index.md

title
Ruby From Other Languages

When you first look at some Ruby code, it will likely remind you of other programming languages you’ve used. This is on purpose. Much of the syntax is familiar to users of Perl, Python, and Java (among other languages), so if you’ve used those, learning Ruby will be a piece of cake.

This document contains two major sections. The first attempts to be a rapid-fire summary of what you can expect to see when going from language X to Ruby. The second section tackles the major language features and how they might compare to what you’re already familiar with.

What to Expect: Language X to Ruby

• To Ruby From C and C++
• To Ruby From Java
• To Ruby From Perl
• To Ruby From Python
• To Ruby From PHP

Important Language Features And Some Gotchas

Here are some pointers and hints on major Ruby features you’ll see while learning Ruby.

Iteration

Two Ruby features that are a bit unlike what you may have seen before, and which take some getting used to, are “blocks” and iterators. Instead of looping over an index (like with C, C++, or pre-1.5 Java), or looping over a list (like Perl’s for (@a) {...}, or Python’s for i in aList: ...), with Ruby you’ll very often instead see

some_list.each do |this_item|
  # We're inside the block.
  # deal with this_item.
end

For more info on each (and its friends collect, find, inject, sort, etc.), see ri Enumerable (and then ri Enumerable#some_method).

Everything has a value

There’s no difference between an expression and a statement. Everything has a value, even if that value is nil. This is possible:

x = 10
y = 11
z = if x < y
      true
    else
      false
    end
z # => true

Symbols are not lightweight Strings

Many Ruby newbies struggle with understanding what Symbols are, and what they can be used for.

Symbols can best be described as identities. A symbol is all about who it is, not what it is. Fire up irb and see the difference:

irb(main):001:0> :george.object_id == :george.object_id
=> true
irb(main):002:0> "george".object_id == "george".object_id
=> false
irb(main):003:0>

The object_id methods returns the identity of an Object. If two objects have the same object_id, they are the same (point to the same Object in memory).

As you can see, once you have used a Symbol once, any Symbol with the same characters references the same Object in memory. For any given two Symbols that represent the same characters, the object_ids match.

Now take a look at the String (“george”). The object_ids don’t match. That means they’re referencing two different objects in memory. Whenever you use a new String, Ruby allocates memory for it.

If you’re in doubt whether to use a Symbol or a String, consider what’s more important: the identity of an object (i.e. a Hash key), or the contents (in the example above, “george”).

Everything is an Object

“Everything is an object” isn’t just hyperbole. Even classes and integers are objects, and you can do the same things with them as with any other object:

# This is the same as
# class MyClass
#   attr_accessor :instance_var
# end
MyClass = Class.new do
  attr_accessor :instance_var
end

Variable Constants

Constants are not really constant. If you modify an already initialized constant, it will trigger a warning, but not halt your program. That isn’t to say you should redefine constants, though.

Naming conventions

Ruby enforces some naming conventions. If an identifier starts with a capital letter, it is a constant. If it starts with a dollar sign ($), it is a global variable. If it starts with @, it is an instance variable. If it starts with @@, it is a class variable.

Method names, however, are allowed to start with capital letters. This can lead to confusion, as the example below shows:

Constant = 10
def Constant
  11
end

Now Constant is 10, but Constant() is 11.

Keyword arguments

Like in Python, since Ruby 2.0 methods can be defined using keyword arguments:

def deliver(from: "A", to: nil, via: "mail")
  "Sending from #{from} to #{to} via #{via}."
end

deliver(to: "B")
# => "Sending from A to B via mail."
deliver(via: "Pony Express", from: "B", to: "A")
# => "Sending from B to A via Pony Express."

The universal truth

In Ruby, everything except nil and false is considered true. In C, Python and many other languages, 0 and possibly other values, such as empty lists, are considered false. Take a look at the following Python code (the example applies to other languages, too):

# in Python
if 0:
  print("0 is true")
else:
  print("0 is false")

This will print “0 is false”. The equivalent Ruby:

# in Ruby
if 0
  puts "0 is true"
else
  puts "0 is false"
end

Prints “0 is true”.

Access modifiers apply until the end of scope

In the following Ruby code,

class MyClass
  private
  def a_method; true; end
  def another_method; false; end
end

You might expect another_method to be public. Not so. The private access modifier continues until the end of the scope, or until another access modifier pops up, whichever comes first. By default, methods are public:

class MyClass
  # Now a_method is public
  def a_method; true; end

  private

  # another_method is private
  def another_method; false; end
end

public, private and protected are really methods, so they can take parameters. If you pass a Symbol to one of them, that method’s visibility is altered.

