From: Xavier Noria <fxn@...>
Date: 2006-11-13T05:22:03+09:00
Subject: Re: Help convert a Perl user to the Ruby Way.

On Nov 12, 2006, at 8:58 PM, Tom Pollard wrote:

> On Nov 12, 2006, at 2:40 PM, Paul Lutus wrote:
>> Let's say we have an instance variable named "hash" that we are  
>> not sure has
>> been initialized at runtime, and we want to create a new hash  
>> where there
>> is none, but only then. Add this line:
>>
>> hash ||= {}
>>
>> Means "if 'hash' is a hash, do nothing, otherwise create a hash".
>
> Strictly speaking, doesn't it mean "if 'hash' is nil (or false),  
> assign to a newly created empty hash"?  You're not testing whether  
> it's actually a hash already.  Also, maybe it's worth reminding the  
> OP that an empty hash is not treated as 'false' in a logical  
> expression in Ruby, as it would be in Perl.  So, if you were to  
> repeatedly evaluate
>
>     hash ||= {}
>
> you'd only create a new Hash the first time.  In Perl, on the other  
> hand, repeatedly evaluating the similar expression
>
>     $hashref ||= {}
>
> would create and assign a new hash repeatedly, until some hash  
> element was assigned.

Well, in that case there's only one assignment indeed, because a  
hashref is true in Perl as well:

   $ perl -wle 'do {$h ||= {}; print $h} for 1..3'
   HASH(0x1800e8c)
   HASH(0x1800e8c)
   HASH(0x1800e8c)

-- fxn