From: Stefano Crocco <stefano.crocco@...>
Date: 2013-02-17T05:19:16+09:00
Subject: Re: How the string concatenation operator hackes(modifies) the string contents of frozen array?

On Sunday 17 February 2013 Love U Ruby wrote
> Hi,
> 
> Find the below code:
> >> b = %w[foo bar gis]
> 
> => ["foo", "bar", "gis"]
> 
> >> b.frozen?
> 
> => false
> 
> >> b.object_id
> 
> => 7298560
> 
> >> b.freeze
> 
> => ["foo", "bar", "gis"]
> 
> >> b.object_id
> 
> => 7298560
> 
> >> b[0] <<  "lish"
> 
> => "foolish"
> 
> >> b.object_id
> 
> => 7298560
> 
> #How the "<<" operator made the change to the string where as being a
> concatenation operator "+" on string couldn't do that,rather prevented
> by the array?
> 
> >> b[0]=b[0] + "lish"
> 
> RuntimeError: can't modify frozen Array
>   from (irb):16:in `[]='
>   from (irb):16
>   from /usr/bin/irb:12:in `<main>'
> 
> --
> Posted via http://www.ruby-forum.com/.

Writing 
b[0] = b[0] + "lish"
you are changing the array: you take the string which was in position 0 in the 
array, throw it away and put another string in its place. Since the array was 
frozen, ruby doesn't allow you to do these things.

When you write
b[0] <<  "lish"
you're taking the string in position 0 in the array and modifying it, not the 
array. To see this, you have to look at the object_id of the string itself not 
of the array. Let's see what happens for a non frozen array:

b = %w[foo bar gis]
=> ["foo", "bar", "gis"]

b[0].object_id
=> 16686000

b[0] << "lish"
=> "foolish"

b[0].object_id
=> 16686000

The array still contains the original object, but the string contents have 
changed. Now, let's see what happens with the other method:

b[0] = b[0] + "lish"
=> "foolish"

b[0].object_id
=> 15515100

As you can see, the object_id has changed, which means that you stored a new 
object in the array, which you can't do if it's frozen.

I hope this helps

Stefano