Ruby Language Operators Operator Precedence and Methods
Example
From highest to lowest, this is the precedence table for Ruby. High precedence operations happen before low precedence operations.
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║ Operators ║ Operations ║ Method? ║
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║ . ║ 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 ║ ║
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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"