Method access

In Java, public means a method is accessible by anyone. protected means the class’s instances, instances of descendant classes, and instances of classes in the same package can access it, but not anyone else, and private means nobody besides the class’s instances can access the method.

Ruby differs slightly. public is, naturally, public. private means the method(s) are accessible only when they can be called without an explicit receiver. Only self is allowed to be the receiver of a private method call.

protected is the one to be on the lookout for. A protected method can be called from a class or descendant class instances, but also with another instance as its receiver. Here is an example (adapted from The Ruby Language FAQ):

class Test
  # public by default
  def identifier
    99
  end

  def ==(other)
    identifier == other.identifier
  end
end

t1 = Test.new  # => #<Test:0x34ab50>
t2 = Test.new  # => #<Test:0x342784>
t1 == t2       # => true

# now make `identifier' protected; it still works
# because protected allows `other' as receiver

class Test
  protected :identifier
end

t1 == t2  # => true

# now make `identifier' private

class Test
  private :identifier
end

t1 == t2
# NoMethodError: private method `identifier' called for #<Test:0x342784>

Classes are open

Ruby classes are open. You can open them up, add to them, and change them at any time. Even core classes, like Fixnum or even Object, the parent of all objects. Ruby on Rails defines a bunch of methods for dealing with time on Fixnum. Watch:

class Fixnum
  def hours
    self * 3600 # number of seconds in an hour
  end
  alias hour hours
end

# 14 hours from 00:00 January 1st
# (aka when you finally wake up ;)
Time.mktime(2006, 01, 01) + 14.hours # => Sun Jan 01 14:00:00

Funny method names

In Ruby, methods are allowed to end with question marks or exclamation marks. By convention, methods that answer questions end in question marks (e.g. Array#empty?, which returns true if the receiver is empty). Potentially “dangerous” methods by convention end with exclamation marks (e.g. methods that modify self or the arguments, exit!, etc.). Not all methods that change their arguments end with exclamation marks, though. Array#replace replaces the contents of an array with the contents of another array. It doesn’t make much sense to have a method like that that doesn’t modify self.

Singleton methods

Singleton methods are per-object methods. They are only available on the Object you defined it on.

class Car
  def inspect
    "Cheap car"
  end
end

porsche = Car.new
porsche.inspect # => Cheap car
def porsche.inspect
  "Expensive car"
end

porsche.inspect # => Expensive car

# Other objects are not affected
other_car = Car.new
other_car.inspect # => Cheap car

Missing methods

Ruby doesn’t give up if it can’t find a method that responds to a particular message. It calls the method_missing method with the name of the method it couldn’t find and the arguments. By default, method_missing raises a NameError exception, but you can redefine it to better fit your application, and many libraries do. Here is an example:

# id is the name of the method called, the * syntax collects
# all the arguments in an array named 'arguments'
def method_missing(id, *arguments)
  puts "Method #{id} was called, but not found. It has " +
       "these arguments: #{arguments.join(", ")}"
end

__ :a, :b, 10
# => Method __ was called, but not found. It has these
# arguments: a, b, 10

The code above just prints the details of the call, but you are free to handle the message in any way that is appropriate.

Message passing, not function calls

A method call is really a message to another object:

# This
1 + 2
# Is the same as this ...
1.+(2)
# Which is the same as this:
1.send "+", 2

Blocks are Objects, they just don’t know it yet

Blocks (closures, really) are heavily used by the standard library. To call a block, you can either use yield, or make it a Proc by appending a special argument to the argument list, like so:

def block(&the_block)
  # Inside here, the_block is the block passed to the method
  the_block # return the block
end
adder = block { |a, b| a + b }
# adder is now a Proc object
adder.class # => Proc

You can create blocks outside of method calls, too, by calling Proc.new with a block or calling the lambda method.

Similarly, methods are also Objects in the making:

method(:puts).call "puts is an object!"
# => puts is an object!

Operators are syntactic sugar

Most operators in Ruby are just syntactic sugar (with some precedence rules) for method calls. You can, for example, override Fixnum’s + method:

class Fixnum
  # You can, but please don't do this
  def +(other)
    self - other
  end
end

You don’t need C++’s operator+, etc.

You can even have array-style access if you define the [] and []= methods. To define the unary + and - (think +1 and -2), you must define the +@ and -@ methods, respectively. The operators below are not syntactic sugar, though. They are not methods, and cannot be redefined:

=, .., ..., not, &&, and, ||, or, ::

In addition, +=, *= etc. are just abbreviations for var = var + other_var, var = var * other_var, etc. and therefore cannot be redefined.