Ruby Programming/Syntax/Operators

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Contents

OperatorsEdit

Complete List and PrecedenceEdit

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 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 does A = A && B Binary 15 Right
&= A &=B does A = A & B Binary 15 Right
||= A ||=B assigns B to A iff A is nil or false Binary 15 Right
= =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.

AssignmentEdit

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:[1]

 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()

ScopeEdit

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

Local ScopeEdit

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 scopeEdit

 $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 scopeEdit

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 scopeEdit

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 accesed 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 scopeEdit

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 gotchasEdit

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 AndEdit

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 OrEdit

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.

  1. http://www.rubyinside.com/what-rubys-double-pipe-or-equals-really-does-5488.html