Ruby Programming/Syntax/Operators

Operators

Complete List and Precedence

Operator	Name/meaning	Arguments	Precedence	Association
!	Boolean NOT	Unary	1	Right
~	Bitwise complement	Unary	1	Right
+	Unary plus (no effect)	Unary	1	Right
**	Exponentiation	Binary	1	Right
-	Unary minus (reverse sign)	Unary	2	Right
*	Multiplication	Binary	3	Left
/	Division	Binary	3	Left
%	Modulo (remainder)	Binary	3	Left
+	Addition or concatenation	Binary	4	Left
-	Subtraction	Binary	4	Left
<<	Bitwise shift left or append (<< and <<- are also used in "here doc" notation)	Binary	5	Left
>>	Bitwise shift right	Binary	5	Left
&	Bitwise AND	Binary	6	Left
|	Bitwise OR	Binary	7	Left
^	Bitwise XOR	Binary	7	Left
<	Less-than	Binary	8	Left
<=	Less-than or equal-to	Binary	8	Left
>	Greater-than	Binary	8	Left
>=	Greater-than or equal to	Binary	8	Left
==	Equal (evaluate to the same value)	Binary	9	N/A
===	"Case equality", "case subsumption" or "three equals" operator. A === B if B is a member of the set of A. Operation varies considerably depending on the data types of A and B.	Binary	9	N/A
!=	Not equal	Binary	9	N/A
=~	Pattern match	Binary	9	N/A
!~	Negative pattern match	Binary	9	N/A
<=>	A <=> B evaluates to -1, 0, or 1; if A is less-than, equal-to, or greater-than B, respectively	Binary	9	N/A
&&	Boolean AND	Binary	10	Left
||	Boolean OR	Binary	11	Left
..	Range creation, boolean flip-flop	Binary	12	N/A
...	Open-ended range creation, boolean flip-flop	Binary	12	N/A
?:	A?B:C evaluates to B if A is true, or C if A is false	Trinary	13	Right
rescue	Modifier for catching exceptions e.g. array[3] rescue nil	Binary	14	Left
=	Assignment	Binary	15	Right
**=	A **=B does A = A ** B	Binary	15	Right
*=	A *=B does A = A * B	Binary	15	Right
/=	A /=B does A = A / B	Binary	15	Right
%=	A %=B does A = A % B	Binary	15	Right
+=	A +=B does A = A + B	Binary	15	Right
-=	A -=B does A = A – B	Binary	15	Right
<<=	A <<=B does A = A << B	Binary	15	Right
>>=	A >>=B does A = A >> B	Binary	15	Right
&&=	A &&=B assigns B to A if A is true or not nil	Binary	15	Right
&=	A &=B does A = A & B	Binary	15	Right
||=	A ||=B assigns B to A if A is nil or false	Binary	15	Right
|=	A |= B does A = A | B	Binary	15	Right
^=	A ^=B does A = A ^ B	Binary	15	Right
defined?	nil if the expression cannot be evaluated (e.g. unset variable)	Unary	16	N/A
not	Boolean NOT	Unary	17	Right
and	Boolean AND	Binary	18	Left
or	Boolean OR	Binary	18	Left
if	Conditional, e.g. print x if x	Binary	19	N/A
unless	Negative conditional, e.g. x = 0 unless x	Binary	19	N/A
while	Loop conditional, e.g. print x += 1 while (x < 10)	Binary	19	N/A
until	Loop conditional, e.g. print x += 1 until (x == 10)	Binary	19	N/A

Higher precedence (lower number in the above table) operators have their immediate arguments evaluated first. Precedence order can be altered with () blocks. For example, because * has higher precedence than +, then:
1 + 2 * 3 == 7
(1 + 2) * 3 == 9

Association direction controls which operators have their arguments evaluated first when multiple operators with the same precedence appear in a row. For example, because - has left association:

1 – 2 – 3 == (1 – 2) – 3 == -1 – 3 == -4

instead of:

1 – 2 – 3 == 1 – (2 – 3) == 1 - -1 == 0

Because ** has right association:

2 ** 3 ** 2 == 2 ** (3 ** 2) == 2 ** 9 == 512

instead of:

2 ** 3 ** 2 == (2 ** 3) ** 2 == 8 ** 2 == 64

{} blocks have lower precedence than the above operators, followed by do/end blocks. Array accesses with [] can be thought of as having a higher precedence than any above operator.

