From: Eleanor McHugh <eleanor@...>
Date: 2009-05-06T09:45:01+09:00
Subject: Re: '=||'

On 6 May 2009, at 00:09, 7stud -- wrote:
> Eleanor McHugh wrote:
>> Are you referring to ||= ? If so it's one of the augmented assignment
>> operators so you won't find it documented separately as it's  
>> syntactic
>> sugar for:
>>
>> x = x || some_other_value
>
> Nope.
>
> h = Hash.new(10)
>
> h["red"] = h["red"] || 20
>
> --output:--
> {"red"=>10}
>
>
> h = Hash.new(10)
> h["blue"] ||= 20
> p h
>
> --output:--
> {}
>
>
> The statement:
>
> x ||= val
>
> is actually equivalent to:
>
> x = val unless x


It seems you've misunderstood what happens under the hood when using  
augmented assignment with tables as '||=' then becomes syntactic sugar  
for 'x[] = x[] || some_other_value' and the assignment is performed  
via '[]=' rather than '='. '[]=' will not create a key if it believes  
it already exists and this is the cause of the behaviour you're seeing.

h = Hash.new(10)
p h["blue"]			=> 10
h["blue"] ||= 20
p h				=> {}

In this case when '||=' invokes the assignment it finds that h["blue"]  
already contains a value because of the default so the hash method  
'[]=' doesn't attempt to create a new key because it appears that the  
key already exists.

Contrast this to:

h = Hash.new(10)
p h["blue"]			=> 10
h["blue"] = nil
p h				=> { "blue" => nil }
h["blue"] ||= 10
p h				=> { "blue" => 10 }
h["blue"] ||= 20
p h				=> { "blue" => 10 }

Here the key has been explicitly set equal to nil and '||=' acts the  
way we'd expect an augmented assignment to work with scalar types.

Finally if no default is set for the table:

h = {}
h["red"] ||= 10
p h				=> {"red" => 10}

The key is always created as expected.


Ellie

Eleanor McHugh
Games With Brains
http://slides.games-with-brains.net
----
raise ArgumentError unless @reality.responds_to? :reason