Ruby programming language

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Image:RubyLogo.gif
Paradigm: multi-paradigm
Appeared in: 1995
Designed by: Yukihiro Matsumoto
Developer: Yukihiro Matsumoto (among others)
Latest release version: 1.8.4
Latest release date: December 24, 2005
Typing discipline: strong, dynamic ("duck")
Major implementations: Ruby, JRuby
Dialects: --
Influenced by: Ada, Perl, Smalltalk, Python, Lisp, Dylan, and CLU
Influenced: --
Operating system: Cross-platform
License: Ruby License and GPL
Website: www.ruby-lang.org

Ruby is a reflective, object-oriented programming language. It combines syntax inspired by Ada and Perl with Smalltalk-like object-oriented features, and also shares some features with Python, Lisp, Dylan and CLU. Ruby is a single-pass interpreted language. Its main implementation is Free software distributed under an open-source license.

Contents

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History

The language was created by Yukihiro "Matz" Matsumoto, who started working on Ruby on February 24, 1993, and released it to the public in 1995. "Ruby" was named after a colleague's birthstone. As of March 2006, the latest stable version is 1.8.4. Ruby 1.9 (with some major changes) is also in development.

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Philosophy

Matz's primary design consideration is to make programmers happy by reducing the menial work they must do, following the principles of good user interface design. [1] He stresses that systems design needs to emphasize human, rather than computer, needs [2]:

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 typically behaves intuitively or as the programmer assumes it should. The phrase did not originate with Matz and, generally speaking, Ruby may more closely follow a paradigm best termed as "Matz's Least Surprise", though many programmers have found it to be close to their own mental model as well.

Matz defined it this way in an interview: "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 therefore 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 surprised me."

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Semantics

Ruby is object-oriented: every bit of data is an object, even classes and types many other languages designate primitive such as integers. 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 everything done in Ruby procedurally (that is, outside of the scope of a particular object) is actually done to an Object instance named 'main'. 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 you to program procedurally (defining functions/variables outside classes makes them part of the root, 'self' Object), with object orientation (everything is an object) or functionally (it has anonymous functions, closures, and continuations; statements all have values, and functions return the last evaluation). It has support for introspection, reflection and meta-programming, as well as support for threads. Ruby features dynamic typing, and supports parametric polymorphism.

According to the Ruby FAQ, "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." [3]

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Features

Ruby currently lacks full support for Unicode, though it has partial support for UTF-8.

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Interaction

The Ruby official distribution also includes "irb", an interactive command-line interpreter which can be used to test code quickly. A session with this interactive program might be: 

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

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 obligatorially 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 brackets. Line breaks are significant and taken as the end of a statement; a semicolon may be equivalently used. Indentation is not significant (unlike Python).

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

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Gotchas and possible surprises

Although Ruby's design is guided by the principle of least surprise, naturally, some features differ from languages such as C or Perl:

  • Names that begin with a capital letter are treated as constants, so local variables should begin with a lowercase letter.
  • 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. 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 (e.g., mismatch).
  • To denote floating point numbers, one must follow with a zero digit (99.0) or an explicit conversion (99.to_f). It is insufficient to append a dot (99.) because numbers are susceptible to method syntax.
  • Lack of a character data type (compare to C, which provides type char for characters). This may cause surprises when slicing strings: "abc"[0] yields 97 (an integer, representing 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.

In addition, some issues with the language itself are commonly raised:

  • In terms of speed, Ruby's performance is inferior to that of many compiled languages (as is any interpreted language) and other major scripting languages such as Python and Perl. However, from release 1.9, Ruby will be bytecode compiled to be executed on the YARV (Yet Another Ruby VM).
  • Omission of parentheses around method arguments may lead to unexpected results. Note that the Ruby developers have stated that omission of parentheses may be disallowed in future Ruby versions; however, much existing literature encourages parenthesis omission.

A good list of "gotchas" may be found in Hal Fulton's book The Ruby Way, pages 48–64. However, since the list in the book pertains to an older version of Ruby (version 1.6), some items have been fixed since the book's publication. For example, retry now works with while, until and for, as well as iterators.

