nil on failure. This convention is also used in Smalltalk, where only the special objects true and false can be used in a boolean expression.
  • Versions prior to 1.9 use plain integers to represent single characters, much like C. This may cause surprises when slicing strings: "abc"[0] yields 97 (the ASCII code of the first character in the string); to obtain "a" use "abc"[0,1] (a substring of length 1) or "abc"[0].chr.
  • The notation statement until expression, unlike other languages' equivalent statements (e.g. do { statement } while (!(expression)); in C/C++/...), actually never runs the statement if the expression is already true. This is because statement until expression is actually syntactic sugar over until expression; statement; end, the equivalent of which in C/C++ is while (!(expression)) { statement; }, just like statement if expression is an equivalent to if (expression) { statement; }. However, the notation begin statement end until expression in Ruby will in fact run the statement once even if the expression is already true, acting similar to the "do-while" of other languages.
  • Because constants are references to objects, changing what a constant refers to generates a warning, but modifying the object itself does not. For example, Greeting << " world!" if Greeting == "Hello" does not generate an error or warning. This is similar to final variables in Java or a const pointer to a non-const object in C++, but Ruby provides the functionality to "freeze" an object, unlike Java.

Some features which differ notably from other languages:

  • The usual operators for conditional expressions, and and or, do not follow the normal rules of precedence: and does not bind tighter than or. Ruby also has expression operators || and && which work as expected.

A list of so-called gotchas may be found in Hal Fulton's book The Ruby Way, 2nd ed (ISBN 0-672-32884-4), Section 1.5. A similar list in the 1st edition pertained to an older version of Ruby (version 1.6), some problems of which have been fixed in the meantime. retry, for example, now works with while, until, and for, as well as iterators.


The Ruby official distribution also includes "irb", an interactive command-line interpreter which can be used to test code quickly. The following code fragment represents a sample session using irb:

$ irb
irb(main):001:0> puts "Hello, World"
Hello, World
=> nil
irb(main):002:0> 1+2
=> 3


The following examples can be run in a Ruby shell such as Interactive Ruby Shell or saved in a file and run from the command line by typing ruby <filename>.

Classic Hello world example:

puts "Hello World!"

Some basic Ruby code:

# Everything, including a literal, is an object, so this works:
-199.abs                                                # 199
"ruby is cool".length                                   # 12
"Your mother is nice.".index("u")                       # 2
"Nice Day Isn't It?".downcase.split("").sort.uniq.join  # " '?acdeinsty"


puts "What's your favorite number?"
number = gets.chomp
output_number = number.to_i + 1
puts output_number.to_s + ' is a bigger and better favorite number.'


There are a variety of methods of defining strings in Ruby

The below conventions are equivalent for double quoted strings:

a = "\nThis is a double quoted string\n"
a = %{\nThis is a double quoted string\n}
a = %Q{\nThis is a double quoted string\n}
a = <<BLOCK
This is a multi-line double quoted string
a = %/\nThis is a double quoted string\n/

The below conventions are equivalent for single quoted strings:

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


Constructing and using an array:

a = [1, 'hi', 3.14, 1, 2, [4, 5]]
puts a[2]           # 3.14
puts a.[](2)        # 3.14
puts a.reverse      # [[4, 5], 2, 1, 3.14, 'hi', 1]
puts a.flatten.uniq # [1, 'hi', 3.14, 2, 4, 5]

Constructing and using an associative array (called hashes in Ruby):

hash = { :water => 'wet', :fire => 'hot' }
puts hash[:fire] # Prints:  hot
hash.each_pair do |key, value| # Or:  hash.each do |key, value|
  puts "#{key} is #{value}"
# Prints:  water is wet
#          fire is hot
hash.delete :water # Deletes :water => 'wet'
hash.delete_if {|key,value| value=='hot'} # Deletes :fire => 'hot'

Blocks and iterators

The two syntaxes for creating a code block:

{ puts "Hello, World!" } # Note the { braces }
do puts "Hello, World!" end

When a code block is created it is always attached to a method as an optional block argument.

Parameter-passing a block to be a closure:

# In an object instance variable (denoted with '@'), remember a block.
def remember(&a_block)
  @block = a_block
# Invoke the above method, giving it a block which takes a name.
remember {|name| puts "Hello, #{name}!"}
# When the time is right (for the object) -- call the closure!"Jon")
# => "Hello, Jon!"

