From: Charles Mills <cmills@...>
Date: 2005-01-28T07:45:54+09:00
Subject: Re: Extending Ruby

Tim Sutherland wrote:
> In article <d4cf71b0050127093664c9bf7b@mail.gmail.com>, Lyle Johnson
wrote:
> >On Fri, 28 Jan 2005 01:56:24 +0900, Asbj�rn Reglund Thorsen
> ><asbjoert@ifi.uio.no> wrote:
> >
(...)
> As an alternative to rb_ary_store(), you can access the buffer
underlying
> the Array directly:
>
>     $result = rb_ary_new2(size);
>     for (int i = 0; i < size; ++i)
>     	RARRAY($result)->ptr[i] = rb_float_new((double) p(i+1)));
>
> The latter is faster since it doesn't have to check things like
>  - 'Do we need to increase the buffer size'
>  - 'Is the buffer shared with another Array'
>  - 'Is the Array frozen'
>  ...etc.

There is a gotcha to this though.
rb_ary_new2(size) returns an array with length==0 and capacity==size.
The example you give above never increases length.
You could fix this with
RARRAY($result)->len = size;
However, this is also a gotcha.  If you put it before the loop then the
array is filled with a bunch of garabage references which will crash
Ruby if the GC is run when rb_float_new() is called.  If you put it
after he loop then all those floats you create could be destroyed by
the GC, causing Ruby to crash later.
rb_ary_store() is the simplest and safest solution.  You could also do:

$result = rb_ary_new2(size);
for ( ; RARRAY($result)->len < size; RARRAY($result)->len++) {
int i = RARRAY($result)->len;
RARRAY($result)->ptr[i] = rb_float_new((double) p(i+1)));
}


-Charlie