What does "||=" do in Ruby 1.9.2?

Question

params[:user][:role_ids] ||= []

What does it do?

ruby -v = 1.9.2p290

Duplicate: [What does `||=` mean in Ruby?](http://StackOverflow.Com/q/995593/), [What does `||=` mean in Ruby?](http://StackOverflow.Com/q/3800957/), [what is `||=` in ruby?](http://StackOverflow.Com/q/3945711/), [Double Pipe Symbols in Ruby Variable Assignment?](http://StackOverflow.Com/q/4500375/), [What does the “`||=`” operand stand for in ruby](http://StackOverflow.Com/q/5124930/), [what does a `||=` mean in Ruby language?](http://StackOverflow.Com/q/5230162/), [Is the ruby operator `||=` intelligent?](http://StackOverflow.Com/q/2989862/), … — Jörg W Mittag, Oct 11 '11 at 01:46
… [What does `||=` mean?](http://StackOverflow.Com/q/7556902/), and probably many others as well. See also [The definitive list of `||=` (OR Equal) threads and pages](http://Ruby-Forum.Com/topic/151660/). *Please*, *please*, *please* don't ask duplicate questions. It just scatters information around the site that would much better be found in just one place. Thank you! — Jörg W Mittag, Oct 11 '11 at 01:47
@JörgWMittag: Why tell a 393 rep user not to ask duplicate questions, rather than 4 people who have more than 3K rep (and therefore have close right privileges) not to answer duplicate questions? — Andrew Grimm, Oct 11 '11 at 02:06

jondavidjohn · Accepted Answer · 2011-10-10T15:19:37.347

5

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"

edited Oct 10 '11 at 15:19

answered Oct 10 '11 at 15:07

jondavidjohn

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score 2 · Answer 2 · answered Oct 10 '11 at 15:07

2

if params[:user][:role_ids]is nil, it gets initialized with [] otherwise params[:user][:role_ids] holds its value further

answered Oct 10 '11 at 15:07

Tudor Constantin

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score 1 · Answer 3 · answered Oct 10 '11 at 15:07

1

If the left-hand value is not yet assigned, assign it to the right-hand value. If it is assigned, keep it as itself. A good explanation can be found on Michael Hartl's RoR tutorial site.

answered Oct 10 '11 at 15:07

eykanal

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score 1 · Answer 4 · answered Oct 10 '11 at 15:08

1

It's the memoize operator and it does one of two things:

If the value on the left of it is not nil, it simply returns the value
If the value on the left of it is nil (or undefined) it sets it.

answered Oct 10 '11 at 15:08

d11wtq

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score 1 · Answer 5 · answered Oct 10 '11 at 15:08

1

It's a conditional assignment in Ruby. You can read more about it here: Ruby Operators

answered Oct 10 '11 at 15:08

Jakob W

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score 1 · Answer 6 · answered Oct 10 '11 at 15:34

It sets a value to the variable if the variable isn't already set. Meaning

class Something
  attr_accessor :some_value

def perform_action
  @some_value ||= "Mom"
  puts @some_value
end

foo = Something.new
foo.perform_action -> "Mom"
foo.some_value = "Dad"
foo.perform_action -> "Dad"

What does "||=" do in Ruby 1.9.2?

6 Answers6

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