What Does ||= (Or-Equals) Mean in Ruby

What does ||= (or-equals) mean in Ruby?

This question has been discussed so often on the Ruby mailing-lists and Ruby blogs that there are now even threads on the Ruby mailing-list whose only purpose is to collect links to all the other threads on the Ruby mailing-list that discuss this issue.

Here's one: The definitive list of ||= (OR Equal) threads and pages

If you really want to know what is going on, take a look at Section 11.4.2.3 "Abbreviated assignments" of the Ruby Language Draft Specification.

As a first approximation,

a ||= b

is equivalent to

a || a = b

and not equivalent to

a = a || b

However, that is only a first approximation, especially if a is undefined. The semantics also differ depending on whether it is a simple variable assignment, a method assignment or an indexing assignment:

a    ||= b
a.c  ||= b
a[c] ||= b

are all treated differently.

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).

Ruby's ||= (or equals) in JavaScript?

Both are absolutely correct, but if you are looking for something that works like ||= in ruby. The first method which is variable = variable || {} is the one you are looking for :)

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.

doubts regarding ||= OR EQUALS operator in ruby

In Ruby, there are two values that are considered logical false. The first is the boolean value false, the other is nil. Anything which is non-nil and not explicitly false is true. The first time though the method, @num is nil, which is treated as false and the logical or portion of ||= needs to be evaluated and ends up assigning the empty array to @num. Since that's now non-nil, it equates to true. Since true || x is true no matter what x is, in future invocations Ruby short circuits the evaluation and doesn't do the assignment.

Ruby |= assignment operator

Bitwise OR assignment.

x |= y

is shorthand for:

x = x | y

(just like x += y is shorthand for x = x + y).

'||=' 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 ||= do in Ruby

This will assign bar to foo if (and only if) foo is nil or false.

EDIT: or false, thanks @mopoke.

Double Pipe Symbols in Ruby Variable Assignment?

It's a conditional assignment. From here:

 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

---

Related Topics