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[ci skip][fix GH-1121] Patch by @clandry94 git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53182 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
454 lines
11 KiB
Text
454 lines
11 KiB
Text
= Assignment
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In Ruby, assignment uses the <code>=</code> (equals sign) character. This
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example assigns the number five to the local variable +v+:
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v = 5
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Assignment creates a local variable if the variable was not previously
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referenced.
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== Local Variable Names
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A local variable name must start with a lowercase US-ASCII letter or a
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character with the eight bit set. Typically local variables are US-ASCII
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compatible since the keys to type them exist on all keyboards.
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(Ruby programs must be written in a US-ASCII-compatible character set. In
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such character sets if the eight bit is set it indicates an extended
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character. Ruby allows local variables to contain such characters.)
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A local variable name may contain letters, numbers, an <code>_</code>
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(underscore or low line) or a character with the eighth bit set.
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== Local Variable Scope
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Once a local variable name has been assigned-to all uses of the name for the
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rest of the scope are considered local variables.
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Here is an example:
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1.times do
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a = 1
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puts "local variables in the block: #{local_variables.join ", "}"
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end
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puts "no local variables outside the block" if local_variables.empty?
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This prints:
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local variables in the block: a
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no local variables outside the block
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Since the block creates a new scope, any local variables created inside it do
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not leak to the surrounding scope.
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Variables defined in an outer scope appear inner scope:
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a = 0
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1.times do
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puts "local variables: #{local_variables.join ", "}"
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end
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This prints:
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local variables: a
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You may isolate variables in a block from the outer scope by listing them
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following a <code>;</code> in the block's arguments. See the documentation
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for block local variables in the {calling
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methods}[rdoc-ref:syntax/calling_methods.rdoc] documentation for an example.
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See also Kernel#local_variables, but note that a +for+ loop does not create a
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new scope like a block does.
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== Local Variables and Methods
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In Ruby local variable names and method names are nearly identical. If you
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have not assigned to one of these ambiguous names ruby will assume you wish to
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call a method. Once you have assigned to the name ruby will assume you wish
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to reference a local variable.
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The local variable is created when the parser encounters the assignment, not
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when the assignment occurs:
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a = 0 if false # does not assign to a
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p local_variables # prints [:a]
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p a # prints nil
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The similarity between method and local variable names can lead to confusing
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code, for example:
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def big_calculation
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42 # pretend this takes a long time
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end
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big_calculation = big_calculation()
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Now any reference to +big_calculation+ is considered a local variable and will
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be cached. To call the method, use <code>self.big_calculation</code>.
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You can force a method call by using empty argument parentheses as shown above
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or by using an explicit receiver like <code>self.</code>. Using an explicit
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receiver may raise a NameError if the method's visibility is not public.
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Another commonly confusing case is when using a modifier +if+:
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p a if a = 0.zero?
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Rather than printing "true" you receive a NameError, "undefined local variable
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or method `a'". Since ruby parses the bare +a+ left of the +if+ first and has
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not yet seen an assignment to +a+ it assumes you wish to call a method. Ruby
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then sees the assignment to +a+ and will assume you are referencing a local
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method.
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The confusion comes from the out-of-order execution of the expression. First
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the local variable is assigned-to then you attempt to call a nonexistent
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method.
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== Instance Variables
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Instance variables are shared across all methods for the same object.
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An instance variable must start with a <code>@</code> ("at" sign or
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commercial at). Otherwise instance variable names follow the rules as local
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variable names. Since the instance variable starts with an <code>@</code> the
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second character may be an upper-case letter.
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Here is an example of instance variable usage:
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class C
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def initialize(value)
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@instance_variable = value
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end
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def value
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@instance_variable
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end
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end
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object1 = C.new "some value"
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object2 = C.new "other value"
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p object1.value # prints "some value"
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p object2.value # prints "other value"
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An uninitialized instance variable has a value of +nil+. If you run Ruby with
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warnings enabled, you will get a warning when accessing an uninitialized
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instance variable.
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The +value+ method has access to the value set by the +initialize+ method, but
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only for the same object.
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== Class Variables
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Class variables are shared between a class, its subclasses and its instances.
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A class variable must start with a <code>@@</code> (two "at" signs). The rest
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of the name follows the same rules as instance variables.
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Here is an example:
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class A
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@@class_variable = 0
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def value
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@@class_variable
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end
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def update
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@@class_variable = @@class_variable + 1
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end
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end
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class B < A
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def update
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@@class_variable = @@class_variable + 2
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end
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end
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a = A.new
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b = B.new
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puts "A value: #{a.value}"
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puts "B value: #{b.value}"
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This prints:
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A value: 0
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B value: 0
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Continuing with the same example, we can update using objects from either
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class and the value is shared:
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puts "update A"
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a.update
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puts "A value: #{a.value}"
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puts "B value: #{b.value}"
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puts "update B"
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b.update
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puts "A value: #{a.value}"
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puts "B value: #{b.value}"
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puts "update A"
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a.update
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puts "A value: #{a.value}"
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puts "B value: #{b.value}"
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This prints:
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update A
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A value: 1
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B value: 1
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update B
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A value: 3
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B value: 3
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update A
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A value: 4
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B value: 4
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Accessing an uninitialized class variable will raise a NameError exception.
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Note that classes have instance variables because classes are objects, so
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try not to confuse class and instance variables.
