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Syntax

What is the difference between an immediate value and a reference?

This section or parts of it might be out-dated or in need of confirmation.

Fixnum, true, nil, and false are implemented as immediate values. With immediate values, variables hold the objects themselves, rather than references to them.

Singleton methods cannot be defined for such objects. Two Fixnums of the same value always represent the same object instance, so (for example) instance variables for the Fixnum with the value 1 are shared between all the 1’s in the system. This makes it impossible to define a singleton method for just one of these.

What is the difference between nil and false?

First the similarity: nil and false are the only two objects that evaluate to false in a boolean context. (In other words: they are the only “falsy” values, all other objects are “truthy”.)

However, nil and false are instances of different classes (NilClass and FalseClass), and have different behavior elsewhere.

We recommend that predicate methods (those whose name ends with a question mark) return true or false. Other methods that need to indicate failure should return nil.

Why is an empty string not false?

Q: An empty string ("") returns true in a conditional expression! In Perl, it’s false.

A: But Ruby is not Perl ;-). It’s very simple: in Ruby, only nil and false are false in conditional contexts.

You can use empty?, compare the string to "", or compare the string’s size or length to 0 to find out if a string is empty.

What does :name mean?

A colon followed by a name generates a Symbol object which corresponds one to one with the identifier. During the duration of a program’s execution the same Symbol object will be created for a given name or string. Symbols can also be created with "name".intern or "name".to_sym.

Symbol objects can represent identifiers for methods, variables, and so on. Some methods, like define_method, method_missing, or trace_var, require a symbol. Other methods, e.g. attr_accessor, send, or autoload, also accept a string.

Due to the fact that they are created only once, Symbols are often used as hash keys. String hash keys would create a new object for every single use, thereby causing some memory overhead. There is even a special syntax for symbol hash keys:

person_1 = { :name => "John", :age => 42 }
person_2 = { name: "Jane", age: 24 }        # alternate syntax

Symbols can also be used as enumeration values or to assign unique values to constants:

status = :open  # :closed, ...

NORTH = :NORTH
SOUTH = :SOUTH

How can I access the value of a symbol?

To get the value of the variable corresponding to a symbol, you can use symbol.to_s or "#{symbol}" to get the name of the variable, and then eval that in the scope of the symbol to get the variable’s contents:

a = "This is the content of `a'"
b = eval("#{:a}")
a.object_id == b.object_id  # => true

You can also use

b = binding.local_variable_get(:a)

If your symbol corresponds to the name of a method, you can use send:

class Demo
  def hello
    "Hello, world"
  end
end

demo = Demo.new
demo.send(:hello)

Or you can use Object#method to return a corresponding Method object, which you may then call:

m = demo.method(:hello)  # => #<Method: Demo#hello>
m.call                   # => "Hello, world"

Is loop a control structure?

Although loop looks like a control structure, it is actually a method defined in Kernel. The block which follows introduces a new scope for local variables.

Ruby doesn’t have a post-test loop

Q: Ruby does not have a do { ... } while construct, so how can I implement loops that test the condition at the end?

Clemens Hintze says: You can use a combination of Ruby’s begin ... end and the while or until statement modifiers to achieve the same effect:

i = 0
begin
  puts "i = #{i}"
  i += 1
end until i > 4

Produces:

i = 0
i = 1
i = 2
i = 3
i = 4

Why can’t I pass a hash literal to a method: p {}?

The {} is parsed as a block, not a Hash constructor. You can force the {} to be treated as an expression by making the fact that it’s a parameter explicit: p({}).

I can’t get def pos=(val) to work!

I have the following code, but I cannot use the method pos = 1.

def pos=(val)
  @pos = val
  puts @pos
end

Methods with = appended must be called with an explicit receiver (without the receiver, you are just assigning to a local variable). Invoke it as self.pos = 1.

What is the difference between '\1' and '\\1'?

They have the same meaning. In a single quoted string, only \' and \\ are transformed and other combinations remain unchanged.

However, in a double quoted string, "\1" is the byte \001 (an octal bit pattern), while "\\1" is the two character string containing a backslash and the character "1".

What is the difference between .. and ...?

.. includes the right hand side in the range, ... does not:

(5..8).to_a   # => [5, 6, 7, 8]
(5...8).to_a  # => [5, 6, 7]

What is the difference between or and ||?

Q: p(nil || "Hello") prints "Hello", while p(nil or "Hello") gives a parse error. Why?

A: or has a very low precedence, p( (nil or "Hello") ) will work.

The precedence of or is for instance also lower than that of =, whereas || has a higher precedence:

foo = nil || "Hello"  # parsed as: foo = (nil || "Hello")
foo  # => "Hello"

# but perhaps surprisingly:

foo = nil or "Hello"  # parsed as: (foo = nil) or "Hello"
foo  # => nil

or (and similarly and) is best used not for combining boolean expressions, but for control flow, like in

do_something  or raise "some error!"

where do_something returns false or nil when an error occurs.

Does Ruby have function pointers?

A Proc object generated by Proc.new, proc, or lambda can be referenced from a variable, so that variable could be said to be a function pointer. You can also get references to methods within a particular object instance using object.method.

What is the difference between load and require?

load will load and execute a Ruby program (*.rb).

require loads Ruby programs as well, but will also load binary Ruby extension modules (shared libraries or DLLs). In addition, require ensures that a feature is never loaded more than once.

Does Ruby have exception handling?

Ruby supports a flexible exception handling scheme:

begin
  statements which may raise exceptions
rescue [exception class names]
  statements when an exception occurred
rescue [exception class names]
  statements when an exception occurred
ensure
  statements that will always run
end

If an exception occurs in the begin clause, the rescue clause with the matching exception name is executed. The ensure clause is executed whether an exception occurred or not. rescue and ensure clauses may be omitted.

If no exception class is designated for a rescue clause, StandardError exception is implied, and exceptions which are in a is_a? relation to StandardError are captured.

This expression returns the value of the begin clause.

The latest exception is accessed by the global variable $! (and so its type can be determined using $!.type).

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