Is the Ruby Operator ||= Intelligent

Is the ruby operator ||= intelligent?

This is extremely easy to test:

class MyCache
  def initialize
    @hash = {}
  end

  def []=(key, value)
    puts "Cache key '#{key}' written"
    @hash[key] = value
  end

  def [](key)
    puts "Cache key '#{key}' read"
    @hash[key]
  end
end

Now simply try the ||= syntax:

cache = MyCache.new
cache["my key"] ||= "my value"  # cache value was nil (unset)
# Cache key 'my key' read
# Cache key 'my key' written

cache["my key"] ||= "my value"  # cache value is already set
# Cache key 'my key' read

So we can conclude that no assignment takes place if the cache key already exists.

The following extract from the Rubyspec shows that this is by design and should not be dependent on the Ruby implementation:

describe "Conditional operator assignment 'obj.meth op= expr'" do
  # ...
  it "may not assign at all, depending on the truthiness of lhs" do
    m = mock("object")
    m.should_receive(:foo).and_return(:truthy)
    m.should_not_receive(:foo=)
    m.foo ||= 42

    m.should_receive(:bar).and_return(false)
    m.should_not_receive(:bar=)
    m.bar &&= 42
  end
  # ...
end

In the same file, there is a similar spec for [] and []= that mandates identical behaviour.

Although the Rubyspec is still a work in progress, it has become clear that the major Ruby implementation projects intend to comply with it.

What does the '||=' operator do in ruby?

what does || do? If you have a and b then a || b is true if and only if either a or b is true. It is the same with ||= this operator combines two operations '=' and '||'. So a ||= b is equivelent to c || c = b

EDIT: so in your context ENV['ENVIRONMENT'] ||= 'test' means that if ENV['ENVIRONMENT'] is not nil and not false it will preserve its value, otherwise it will become 'test' and after that the new value of ENV['ENVIRONMENT'] is assigned to RACK_ENV

What does the ||= operand stand for in ruby

It is the shorthand for a logical OR operation. It is equivalent to:

a || a = b

Note: The above code sample has been corrected to reflect the true (if unintuitive) behavior if expanding a ||= b. Thanks to the people who pointed that out for me. Here is the source

if a evaluates to true it will remain as is, otherwise b will be assigned to a. In ruby nil evaluates to false, so you can see how this is useful for lazy loading and default value assignment.

What does ||= mean?

Basically, a ||= b means assign b to a if a is null or undefined or false (i.e. false-ish value in ruby), it is similar to a = b unless a, except it will always evaluate to the final value of a (whereas a = b unless a would result in nil if a was true-ish).

'||=' operator in Ruby

It's an assignment operator for 'Conditional Assignment'

See here -> http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Operators

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 of the expression:

x = x || "default" 

EDIT:

After seeing OP's edit, the example is just an extension of this, meaning:

car = method_1 || method_2 || method_3 || method_4

Will assign the first non-nil or non-false return value of method_1, method_2, method_3, method_4 (in that order) to car or it'll retain its old value.

What does ||= mean?

It is an assignment operator which means: or assign this value to a variable.

So if you did something like x ||= ythis meansx || x = y so if x is nil or false set x to be the value of y.

What does ||= do in Ruby 1.9.2?

It assigns [] to params["user][:role_ids] if params["user][:role_ids] is nil or another falsy value...

Otherwise, it retains the original value of params["user][:role_ids]

Example

variable = nil

variable ||= "string"

puts variable # "string"

variable2 = "value"

variable2 ||= "string"

puts variable2 # "value"

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