Thanks for all your responses; I learned a bit.
1. I wasn't clear on $_ in my email; that's being read elsewhere in program so
it's already set by the time print_lines is called.
2. Will you bet your life on this equivalence: "not /^\s*$/ == /\S/"? I
believe it's safer to negate an EXPR than to find some other EXPR2 that is
equivalent to 'not EXPR'.
3. Hadn't thought about xor; good to know.
4. As for the generalized case, I learned about using refs. Anonymous subs
also work.
my $re = sub { return /^\s*$/; };
my $nre = sub { return not &$re; };
my $expr = $blank ? $re : $nre;
do ... while (&$expr and not eof);
But I'm not sure which is faster though.
Thanks for all your help.
--- Jeff 'japhy' Pinyan <[EMAIL PROTECTED]> wrote:
> On Jan 27, Rob Dixon said:
>
> >Sam wrote:
> >>
> >> sub print_lines_ok {
> >> my ($output, $blank_lines) = @_;
> >> if ($blank_lines) {
> >> do {
> >> print if $output;
> >> $_ = <>;
> >> } while (/^\s*$/ and not eof);
> >> } else {
> >> do {
> >> print if $output;
> >> $_ = <>;
> >> } while (not /^\s*$/ and not eof);
> >> }
> >> }
> >>
> >>
> >> sub print_lines_ideal {
> >> my ($output, $blank_lines) = @_;
> >> my $expr = $blank_lines ? '/^\s*$/' : 'not /^\s*$/';
> >>
> >> do {
> >> print if $output;
> >> $_ = <>;
> >> } while (eval $expr and not eof); # works, but not fast.
> >> # Can I move eval out of loop?
> >> }
>
> You could just say
>
> do ... while (($blank ? /^\s*$/ : not /^\s*$/) and not eof);
>
> which could be written with 'xor' as Rob shows below:
>
> do ... while ($blank xor not /^\s*$/) and not eof;
>
> and 'not /^\s*$/' can be written as /\S/, which results in Rob's code.
>
> >I don't think you want what you've written, as the first call to 'print'
> >is before anything has been read from the file. This should come close:
> >
> > sub print_lines {
> > my ($output, $blank_lines) = @_;
> > while (<>) {
> > last if $blank_lines xor /\S/;
> > print if $output;
> > }
> > }
>
> That might work in this specific case, but if you want to be able to
> change the course of events like that without constant re-evaluation, I'd
> suggest using code references:
>
> my $code = \&first;
>
> sub first {
> # code for the first case
> if (some condition) { $code = \&second }
> }
>
> sub second {
> # code for the second case
> }
>
> $code->();
>
> --
> Jeff "japhy" Pinyan [EMAIL PROTECTED] http://www.pobox.com/~japhy/
> RPI Acacia brother #734 http://www.perlmonks.org/ http://www.cpan.org/
> <stu> what does y/// stand for? <tenderpuss> why, yansliterate of course.
> [ I'm looking for programming work. If you like my work, let me know. ]
>
>
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