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Re: Hmmm: while(), file handle and $_ gotcha

by moritz (Cardinal)
on Mar 06, 2009 at 12:41 UTC ( #748838=note: print w/replies, xml ) Need Help??

in reply to Hmmm: while(), file handle and $_ gotcha

Bare <DATA> outside a while doesn't assign to $_, which is why the output doesn't change in the inner loop. It's the while that does the rewriting trick to while (defined($_=<DATA>)) { ...}, as described in perlsyn.

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Re^2: Hmmm: while(), file handle and $_ gotcha
by roboticus (Chancellor) on Mar 06, 2009 at 12:59 UTC

    OK, that explains it. (I had an incorrect mental model: I was thinking that <FH> always put the data somewhere, using $_ unless otherwise specified. I don't know how I came to think that, as in hindsight that would be clearly stupid. Oh, well--live and learn!)

    So the correct fix to my program would is to change the inner loop from:

    for my $cnt (0 .. 3) { my $t=<DATA>; print $t; }


    for my $cnt (0 .. 3) { $_=<DATA>; print; }

    so I can avoid unnecessary variables and clutter.

    Thank you!

      for my $cnt (0 .. 3) { print scalar(<DATA>); }

        Cool! And then perhaps:

        while (<DATA>) { if ($. % 10 == 1) { print; print scalar(<DATA>) for(0..3); } }

      Hold up. How does one variable - two, if you did what I do and use a variable for the outer read as well - equate to clutter? I'd probably remove the unused $cnt if I was going to knock clutter out.

      use Modern::Perl; while (my $line = <DATA>) { if ( $. % 10 == 1 ) { print $line; } for (0 .. 3) { my $important_line = <DATA>; print $important_line; } }

      I realize this is a matter of personal aesthetics, of course. I was just a little confused how declaring and using a variable (and avoiding weird behavior from <DATA>) was better than declaring an unused variable and getting the weird behavior.

        Using a variable is even required if you want to add error checking.

        while (my $line = <DATA>) { if ( $. % 10 == 1 ) { print $line; } for (0 .. 3) { defined( my $important_line = <DATA> ) or die("Premature EOF\n"); print $important_line; } }

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