[REBOL] Re: REBOL Enhancement Proposals (REPs)
From: joel:neely:fedex at: 27-Sep-2001 13:38
Hi, Ladislav,
Ladislav Mecir wrote:
> > >
> > > BTW, is your Perl implementation correct?
> > >
> > > > ... That is, the equivalent of the Perl
> > > >
> > > > for (my $i = 1; $i < $#a; ++$i) {
> > > > $a[$i] = ($a[$i-1] + $a[$i] + $a[$i+1])/3;
> > > > }
> > > >
> >
> > Yes, AFAICT. It even agrees with uSoft XL! ;-)
> >
> > What looked suspicious to you?
> >
> > -jn-
> >
>
> I don't know Perl enough, but the difference between:
>
> ;$i < $#a; for Perl and the C ; i < sizeof(a)/sizeof(double) - 2;
>
> looked suspicious to me.
>
The bug is in my composed-at-the-keyboard version for TLWMNBN.
Both Perl and TLWMNBN use zero-origin indexing, which means that
an array of length N has indices 0 .. N-1. This, in turn means
that the "interior" elements have indices 1 .. N-2.
The Perl construct $#a provides the last index of the array @a,
so from 1 while < $#a really is correct. The other version
should have read (in part)
... < sizeof(a)/sizeof(double) - 1 ...
(or the equivalent
... <= sizeof(a)/sizeof(double) - 2 ...
meaning that my typo was either the substitution of "2" for "1" or
the omission of "=" ;-)
You can draw your own conclusions about which of Perl and TLWMNBN
I spend more time in (or pay more attention to) these days!
Thanks for catching the error!
-jn-
--
This sentence contradicts itself -- no actually it doesn't.
-- Doug Hofstadter
joel<dot>neely<at>fedex<dot>com
