From: Dave Aronson <rubytalk2dave@...>
Date: 2011-09-16T07:33:30+09:00
Subject: Re: Some newbie questions

On Thu, Sep 15, 2011 at 15:46, Vladimir Van Bauenhoffer
<cluny_gisslaren@hotmail.com> wrote:

> 1. What does @ mean in Ruby? Im sure I have seen variables like this:
> @variable. A new datatype like the :symbol perhaps?

Someone beat me to this one, so I'll just preemptively add, @@ means
it's a class variable.

> 2. Hash.new {0}
>   anotherhash.values.each {|x| thecreatedhash[x] += 1}
>
> What makes the hash created above different from a hash created the
> normal way? I can execute the block on a hash created with that method
> but not a normal empty hash created manually.

You've already clarified that "normal" means {}.  But what I don't get
is how you're using it.  Do you mean that:

  thecreatedhash = Hash.new {0}
  anotherhash.values.each {|x| thecreatedhash[x] += 1}

yields different results from:

  thecreatedhash = {}
  anotherhash.values.each {|x| thecreatedhash[x] += 1}

?  That boils down to the difference between "Hash.new {0}" and "{}".
The latter will return an empty hash such that uninitialized slots
have a nil value, as usual.  The former is a good bit more
complicated.  There is documentation at:

  http://www.ruby-doc.org/core/classes/Hash.html#M000718

At first glance the docs would lead me to believe that the
uninitialized slots should return the value {0}, using the
Hash.new(object) syntax, but the braces in fact make it take that as a
code block, even though it's not using the args, so it's using the
third syntax (Hash.new {|hash, key| block }).  This means it returns
0, the result of that code block, for all uninitialized slots.
Occasional confusion between hashes and code blocks is one of the
subtleties of Ruby.

So, in the former case, anotherhash would cause corresponding slots of
thecreatedhash to be set to 1, because they start at 0 and get
incremented.  In the latter, they'd start as nil, and you can't add 1
to that, so you'd get a NoMethodError.  Is ths the behavior you're
seeing?

-Dave

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