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Examples

Some basic Ruby code: 

# Everything, including literals, is an object, so this works:
-199.abs                                       # 199
"ruby is cool".length                          # 12
"Rick".index("c")                              # 2
"Nice Day Isn't It?".split(//).uniq.sort.join  # " '?DINaceinsty"
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Collections

Constructing and using an array: 

a = [1, 'hi', 3.14, 1, 2, [4, 5]]

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

Constructing and using a hash: 

hash = {'water' => 'wet', 'fire' => 'hot'}
puts hash['fire']       # Prints:  hot

hash.each_pair do |key, value| 
  puts "#{key} is #{value}"
end

# Prints:             water is wet
#                     fire is hot

hash.delete_if {|key, value| key == 'water'}   # Deletes 'water' => 'wet'
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Blocks and iterators

The two syntaxes for creating a code block: 

{ puts "Hello, World!" }

do puts "Hello, World!" end

Passing a block as a parameter (to be a closure): 

def remember(&p)
  @block = p
end
# Invoke the method, giving it a block that takes a name.
remember {|name| puts "Hello, " + name + "!"}

# When the time is right -- call the closure!
@block.call("John")
# Prints "Hello, John!"

Ruby code corresponding to the fragment in the article Python programming language demonstrating closures: 

def foo(initial_value=0)
  var = initial_value
  return Proc.new {|x| var = x}, Proc.new { var }
end

setter, getter = foo
setter.call(21)
getter.call # => 21

Yielding program flow to a block provided at the location of the call: 

def bfs(e)
  q = []
  e.mark
  yield e
  q.push e
  while not q.empty?
    u = q.shift
    u.edge_iterator do |v|
    if not v.marked?
      v.mark
      yield v
      q.push v
    end  
  end
end
bfs(e) {|v| puts v}

Iterating over enumerations and arrays using blocks: 

a = [1, 'hi', 3.14]
a.each {|item| puts item}          # Prints each element
(3..6).each {|num| puts num}       # Prints the numbers 3 through 6

Blocks work with many built-in methods: 

File.open('file.txt', 'w+b') do |file|
  file.puts 'Wrote some text.'
end                             # File automatically closed here

Or: 

IO.readlines('file.txt') do |line|
  # Process each line, here.
end

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

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

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 meta-programming in Ruby: "attr" 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
  def initialize(name, age)
    @name, @age = name, age
  end

  def <=>(person)
    @age <=> person.age
  end

  def to_s
    "#{@name} (#{@age})"
  end

  attr_reader :name, :age
end

group = [ Person.new("John", 20), 
          Person.new("Markus", 63), 
          Person.new("Ash", 16) 
        ]

puts group.sort.reverse

The above prints three names in reverse age order: 

Markus (63)
John (20)
Ash (16)
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Exceptions

An exception is raised with a raise call: 

raise

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!"

Exceptions are handled by the rescue clause. Such a clause can catch exceptions that inherit from StandardError: 

begin
  # Do something
rescue
  # Handle exception
end

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

 begin
   # Do something
 rescue Exception
   # Handle exception
 end

Or particular exceptions: 

begin
  # ...
rescue RuntimeError 
  # handling
end

Finally, it is possible to specify that the exception object be made available to the handler clause: 

begin
  # ...
rescue RuntimeError => e
  # handling, possibly involving e
end

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

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More examples

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

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Implementations

Ruby has two main implementations: the official Ruby interpreter, which is the most widely used, and JRuby, a Java-based implementation. The Ruby interpreter has been ported to many platforms, including Unix, Microsoft Windows, DOS, Mac OS X, OS/2, Amiga and many more.

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Operating systems

Ruby is available for the following operating systems:

Other ports may also exist.

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Licensing terms

The Ruby interpreter and libraries are distributed disjointedly under the free and open source licenses GPL and Ruby License [4].

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Applications

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

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See also

Template:Portalpar

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External links

Template:Book

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