The operators ** through !~ can be overridden (defined for new classes, or redefined for existing operations).

Note that rescue, if, unless, while, and until are operators when used as modifiers in one-liners (as in the above examples) but can also be used as keywords.

Other operators

The dot operator . is used for calling methods on objects, also known as passing a message to the object.

Ruby 2.3.0 introduced the safe navigation operator &., also known as the "lonely operator". This allows replacing

x = foo && foo.bar && foo.bar.baz

with

x = foo&.bar&.baz

An equivalent .dig() method was introduced for hashes and arrays:

hash_variable.dig(:foo, :bar, :baz)
array_variable.dig(1, 0, 2)

are safer versions of:

hash_variable[:foo][:bar][:baz]
array_variable[1][0][2]

The safe navigation operator will raise an error if a requested method, key, or index is not available; unlike the technique of using try() for this purpose, which will return nil.

Yukihiro Matsumoto considered ! ?. and .? before settling on &. because:

• ?. conflicts with *?
• ?. is used by other languages, thus .? is confusingly similar but different
• ! conflicts with "not" logic
• ? is already used by convention for functions that return booleans
• &. is reminiscent of the && syntax the operator is replacing

!! is sometimes seen, but this is simply the ! operator twice. It is used to force the following expression to evaluate to a boolean. This technique is considered non-idiomatic and poor programming practice, because there are more explicit ways to force such a conversion (which is rarely needed to begin with).

Assignment

Assignment in Ruby is done using the equal operator "=". This is both for variables and objects, but since strings, floats, and integers are actually objects in Ruby, you're always assigning objects.

Examples:

  myvar = 'myvar is now this string'
  var = 321
  dbconn = Mysql::new('localhost','root','password')

Self assignment

  x = 1           #=>1
  x += x          #=>2
  x -= x          #=>0
  x += 4          #=>x was 0 so x= + 4 # x is positive 4
  x *= x          #=>16
  x **= x         #=>18446744073709551616 # Raise to the power
  x /= x          #=>1

A frequent question from C and C++ types is "How do you increment a variable? Where are ++ and -- operators?" In Ruby, one should use x+=1 and x-=1 to increment or decrement a variable.

  x = 'a'
  x.succ!         #=>"b" : succ! method is defined for String, but not for Integer types

Multiple assignments

Examples:

  var1, var2, var3 = 10, 20, 30
  var1           #=> 10
  var2           #=> 20
  var3           #=> 30

  myArray=%w(John Michel Fran Doug) # %w() can be used as syntactic sugar to simplify array creation
  var1,var2,var3,var4=*myArray
  puts var1           #=>John
  puts var4           #=>Doug

  names,school=myArray,'St. Whatever'
  names               #=>["John", "Michel", "Fran", "Doug"]
  school              #=>"St. Whatever"

Conditional assignment

  x = find_something() #=>nil
  x ||= "default"      #=>"default" : value of x will be replaced with "default", but only if x is nil or false
  x ||= "other"        #=>"default" : value of x is not replaced if it already is other than nil or false

Operator ||= is a shorthand form that closely resembles the expression:

 x = x || "default"

Operator ||= can be shorthand for code like:

  x = "(some fallback value)" unless respond_to? :x or x

In same way &&= operator works:

  x = get_node() #=>nil
  x &&= x.next_node #=> nil : x will be set to x.next_node, but only if x is NOT nil or false
  x = get_node() #=>Some Node
  x &&= x.next_node #=>Next Node

Operator &&= is a shorthand form of the expression:

 x && x = x.get_node()

Scope

In Ruby there's a local scope, a global scope, an instance scope, and a class scope.

Local Scope

Example:

 var = 2
 4.times do |x|
   puts x = x*var
 end
 #=>
 0
 2
 4
 6
 puts x
 #=> undefined local variable or method `x' for main:Object (NameError)

This error appears because this x(toplevel) is not the x(local) inside the do..end block the x(local) is a local variable to the block, whereas when trying the puts x(toplevel) we're calling a x variable that is in the top level scope, and since there's not one, Ruby protests.

Global scope

 $global = 0
 4.times do |var|
   $global = $global + var
   puts "var #{var}  global #{$global}"
 end
 #=>
 var 0  global 0
 var 1  global 1
 var 2  global 3
 var 3  global 6
 
 puts $global
 #=> 6

This output is given because prefixing a variable with a dollar sign makes the variable a global.