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== Global Variables
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Global variables are accessible everywhere.
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Global variables start with a <code>$</code> (dollar sign). The rest of the
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name follows the same rules as instance variables.
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Here is an example:
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$global = 0
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class C
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puts "in a class: #{$global}"
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def my_method
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puts "in a method: #{$global}"
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$global = $global + 1
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$other_global = 3
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end
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end
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C.new.my_method
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puts "at top-level, $global: #{$global}, $other_global: #{$other_global}"
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This prints:
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in a class: 0
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in a method: 0
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at top-level, $global: 1, $other_global: 3
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An uninitialized global variable has a value of +nil+.
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Ruby has some special globals that behave differently depending on context
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such as the regular expression match variables or that have a side-effect when
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assigned to. See the {global variables documentation}[rdoc-ref:globals.rdoc]
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for details.
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== Assignment Methods
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You can define methods that will behave like assignment, for example:
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class C
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def value=(value)
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@value = value
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end
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end
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c = C.new
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c.value = 42
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Using assignment methods allows your programs to look nicer. When assigning
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to an instance variable most people use Module#attr_accessor:
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class C
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attr_accessor :value
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end
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When using method assignment you must always have a receiver. If you do not
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have a receiver, Ruby assumes you are assigning to a local variable:
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class C
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attr_accessor :value
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def my_method
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value = 42
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puts "local_variables: #{local_variables.join ", "}"
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puts "@value: #{@value.inspect}"
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end
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end
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C.new.my_method
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This prints:
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local_variables: value
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@value: nil
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To use the assignment method you must set the receiver:
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class C
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attr_accessor :value
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def my_method
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self.value = 42
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puts "local_variables: #{local_variables.join ", "}"
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puts "@value: #{@value.inspect}"
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end
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end
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C.new.my_method
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This prints:
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local_variables:
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@value: 42
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== Abbreviated Assignment
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You can mix several of the operators and assignment. To add 1 to an object
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you can write:
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a = 1
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a += 2
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p a # prints 3
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This is equivalent to:
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a = 1
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a = a + 2
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p a # prints 3
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You can use the following operators this way: <code>+</code>, <code>-</code>,
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<code>*</code>, <code>/</code>, <code>%</code>, <code>**</code>,
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<code>&</code>, <code>|</code>, <code>^</code>, <code><<</code>,
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<code>>></code>
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There are also <code>||=</code> and <code>&&=</code>. The former makes an
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assignment if the value was +nil+ or +false+ while the latter makes an
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assignment if the value was not +nil+ or +false+.
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Here is an example:
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a ||= 0
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a &&= 1
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p a # prints 1
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Note that these two operators behave more like <code>a || a = 0</code> than
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<code>a = a || 0</code>.
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== Implicit Array Assignment
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You can implicitly create an array by listing multiple values when assigning:
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a = 1, 2, 3
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p a # prints [1, 2, 3]
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This implicitly creates an Array.
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You can use <code>*</code> or the "splat" operator or unpack an Array when
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assigning. This is similar to multiple assignment:
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a = *[1, 2, 3]
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p a # prints [1, 2, 3]
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You can splat anywhere in the right-hand side of the assignment:
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a = 1, *[2, 3]
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p a # prints [1, 2, 3]
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== Multiple Assignment
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You can assign multiple values on the right-hand side to multiple variables:
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a, b = 1, 2
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p a: a, b: b # prints {:a=>1, :b=>2}
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In the following sections any place "variable" is used an assignment method,
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instance, class or global will also work:
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def value=(value)
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p assigned: value
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end
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self.value, $global = 1, 2 # prints {:assigned=>1}
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p $global # prints 2
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You can use multiple assignment to swap two values in-place:
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old_value = 1
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new_value, old_value = old_value, 2
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p new_value: new_value, old_value: old_value
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# prints {:new_value=>1, :old_value=>2}
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If you have more values on the right hand side of the assignment than variables
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on the left hand side, the extra values are ignored:
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a, b = 1, 2, 3
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p a: a, b: b # prints {:a=>1, :b=>2}
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You can use <code>*</code> to gather extra values on the right-hand side of
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the assignment.
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a, *b = 1, 2, 3
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p a: a, b: b # prints {:a=>1, :b=>[2, 3]}
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The <code>*</code> can appear anywhere on the left-hand side:
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*a, b = 1, 2, 3
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p a: a, b: b # prints {:a=>[1, 2], :b=>3}
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But you may only use one <code>*</code> in an assignment.
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== Array Decomposition
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Like Array decomposition in {method arguments}[rdoc-ref:syntax/methods.rdoc]
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you can decompose an Array during assignment using parenthesis:
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(a, b) = [1, 2]
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p a: a, b: b # prints {:a=>1, :b=>2}
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You can decompose an Array as part of a larger multiple assignment:
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a, (b, c) = 1, [2, 3]
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p a: a, b: b, c: c # prints {:a=>1, :b=>2, :c=>3}
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Since each decomposition is considered its own multiple assignment you can use
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<code>*</code> to gather arguments in the decomposition:
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a, (b, *c), *d = 1, [2, 3, 4], 5, 6
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p a: a, b: b, c: c, d: d
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# prints {:a=>1, :b=>2, :c=>[3, 4], :d=>[5, 6]}
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