||=

ruby-talk
1

what does ||= do?

···

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Posted via http://www.ruby-forum.com/.

2

irb is your friend :slight_smile:

Todd

···

On Wed, Jun 11, 2008 at 3:59 PM, Justin To <tekmc@hotmail.com> wrote:

what does ||= do?

3

what does ||= do?

a ||= b is shorthand for a = a || b

is the "or" operator - a || b evaluates to a unless a is nil or

false, in which case it evaluates to b

a ||= b is most commonly used to say "if a hasn't already been set
(and is therefore nil), set it to b, else leave it alone"

martin

···

On Wed, Jun 11, 2008 at 1:59 PM, Justin To <tekmc@hotmail.com> wrote:

4

The expression:

    x ||= y

is shorthand for:

    x = x || y

where "||" is the OR-operator. If x evaluates as false or nil, the
value of y will be assigned to x; otherwise, x keeps its original
value. So, for example,

    x = nil
    x ||= 42 # now x has value 42
    x ||= 23 # x stays at 42

Hope this helps,

Lyle

···

On Wed, Jun 11, 2008 at 3:59 PM, Justin To <tekmc@hotmail.com> wrote:

what does ||= do?

5

Justin To wrote:

what does ||= do?

In other languages you would code this something like:

if a == nil
  a = b
end

by
TheR

···

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Posted via http://www.ruby-forum.com/\.

6

Just to short-circuit the discussion:

a ||= b

is actually implemented as if:

a || a=b

Search for recent threads or find David A. Black's blog.

-Rob

Rob Biedenharn http://agileconsultingllc.com
Rob@AgileConsultingLLC.com

···

On Jun 11, 2008, at 5:12 PM, Martin DeMello wrote:

On Wed, Jun 11, 2008 at 1:59 PM, Justin To <tekmc@hotmail.com> wrote:

what does ||= do?

a ||= b is shorthand for a = a || b

>> is the "or" operator - a || b evaluates to a unless a is nil or
false, in which case it evaluates to b

a ||= b is most commonly used to say "if a hasn't already been set
(and is therefore nil), set it to b, else leave it alone"

martin

7

Actually, it is like:

   x || x=y

Refer to:
http://dablog.rubypal.com/2008/3/25/a-short-circuit-edge-case

-Rob

···

On Jun 11, 2008, at 5:12 PM, Lyle Johnson wrote:

On Wed, Jun 11, 2008 at 3:59 PM, Justin To <tekmc@hotmail.com> wrote:

what does ||= do?

The expression:

   x ||= y

is shorthand for:

   x = x || y

where "||" is the OR-operator. If x evaluates as false or nil, the
value of y will be assigned to x; otherwise, x keeps its original
value. So, for example,

   x = nil
   x ||= 42 # now x has value 42
   x ||= 23 # x stays at 42

Hope this helps,

Lyle

8

Not exactly.

a = 2
a ||= b #where b is not defined
p a #we get 2
p b #we get NameError

Not only that, but there are other subtleties that many gurus on this
list have pointed out.

Todd

···

On Wed, Jun 11, 2008 at 4:12 PM, Lyle Johnson <lyle@lylejohnson.name> wrote:

On Wed, Jun 11, 2008 at 3:59 PM, Justin To <tekmc@hotmail.com> wrote:

what does ||= do?

The expression:

   x ||= y

is shorthand for:

   x = x || y

where "||" is the OR-operator. If x evaluates as false or nil, the
value of y will be assigned to x; otherwise, x keeps its original
value. So, for example,

   x = nil
   x ||= 42 # now x has value 42
   x ||= 23 # x stays at 42

9

careful..

this is really...

if not a
   a = b
end

(it will assign b if a resolves to false as a boolean (i.e. either nil or false)

···

On Jun 12, 2008, at 1:57 AM, Damjan Rems wrote:

Justin To wrote:

what does ||= do?

In other languages you would code this something like:

if a == nil
a = b
end

by
TheR
--
Posted via http://www.ruby-forum.com/\.

10

(need to finish replies before clicking "send")

this really is equivalent to a = a || b, just like a += 1 is equivalent to a = a + 1 (same for -=, etc.)

that's why you get b if a was false

···

On Jun 12, 2008, at 1:57 AM, Damjan Rems wrote:

Justin To wrote:

what does ||= do?

In other languages you would code this something like:

if a == nil
a = b
end

by
TheR
--
Posted via http://www.ruby-forum.com/\.

11

this really is equivalent to a = a || b, just like a += 1 is equivalent to a = a + 1 (same for -=, etc.

However, be aware that this particular operator has an optimization making it behave like

a || a = b

instead. Example problem:

>> h = Hash.new() { "default" } # Makes a Hash with "default" as the default value instead of nil
=> {}
>> h[:a]
=> "default"
>> h.has_key? :a
=> false

>> h[:a] ||= "not default"
=> "default"
>> h[:a]
=> "default"
>> h.has_key? :a
=> false

>> h[:a] = h[:a] || "not default"
=> "default"
>> h[:a]
=> "default"
>> h.has_key? :a
=> true

···

On Jun 13, 2008, at 0:08, Mike Cargal wrote:
>>

--
# Mikael Høilund
def method_missing(m, a=0) a +
m.to_s[/[a-z]+/].size * 2; end
p What is the meaning of life?