ruby--ruby/doc/syntax/assignment.rdoc

= Assignment

In Ruby, assignment uses the <code>=</code> (equals sign) character.  This
example assigns the number five to the local variable +v+:

  v = 5

Assignment creates a local variable if the variable was not previously
referenced.

== Local Variable Names

A local variable name must start with a lowercase US-ASCII letter or a
character with the eight bit set.  Typically local variables are US-ASCII
compatible since the keys to type them exist on all keyboards.

(Ruby programs must be written in a US-ASCII-compatible character set.  In
such character sets if the eight bit is set it indicates an extended
character.  Ruby allows local variables to contain such characters.)

A local variable name may contain letters, numbers, an <code>_</code>
(underscore or low line) or a character with the eighth bit set.

== Local Variable Scope

Once a local variable name has been assigned-to all uses of the name for the
rest of the scope are considered local variables.

Here is an example:

  1.times do
    a = 1
    puts "local variables in the block: #{local_variables.join ", "}"
  end

  puts "no local variables outside the block" if local_variables.empty?

This prints:

  local variables in the block: a
  no local variables outside the block

Since the block creates a new scope, any local variables created inside it do
not leak to the surrounding scope.

Variables defined in an outer scope appear inner scope:

  a = 0

  1.times do
    puts "local variables: #{local_variables.join ", "}"
  end

This prints:

  local variables: a

You may isolate variables in a block from the outer scope by listing them
following a <code>;</code> in the block's arguments.  See the documentation
for block local variables in the {calling
methods}[rdoc-ref:syntax/calling_methods.rdoc] documentation for an example.

See also Kernel#local_variables, but note that a +for+ loop does not create a
new scope like a block does.

== Local Variables and Methods

In Ruby local variable names and method names are nearly identical.  If you
have not assigned to one of these ambiguous names ruby will assume you wish to
call a method.  Once you have assigned to the name ruby will assume you wish
to reference a local variable.

The local variable is created when the parser encounters the assignment, not
when the assignment occurs:

  a = 0 if false # does not assign to a

  p local_variables # prints [:a]

  p a # prints nil

The similarity between method and local variable names can lead to confusing
code, for example:

  def big_calculation
    42 # pretend this takes a long time
  end

  big_calculation = big_calculation()

Now any reference to +big_calculation+ is considered a local variable and will
be cached.  To call the method, use <code>self.big_calculation</code>.

You can force a method call by using empty argument parentheses as shown above
or by using an explicit receiver like <code>self.</code>.  Using an explicit
receiver may raise a NameError if the method's visibility is not public.

Another commonly confusing case is when using a modifier +if+:

  p a if a = 0.zero?

Rather than printing "true" you receive a NameError, "undefined local variable
or method `a'".  Since ruby parses the bare +a+ left of the +if+ first and has
not yet seen an assignment to +a+ it assumes you wish to call a method.  Ruby
then sees the assignment to +a+ and will assume you are referencing a local
method.

The confusion comes from the out-of-order execution of the expression.  First
the local variable is assigned-to then you attempt to call a nonexistent
method.

== Local Variables and eval

Using +eval+ to evaluate Ruby code will allow access to local variables defined
in the same scope, even if the local variables are not defined until after the
call to +eval+.  However, local variables defined inside the call to +eval+
will not be reflected in the surrounding scope.  Inside the call to +eval+,
local variables defined in the surrounding scope and local variables defined
inside the call to +eval+ will be accessible.  However, you will not be able
to access local variables defined in previous or subsequent calls to +eval+ in
the same scope. Consider each +eval+ call a separate nested scope. Example:

  def m
    eval "bar = 1"
    lvs = eval "baz = 2; ary = [local_variables, foo, baz]; x = 2; ary"
    eval "quux = 3"
    foo = 1
    lvs << local_variables
  end

  m
  # => [[:baz, :ary, :x, :lvs, :foo], nil, 2, [:lvs, :foo]]

== Instance Variables

Instance variables are shared across all methods for the same object.

An instance variable must start with a <code>@</code> ("at" sign or
commercial at).  Otherwise instance variable names follow the rules as local
variable names.  Since the instance variable starts with an <code>@</code> the
second character may be an upper-case letter.

Here is an example of instance variable usage:

  class C
    def initialize(value)
      @instance_variable = value
    end

    def value
      @instance_variable
    end
  end

  object1 = C.new "some value"
  object2 = C.new "other value"

  p object1.value # prints "some value"
  p object2.value # prints "other value"

An uninitialized instance variable has a value of +nil+.  If you run Ruby with
warnings enabled, you will get a warning when accessing an uninitialized
instance variable.

The +value+ method has access to the value set by the +initialize+ method, but
only for the same object.

== Class Variables

Class variables are shared between a class, its subclasses and its instances.

A class variable must start with a <code>@@</code> (two "at" signs).  The rest
of the name follows the same rules as instance variables.

Here is an example:

  class A
    @@class_variable = 0

    def value
      @@class_variable
    end

    def update
      @@class_variable = @@class_variable + 1
    end
  end

  class B < A
    def update
      @@class_variable = @@class_variable + 2
    end
  end

  a = A.new
  b = B.new

  puts "A value: #{a.value}"
  puts "B value: #{b.value}"

This prints:

  A value: 0
  B value: 0

Continuing with the same example, we can update using objects from either
class and the value is shared:

  puts "update A"
  a.update

  puts "A value: #{a.value}"
  puts "B value: #{b.value}"

  puts "update B"
  b.update

  puts "A value: #{a.value}"
  puts "B value: #{b.value}"

  puts "update A"
  a.update

  puts "A value: #{a.value}"
  puts "B value: #{b.value}"

This prints:

  update A
  A value: 1
  B value: 1
  update B
  A value: 3
  B value: 3
  update A
  A value: 4
  B value: 4

Accessing an uninitialized class variable will raise a NameError exception.

