The syntax of the Ruby programming language is broadly similar to that of Perl and Python. 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 sigil. When used, the sigil changes the semantics of scope of the variable. For practical purposes there is no distinction between expressions and statements. 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 (attr_writer, attr_reader, etc.). Unlike the "getter" and "setter" methods of other languages like C++ or Java, accessor methods in Ruby can be created with a single line of code via metaprogramming; 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 C# and VB.NET 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 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 the class; rather, one passes a message to the class and receives a response.
Interactive sessions 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:
Some basic Ruby code:
Input:
Conversions:
Strings There are a variety of ways to define strings in Ruby. The following assignments are equivalent:
Strings support variable interpolation:
The following assignments are equivalent and produce raw strings:
Collections Constructing and using an array:
Constructing and using an associative array (in Ruby, called a hash):
Control structures If statement:
Blocks and iterators The two syntaxes for creating a code block:
A code block can be passed to a method as an optional block argument. Many built-in methods have such arguments:
Parameter-passing a block to be a closure:
Creating an anonymous function:
Returning closures from a method:
Yielding the flow of program control to a block that was provided at calling time:
Iterating over enumerations and arrays using blocks:
A method such as inject can accept both a parameter and a block. The inject method iterates over each member of a list, performing some function on it while retaining an aggregate. This is analogous to the foldl function in functional programming languages. For example:
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, which then becomes sum 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):
Or invoke a method on each item (map is a synonym for collect):
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 metaprogramming in Ruby: attr_accessor defines getter and setter methods of instance variables, but attr_reader only getter methods. The last evaluated statement in a method is its return value, allowing the omission of an explicit return statement.
The preceding code prints three names in reverse age order:
Person is a constant and is a reference to a Class object.
Open classes 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 Time class:
Adding methods to previously defined classes is often called monkey-patching. 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 refinements to reduce the potentially negative consequences of monkey-patching, by limiting the scope of the patch to particular areas of the code base.
Exceptions An exception is raised with a raise call:
An optional message can be added to the exception:
Exceptions can also be specified by the programmer:
Alternatively, an exception instance can be passed to the raise method:
This last construct is useful when raising an instance of a custom exception class featuring a constructor that takes more than one argument:
Exceptions are handled by the rescue clause. Such a clause can catch exceptions that inherit from StandardError. Other flow control keywords that can be used when handling exceptions are else and ensure:
It is a common mistake to attempt to catch all exceptions with a simple rescue clause. To catch all exceptions one must write:
Or catch particular exceptions:
It is also possible to specify that the exception object be made available to the handler clause:
Alternatively, the most recent exception is stored in the magic global $!. Several exceptions can also be caught:
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