From: jzakiya@...
Date: 2005-04-13T13:04:37+09:00
Subject: Re: Array#last_index

Martin DeMello wrote:
> Robert Klemme <bob.news@gmx.net> wrote:
> >
> > "Trans" <transfire@gmail.com> schrieb im Newsbeitrag
> > news:1113074095.070106.22910@l41g2000cwc.googlegroups.com...
> > > > Can you not just use -1 as the index?
> > >
> > > No. Because, it is not always simply a matter of getting to the
last
> > > element. If it were then #last itself would suffice. The use case
here
> > > is when the last index is needed.
> >
> > Care to unveil the use case?  I never neede that myself - just
> > wondering...
>
> I've written C code that started with a pointer at either end of an
> array and walked them towards each other.
>
> martin

Here's the source code from Array.c for Array#last:

static VALUE
rb_ary_last(argc, argv, ary)
    int argc;
    VALUE *argv;
    VALUE ary;
{
    if (argc == 0) {
	if (RARRAY(ary)->len == 0) return Qnil;
	return RARRAY(ary)->ptr[RARRAY(ary)->len-1];
    }
    else {
	VALUE nv, result;
	long n, i;

	rb_scan_args(argc, argv, "01", &nv);
	n = NUM2LONG(nv);
	if (n > RARRAY(ary)->len) n = RARRAY(ary)->len;
	result = rb_ary_new2(n);
	for (i=RARRAY(ary)->len-n; n--; i++) {
	    rb_ary_push(result, RARRAY(ary)->ptr[i]);
	}
	return result;
    }
}

Here's my modification of it to produce Array#last_index.

static VALUE
rb_ary_last_index(argc, argv, ary)
    int argc;
    VALUE *argv;
    VALUE ary;
{
	VALUE nv, result;
	long n;

	rb_scan_args(argc, argv, "01", &nv);
	n = NUM2LONG(nv);
	if (n > RARRAY(ary)->len) n = RARRAY(ary)->len;
	return n;
}

Could someone verify/modify/test this to see if it works.

Jabari