Creating an anonymous function:

proc {|arg| print arg} {|arg| print arg}
lambda {|arg| print arg}

Returning closures from a method:

def create_set_and_get(initial_value=0) # Note the default value of 0
  closure_value = initial_value
  return {|x| closure_value = x}, { closure_value }
setter, getter = create_set_and_get  # ie. returns two values # => 21
#You can also use a parameter variable as a binding for the closure. 
#So the above can be rewritten as...
def create_set_and_get(closure_value=0)
  proc {|x| closure_value = x } , proc { closure_value }

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

def use_hello
  yield "hello"
# Invoke the above method, passing it a block.
use_hello {|string| puts string} # => 'hello'

Iterating over enumerations and arrays using blocks:

array = [1, 'hi', 3.14]
array.each {|item| puts item }
# => 1
# => 'hi'
# => 3.14
array.each_index {|index| puts "#{index}: #{array[index]}" }
# => 0: 1
# => 1: 'hi'
# => 2: 3.14
(3..6).each {|num| puts num }
# => 3
# => 4
# => 5
# => 6

A method such as inject() can accept both a parameter and a block. Inject iterates over each member of a list, performing some function on while retaining an aggregate. This is analogous to the foldl function in functional programming languages. For example:

[1,3,5].inject(10) {|sum, element| sum + element} # => 19

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

Blocks work with many built-in methods:'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
# => Wrote some text.

Using an enumeration and a block to square the numbers 1 to 10:

(1..10).collect {|x| x*x} # => [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]


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

class Person
  attr_reader :name, :age
  def initialize(name, age)
    @name, @age = name, age
  def <=>(person) # Comparison operator for sorting
    @age <=> person.age
  def to_s
    "#@name (#@age)"
group = ["Bob", 33),"Chris", 16),"Ash", 23) 
puts group.sort.reverse

The above prints three names in reverse age order:

Bob (33)
Ash (23)
Chris (16)

Open classes

In Ruby, classes are never closed: you can always add methods to an existing class. This applies to the classes you write as well as the standard, built-in classes. All you have to do is open up a class definition for an existing class, and the new contents you specify will be added to whatever's there. A simple example of adding a new method to the standard library's Time class:

# re-open Ruby's Time class
class Time
  def yesterday
    self - 86400
today = # => Thu Aug 14 16:51:50 +1200 2008
yesterday = today.yesterday # => Wed Aug 13 16:51:50 +1200 2008

Adding methods to previously defined classes is often called monkey-patching. This practice, however, can lead to possible collisions of behavior and subsequent unexpected results, and is a concern for code scalability if performed recklessly.


An exception is raised with a raise call:


An optional message can be added to the exception:

raise "This is a message"

You can also specify which type of exception you want to raise:

raise ArgumentError, "Illegal arguments!"

Alternatively, you can pass an exception instance to the raise method:

raise"Illegal arguments!")

This last construct is useful when you need to raise a custom exception class featuring a constructor which takes more than one argument:

class ParseError < Exception
  def initialize input, line, pos
    super "Could not parse '#{input}' at line #{line}, position #{pos}"
raise"Foo", 3, 9)

Exceptions are handled by the rescue clause. Such a clause can catch exceptions which inherit from StandardError. Also supported for use with exceptions are else and ensure

# Do something
# Handle exception
# Do this if no exception was raised
# Do this whether or not an exception was raised

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

# Do something
rescue Exception # don't write just rescue -- that only catches StandardError, a subclass of Exception
# Handle exception

Or catch particular exceptions:

# ...
rescue RuntimeError 
# handling

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

# ...
rescue RuntimeError => e
# handling, possibly involving e, such as "print e.to_s"

Alternatively, the most recent exception is stored in the magic global $!.

You can also catch several exceptions:

# ...
rescue RuntimeError, Timeout::Error => e
# handling, possibly involving e


Ruby provides a programmer the ability to add to or modify methods in the language's standard library during runtime, as well for a Ruby program to modify itself during its operation without generating source code, techniques known as metaprogramming.

For example, like Lisp, Ruby can evaluate its own code via eval:

eval "a=[1,2,3]; puts a.join"

This will produce the following output:


Note, however, that this is an unsafe operation in most cases as it can allow for dangerous code to be executed. Finer control over the scope of the execution is provided using class_eval and instance_eval.

A common metaprogramming pattern in Ruby is to use the metaclass of the object to define behavior at runtime.

