Operator Precedence and Methods

suggest change

From highest to lowest, this is the precedence table for Ruby. High precedence operations happen before low precedence operations.

╔═══════════════════════╦════════════════════════════════════════╦═════════╗
║ Operators             ║                 Operations             ║ Method? ║
╠═══════════════════════╬════════════════════════════════════════╬═════════╣
║ .                     ║ Method call (e.g. foo.bar)             ║         ║
║ []  []=               ║ Bracket Lookup, Bracket Set            ║    ✓¹   ║
║ ! ~ +                 ║ Boolean NOT, complement, unary plus    ║    ✓²   ║
║ **                    ║ Exponentiation                         ║    ✓    ║
║ -                     ║ Unary minus                            ║    ✓²   ║
║ * / %                 ║ Multiplication, division, modulo       ║    ✓    ║
║ + -                   ║ Addition, subtraction                  ║    ✓    ║
║ << >>                 ║ Bitwise shift                          ║    ✓    ║
║ &                     ║ Bitwise AND                            ║    ✓    ║
║ | ^                   ║ Bitwise OR, Bitwise XOR                ║    ✓    ║
║ < <= >= >             ║ Comparison                             ║    ✓    ║
║ <=> == != === =~ !~   ║ Equality, pattern matching, comparison ║    ✓³   ║
║ &&                    ║ Boolean AND                            ║         ║
║ ||                    ║ Boolean OR                             ║         ║
║ .. ...                ║ Inclusive range, Exclusive range       ║         ║
║ ? :                   ║ Ternary operator                       ║         ║
║ rescue                ║ Modifier rescue                        ║         ║
║ = += -=               ║ Assignments                            ║         ║
║ defined?              ║ Defined operator                       ║         ║
║ not                   ║ Boolean NOT                            ║         ║
║ or and                ║ Boolean OR, Boolean AND                ║         ║
║ if unless while until ║ Modifier if, unless, while, until      ║         ║
║ { }                   ║ Block with braces                      ║         ║
║ do end                ║ Block with do end                      ║         ║
╚═══════════════════════╩════════════════════════════════════════╩═════════╝

Unary + and unary - are for +obj, -obj or -(some_expression).

Modifier-if, modifier-unless, etc. are for the modifier versions of those keywords. For example, this is a modifier-unless expression:

a += 1 unless a.zero?

Operators with a ✓ may be defined as methods. Most methods are named exactly as the operator is named, for example:

class Foo
  def **(x)
    puts "Raising to the power of #{x}"
  end
  def <<(y)
    puts "Shifting left by #{y}"
  end
  def !
    puts "Boolean negation"
  end
end

Foo.new ** 2     #=> "Raising to the power of 2"
Foo.new << 3     #=> "Shifting left by 3"
!Foo.new         #=> "Boolean negation"

¹ The Bracket Lookup and Bracket Set methods ([] and []=) have their arguments defined after the name, for example:

class Foo
  def [](x)
    puts "Looking up item #{x}"
  end
  def []=(x,y)
    puts "Setting item #{x} to #{y}"
  end
end

f = Foo.new
f[:cats] = 42    #=> "Setting item cats to 42"
f[17]            #=> "Looking up item 17"

² The “unary plus” and “unary minus” operators are defined as methods named +@ and -@, for example

class Foo
  def -@
    puts "unary minus"
  end
  def +@
    puts "unary plus"
  end
end

f = Foo.new
+f               #=> "unary plus"
-f               #=> "unary minus"

³ In early versions of Ruby the inequality operator != and the non-matching operator !~ could not be defined as methods. Instead, the method for the corresponding equality operator == or matching operator =~ was invoked, and the result of that method was boolean inverted by Ruby.

If you do not define your own != or !~ operators the above behavior is still true. However, as of Ruby 1.9.1, those two operators may also be defined as methods:

class Foo
  def ==(x)
    puts "checking for EQUALITY with #{x}, returning false"
    false
  end
end

f = Foo.new
x = (f == 42)    #=> "checking for EQUALITY with 42, returning false"
puts x           #=> "false"
x = (f != 42)    #=> "checking for EQUALITY with 42, returning false"
puts x           #=> "true"

class Foo
  def !=(x)
    puts "Checking for INequality with #{x}"
  end
end

f != 42          #=> "checking for INequality with 42"

Found a mistake? Have a question or improvement idea? Let me know.

Table Of Contents