Note that classes have instance variables because classes are objects, so
try not to confuse class and instance variables.

== Global Variables

Global variables are accessible everywhere.

Global variables start with a <code>$</code> (dollar sign).  The rest of the
name follows the same rules as instance variables.

Here is an example:

  $global = 0

  class C
    puts "in a class: #{$global}"

    def my_method
      puts "in a method: #{$global}"

      $global = $global + 1
      $other_global = 3
    end
  end

  C.new.my_method

  puts "at top-level, $global: #{$global}, $other_global: #{$other_global}"

This prints:

  in a class: 0
  in a method: 0
  at top-level, $global: 1, $other_global: 3

An uninitialized global variable has a value of +nil+.

Ruby has some special globals that behave differently depending on context
such as the regular expression match variables or that have a side-effect when
assigned to.  See the {global variables documentation}[rdoc-ref:globals.rdoc]
for details.

== Assignment Methods

You can define methods that will behave like assignment, for example:

  class C
    def value=(value)
      @value = value
    end
  end

  c = C.new
  c.value = 42

Using assignment methods allows your programs to look nicer.  When assigning
to an instance variable most people use Module#attr_accessor:

  class C
    attr_accessor :value
  end

When using method assignment you must always have a receiver.  If you do not
have a receiver, Ruby assumes you are assigning to a local variable:

  class C
    attr_accessor :value

    def my_method
      value = 42

      puts "local_variables: #{local_variables.join ", "}"
      puts "@value: #{@value.inspect}"
    end
  end

  C.new.my_method

This prints:

  local_variables: value
  @value: nil

To use the assignment method you must set the receiver:

  class C
    attr_accessor :value

    def my_method
      self.value = 42

      puts "local_variables: #{local_variables.join ", "}"
      puts "@value: #{@value.inspect}"
    end
  end

  C.new.my_method

This prints:

  local_variables:
  @value: 42

== Abbreviated Assignment

You can mix several of the operators and assignment.  To add 1 to an object
you can write:

  a = 1

  a += 2

  p a # prints 3

This is equivalent to:

  a = 1

  a = a + 2

  p a # prints 3

You can use the following operators this way:  <code>+</code>, <code>-</code>,
<code>*</code>, <code>/</code>, <code>%</code>, <code>**</code>,
<code>&</code>, <code>|</code>, <code>^</code>, <code><<</code>,
<code>>></code>

There are also <code>||=</code> and <code>&&=</code>.  The former makes an
assignment if the value was +nil+ or +false+ while the latter makes an
assignment if the value was not +nil+ or +false+.

Here is an example:

  a ||= 0
  a &&= 1

  p a # prints 1

Note that these two operators behave more like <code>a || a = 0</code> than
<code>a = a || 0</code>.

== Implicit Array Assignment

You can implicitly create an array by listing multiple values when assigning:

  a = 1, 2, 3

  p a # prints [1, 2, 3]

This implicitly creates an Array.

You can use <code>*</code> or the "splat" operator or unpack an Array when
assigning.  This is similar to multiple assignment:

  a = *[1, 2, 3]

  p a # prints [1, 2, 3]

You can splat anywhere in the right-hand side of the assignment:

  a = 1, *[2, 3]

  p a # prints [1, 2, 3]

== Multiple Assignment

You can assign multiple values on the right-hand side to multiple variables:

  a, b = 1, 2

  p a: a, b: b # prints {:a=>1, :b=>2}

In the following sections any place "variable" is used an assignment method,
instance, class or global will also work:

  def value=(value)
    p assigned: value
  end

  self.value, $global = 1, 2 # prints {:assigned=>1}

  p $global # prints 2

You can use multiple assignment to swap two values in-place:

  old_value = 1

  new_value, old_value = old_value, 2

  p new_value: new_value, old_value: old_value
  # prints {:new_value=>1, :old_value=>2}

If you have more values on the right hand side of the assignment than variables
on the left hand side, the extra values are ignored:

  a, b = 1, 2, 3

  p a: a, b: b # prints {:a=>1, :b=>2}

You can use <code>*</code> to gather extra values on the right-hand side of
the assignment.

  a, *b = 1, 2, 3

  p a: a, b: b # prints {:a=>1, :b=>[2, 3]}

The <code>*</code> can appear anywhere on the left-hand side:

  *a, b = 1, 2, 3

  p a: a, b: b # prints {:a=>[1, 2], :b=>3}

But you may only use one <code>*</code> in an assignment.

== Array Decomposition

Like Array decomposition in {method arguments}[rdoc-ref:syntax/methods.rdoc]
you can decompose an Array during assignment using parenthesis:

  (a, b) = [1, 2]

  p a: a, b: b # prints {:a=>1, :b=>2}

You can decompose an Array as part of a larger multiple assignment:

  a, (b, c) = 1, [2, 3]

  p a: a, b: b, c: c # prints {:a=>1, :b=>2, :c=>3}

Since each decomposition is considered its own multiple assignment you can use
<code>*</code> to gather arguments in the decomposition:

  a, (b, *c), *d = 1, [2, 3, 4], 5, 6

  p a: a, b: b, c: c, d: d
  # prints {:a=>1, :b=>2, :c=>[3, 4], :d=>[5, 6]}