Instance scope

Within methods of a class, you can share variables by prefixing them with an @.

 class A
   def setup
     @instvar = 1
   end
   def go
     @instvar = @instvar*2
     puts @instvar
   end
 end
 instance = A.new
 instance.setup
 instance.go
 #=> 2
 instance.go
 #=> 4

Class scope

A class variable is one that is like a "static" variable in Java. It is shared by all instances of a class.

 class A
   @@classvar = 1
   def go
     @@classvar = @@classvar*2
     puts @@classvar
   end
 end
 instance = A.new
 instance.go
 #=> 2
 instance = A.new
 instance.go
 #=> 4 -- variable is shared across instances

Here's a demo showing the various types:

$variable
 class Test
   def initialize(arg1='kiwi')
     @instvar=arg1
     @@classvar=@instvar+' told you so!!'
     localvar=@instvar
   end
   def print_instvar
     puts @instvar
   end
   def print_localvar
     puts @@classvar
     puts localvar
   end
 end
 var=Test.new
 var.print_instvar              #=>"kiwi", it works because a @instance_var can be accessed inside the class
 var.print_localvar             #=>undefined local variable or method 'localvar' for #<Test:0x2b36208 @instvar="kiwi"> (NameError).

This will print the two lines "kiwi" and "kiwi told you so!!", then fail with a undefined local variable or method 'localvar' for #<Test:0x2b36208 @instvar="kiwi"> (NameError). Why? Well, in the scope of the method print_localvar there doesn't exists localvar, it exists in method initialize(until GC kicks it out). On the other hand, class variables '@@classvar' and '@instvar' are in scope across the entire class and, in the case of @@class variables, across the children classes.

 class SubTest < Test
   def print_classvar
     puts @@classvar
   end
 end
 newvar=SubTest.new              #newvar is created and it has @@classvar with the same value as the var  instance of Test!!
 newvar.print_classvar           #=>kiwi told you so!! 

Class variables have the scope of parent class AND children, these variables can live across classes, and can be affected by the children actions ;-)

 class SubSubTest < Test
   def print_classvar
     puts @@classvar
   end
   def modify_classvar
     @@classvar='kiwi kiwi waaai!!'
   end
 end
 subtest=SubSubTest.new
 subtest.modify_classvar          #lets add a method that modifies the contents of @@classvar in  SubSubTest
 subtest.print_classvar

This new child of Test also has @@classvar with the original value newvar.print_classvar. The value of @@classvar has been changed to 'kiwi kiwi waaai!!' This shows that @@classvar is "shared" across parent and child classes.

Default scope

When you don't enclose your code in any scope specifier, ex:

 @a = 33

it affects the default scope, which is an object called "main".

For example, if you had one script that says

@a = 33
require 'other_script.rb'

and other_script.rb says

puts @a
 #=> 33

They could share variables.

Note however, that the two scripts don't share local variables.

Local scope gotchas

Typically when you are within a class, you can do as you'd like for definitions, like.

class A
  a = 3
  if a == 3

    def go
       3
    end
  else
    def go
       4
    end
  end

end

And also, procs "bind" to their surrounding scope, like

 a = 3
 b = proc { a }
 b.call # 3 -- it remembered what a was

However, the "class" and "def" keywords cause an *entirely new* scope.

class A
  a = 3
  def go
     return a # this won't work!
  end
end

You can get around this limitation by using define_method, which takes a block and thus keeps the outer scope (note that you can use any block you want, to, too, but here's an example).

class A
   a = 3
  define_method(:go) {
      a
  }
end

Here's using an arbitrary block

a = 3
PROC = proc { a } # gotta use something besides a local
# variable because that "class" makes us lose scope.

class A
  define_method(:go, &PROC)
end

or here

 class A
 end
a = 3
 A.class_eval do
   define_method(:go) do
       puts a
   end
end

Logical And

The binary "and" operator will return the logical conjunction of its two operands. It is the same as "&&" but with a lower precedence. Example:

a = 1
b = 2
c = nil
puts "yay all my arguments are true" if a and b
puts "oh no, one of my argument is false" if a and c

Logical Or

The binary "or" operator will return the logical disjunction of its two operands. It is the same as "||" but with a lower precedence. Example:

a = nil
b = "foo"
c = a || b  # c is set to "foo" it's the same as saying c = (a || b)
c = a or b  # c is set to nil   it's the same as saying (c = a) || b which is not what you want.