Re: even.pl solutions

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From:

Jeff 'japhy' Pinyan

Date:

January 29, 2002 10:39

Subject:

Re: even.pl solutions

Message ID:

Pine.GSO.4.21.0201291328320.27903-100000@crusoe.crusoe.net

On Jan 29, Stephen Turner said:

>On Tue, 29 Jan 2002, Jeff 'japhy' Pinyan wrote:
>> 
>> Well, because $| is the magical flip-flop variable, so long as you -- it
>> and not ++ it.
>
>This I didn't know. Where is this documented? man perlvar (nope)? Camel book
>page 237 or 670 (NAFAICS)?

Well, it's "documented" in mg.c, around line 786.  Here's an excerpt from 
Perl_magic_get():

    case '|':
        sv_setiv(sv, (IV)(IoFLAGS(GvIOp(PL_defoutgv)) & IOf_FLUSH) != 0 );
        break;

We see that it's setting it to (...) != 0, which will be either 1 or 0.

And here's an excerpt from Perl_magic_set() (around line 1987):

    case '|':
        {   
            IO *io = GvIOp(PL_defoutgv);
            if(!io)
              break;
            if ((SvIOK(sv) ? SvIVX(sv) : sv_2iv(sv)) == 0)
                IoFLAGS(io) &= ~IOf_FLUSH;
            else {
                if (!(IoFLAGS(io) & IOf_FLUSH)) {
                    PerlIO *ofp = IoOFP(io);
                    if (ofp)
                        (void)PerlIO_flush(ofp);
                    IoFLAGS(io) |= IOf_FLUSH;
                }
            }
        }
        break;

So if the value we are setting $| to is ZERO, flushing is turned off; for
all other values, flushing is turned on.  THEN, when we look at the code
for Perl_magic_get(), and we notice that it merely returns 1 or 0 based on
whether flushing is off or on.  So it retains absolutely nothing about the
value you gave $|.

So how does $|-- work?  $| starts at 0.  $|-- is like $| = $| - 1, which
is like $| = 0 - 1, which internally turns autoflush ON.  When we do $|--
again, it's like $| = $| - 1 again, except now $| returns 1 instead of 0,
so the value is $| = 1 - 1, or 0.

Hooray for flip-flops.

-- 
Jeff "japhy" Pinyan      japhy@pobox.com      http://www.pobox.com/~japhy/
RPI Acacia brother #734   http://www.perlmonks.org/   http://www.cpan.org/
** Look for "Regular Expressions in Perl" published by Manning, in 2002 **
<stu> what does y/// stand for?  <tenderpuss> why, yansliterate of course.

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