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comp.lang.ruby

What does this mean? ||=

Justin Ko

1/12/2007 7:52:00 PM

I'm looking at this method and don't understand what ||= means:

def current_account
@account ||= Account.find(session[:account_id])
end

I have the ruby for rails book and can't find anything on it.

--
Posted via http://www.ruby-....
7 Answers

Alexandru Popescu

1/12/2007 8:00:00 PM

0

var ||= value is

var = var || value

/alex
--
w( the_mindstorm )p.


On 1/12/07, Justin Ko <jko170@yahoo.com> wrote:
> I'm looking at this method and don't understand what ||= means:
>
> def current_account
> @account ||= Account.find(session[:account_id])
> end
>
> I have the ruby for rails book and can't find anything on it.
>
> --
> Posted via http://www.ruby-....
>
>

Keith Fahlgren

1/12/2007 8:37:00 PM

0

On 1/12/07, Pedro Fortuny Ayuso <pfortuny@gmail.com> wrote:
> It does the following in this order:
>
> if @account HAS A VALUE (is not nil) then DO NOTHING
> otherwise, compute Account.find(...) and set its return value into
> @account.

Just to be explicit, it's both 'nil' or 'false' that will be reset to
the new value:

irb(main):001:0> f = false
=> false
irb(main):003:0> f ||= 1
=> 1
irb(main):004:0> f
=> 1 # was set
irb(main):002:0> n = nil
=> nil
irb(main):005:0> n ||= 1
=> 1
irb(main):006:0> n
=> 1 # was set
irb(main):007:0> t = true
=> true
irb(main):008:0> t ||= 1
=> true
irb(main):009:0> t
=> true # wasn't set



HTH,
Keith

Robert Worley

1/12/2007 9:33:00 PM

0

If @account is nil it'll look in the database to find an account using
the account id from the session and return it; otherwise it'll just
return the current value of @account.

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Chris Gernon

1/12/2007 9:36:00 PM

0

Justin Ko wrote:
> I'm looking at this method and don't understand what ||= means:

Much like 'x += 1' is a shortcut for 'x = x + 1', 'x ||= 1' is a
shortcut for 'x = x || 1'.

The significance of this is that the || method doesn't simply return a
boolean value; it checks the first argument, returns it if it evaluates
to 'true' (i.e. is something other than false or nil), and otherwise
returns the second argument. So it's commonly used as a shortcut for an
if/then statement, checking if the first argument is nil (or false). So
the following are all equivalent:

if @account
@account
else
Account.find(session[:account_id])
end

is the same as:

@account = @account || Account.find(session[:account_id])

is the same as:

@account ||= Account.find(session[:account_id])

Hope this helps!

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

dblack

1/12/2007 11:55:00 PM

0

Justin Ko

1/13/2007 4:12:00 AM

0

Great explanation! Thanks.

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Bertram Scharpf

1/13/2007 4:38:00 AM

0

Hi Justin,

Am Samstag, 13. Jan 2007, 04:52:29 +0900 schrieb Justin Ko:
> I'm looking at this method and don't understand what ||= means:
>
> def current_account
> @account ||= Account.find(session[:account_id])
> end

A scheme I practice a lot:

def meth parm = nil
parm ||= "default"
...
end

...

a = some_calculation
obj.meth a # a may be nil and still hits the default value

Bertram


--
Bertram Scharpf
Stuttgart, Deutschland/Germany
http://www.bertram-...