# A new class is created with no user-defined methods
class Test
# Create two instances of the class
t1, t2 =,
# Open the first class' metaclass and define a method for that instance
class << t1
  def inst_method arg
    puts arg
t1.inst_method "test!"
t2.inst_method "test!"

The second to last statement will produce "test!" while the last statement should produce a NoMethodError, since the metaclass is defined on a by-instance basis.

More examples

More sample Ruby code is available as algorithms in the following articles:


The newest version of Ruby, the recently released version 1.9, has a single working implementation written in C that utilizes a Ruby-specific virtual machine.

Ruby version 1.8 has two main implementations: The official Ruby interpreter often referred to as the Matz's Ruby Interpreter or MRI, which is the most widely used, and JRuby, a Java-based implementation that runs on the Java Virtual Machine.

There are other less-known or upcoming implementations such as Cardinal (an implementation for the Parrot virtual machine), IronRuby (alpha version available since July 24, 2008[20]), MacRuby (0.5 experimental), MagLev, Rubinius, Ruby.NET, XRuby and HotRuby (runs Ruby source code on a web browser and Flash).

The maturity of Ruby implementations tends to be measured by their ability to run the Ruby on Rails (Rails) framework, because it is a complex framework to implement, and it uses a lot of Ruby-specific features. The point when a particular implementation achieves this goal is called The Rails singularity. As of January 2009, only the reference implementation (MRI) and JRuby are able to run Rails unmodified in a production environment.[21] IronRuby[22][23] and Rubinius[24] starting to be able to run Rails test cases, but they still are far from being production-ready.

Ruby is available on many operating systems such as Linux, Mac OS X, Microsoft Windows, Windows CE and most flavors of Unix.

Ruby 1.9 has recently been ported onto Symbian OS 9.x[25].

Repositories and libraries

The Ruby Application Archive (RAA), as well as RubyForge, serve as repositories for a wide range of Ruby applications and libraries, containing more than seven thousand items. Although the number of applications available does not match the volume of material available in the Perl or Python community, there are a wide range of tools and utilities which serve to foster further development in the language.

RubyGems has become the standard package manager for Ruby libraries. It is very similar in purpose to Perl's CPAN, although its usage is more like apt-get.

Recently, many new and existing libraries have found a home on GitHub, which is focused on Git and used to have native support for RubyGems packaging.[26]

See also


  1. Ruby-Lang About Ruby
  2. 2.0 2.1 An Interview with the Creator of Ruby
  3. 3.0 3.1 3.2 History of Ruby
  4. "[FYI: historic] The decisive moment of the language name Ruby. (Re: [ANN] ruby 1.8.1)" - Email from Hiroshi Sugihara to ruby-talk
  5. "Re: the name of Ruby?" - Email from Yukihiro Matsumoto to ruby-talk
  6. More archeolinguistics: unearthing proto-Ruby
  7. "Re: history of ruby" - Email from Yukihiro Matsumoto to ruby-talk
  8. "TUTORIAL - ruby's features" - Email From Yukihiro Matsumoto to ruby-list
  9. The Ruby Programming Language by Yukihiro Matsumoto on 2000-06-12 (
  10. 10.0 10.1 10.2 The Philosophy of Ruby, A Conversation with Yukihiro Matsumoto, Part I by Bill Venners on 2003-09-29 (Artima Developer)
  11. Ruby Weekly News 23rd - 29th May 2005
  12. [1]
  13. [2]
  14. [3]
  15. [4]
  16. [5]
  17. Green threads
  18. Ruby FAQ
  19. In Ruby's syntax, statement is just a special case of an expression which cannot appear as an argument (e.g. multiple assignment).
    statement [...] can not be part of expression unless grouped within parentheses.
  20. Lam, John (2008-07-24). "IronRuby at OSCON". Retrieved 2008-08-04. "We’re shipping our first binary release. In this package, we’re taking a “batteries included” approach and shipping the Ruby standard libraries in it" 
  21. Charles Nutter (2008-04-27). "Promise and Peril for Alternative Ruby Impls". Retrieved 2008-06-01. 
  22. John Lam (2008-05-25). "IronRuby / Rails Question". Retrieved 2008-05-25. 
  23. John Lam (2008-05-30). "IronRuby and Rails". Retrieved 2008-06-01. 
  24. Evan Phoenix (2008-05-17). "Rails on Rubinius". Retrieved 2008-05-25. 
  25. "Ruby 1.9 for Symbian OS". Retrieved 2008-12-03. 
  26. Gem Building is Defunct, GitHub Blog


External links

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