in reply to Re: blank lines up to a point
in thread blank lines up to a point

Never say never :)   0 as a second condition can be true. 
use strict;
use warnings;

my $firsttime = 1;

my $line1 = <DATA>;
print neverflop();
$firsttime = 0;

my $line2 = <DATA>;
print neverflop();

scalar tell(STDIN);
print neverflop();
print neverflop();

sub neverflop {
   ($firsttime .. 0) ? "flip" : "flop";
}
__DATA__
line1
line2
line3
__END__
Output:
flipflipflipflop

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Re^3: blank lines up to a point
by Random_Walk (Prior) on Sep 15, 2004 at 11:09 UTC

    I get an error from this unless I alter the tell line to my $discard=tell(STDIN); then indeed it gives the advertised output (I am on an old perl version here, client requirements)

    ./flipflop
Useless use of tell in void context at ./flipflop line 14.
perl -v 
This is perl, version 5.005_03 built for sun4-solaris

...after the above edit....

root@tivpre-master:/home/robinp # ./fixedflop
flipflipflipflop

    [download]

    that aside how does this work ? the docco says tell normaly returns -1 against STDIN, what dark juju is going on to make 0 true and what else could I use that I can trust never to be true ?

    Updated

    It took some digging but I've got it now. the binary range operator .. will compare a given constant value to the value of the current input line number $. (actualy int(EXPR)==int($.) I was naively using 0 in its meaning of false and you cunningly reset $. to 0 (with tell(STDIN)) so 0 got matched. I offer up a fixed sub to avoid this gotcha. 
    sub logit {
    local $.=1;
    my $line  = shift @_;
    $line =~ s/\0//g;
    return unless (/\S/..0);
    print $line
}

    [download]

    Many Thanks,
    R.

[reply]
[d/l]
[select]
Re^3: blank lines up to a point
by periapt (Hermit) on Sep 15, 2004 at 13:59 UTC
    This works as a quirk of the flip/flop operator. From the documentation "If either operand of scalar ``..'' is a constant expression, that operand is implicitly compared to the $. variable, the current line number." The statement scalar tell(STDIN) must set $. = 0 (although I don't know why) changing the line from scalar tell(STDIN); to $. = 0; gives the same result. 

    use strict;
use warnings;

my $firsttime = 1;

my $line1 = <DATA>;
print neverflop();
$firsttime = 0;

my $line2 = <DATA>;
print neverflop();

#scalar tell(STDIN);
$. = 0;
print neverflop();
print neverflop();

sub neverflop {
   my $statecount = ($firsttime .. 0);
   ($firsttime .. 0) ? "flip$statecount\t" : "flop$statecount\t";
}

__DATA__
line1
line2
line3

__END__
flip1   flip2   flip3E0 flop

    [download]

    PJ
    use strict; use warnings; use diagnostics; (if needed)
[reply]
[d/l]
      $. is stored per-filehandle, and automatically refers to the last input file. Any of readline, tell, seek, sysseek, or eof will change the last input file.

      You could just use:

      /\S/ .. my $false

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      instead.
[reply]
[d/l]
        Can any operator that inspires this much confusion be a GoodThing (TM)?

        Don't answer that. It's a rhetorical question.

[reply]

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