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What are all the false values in Perl ((conditional, boolean, whitespace, unprintable characters)

by princepawn (Parson)
on Sep 22, 2000 at 19:16 UTC ( #33638=perlquestion: print w/ replies, xml ) Need Help??
princepawn has asked for the wisdom of the Perl Monks concerning the following question:

The following simple subroutine:
sub log_msg { my $log_msg = shift; print "value $script::debug"; if ($script::debug) { print sprintf "(%s) $log_msg \n", shell_date; } }
still prints log messages when the printed value of $script::debug is 0.

So this yields two questions, and I would like an answer to both:

  • What are all the false values in Perl? I know of the following:
    1. "0"
    2. ""
    3. 0
    4. 0.0
  • How do one brute-force print every character in a string even if it is typically unprintable? That is, my guess is that things are being printed even when $script::debug appears to be zero when in fact there may be some hidden control char or something in the string.
  • Comment on What are all the false values in Perl ((conditional, boolean, whitespace, unprintable characters)
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    Re: What are all the false values in Perl ((conditional
    by mrmick (Curate) on Sep 22, 2000 at 19:29 UTC
      I may be mistaken, but I think that once something is evaluated in scalar context, I believe it's context is set to 'TRUE'. You print statement has set it to true whether or not the value is true.

      Mick
    Re: What are all the false values in Perl ((conditional
    by swiftone (Curate) on Sep 22, 2000 at 19:36 UTC
      You forgot undef, which is also false.

      To check to see if there are characters in $script::debug that aren't printing you can always check the length of $string::debug.

      Update: Just thought of this: are you putting a \n in the variable? Because "0\n" is not "0".

    Re: What are all the false values in Perl ((conditional
    by Fastolfe (Vicar) on Sep 22, 2000 at 19:41 UTC
      I believe an empty string, the undefined value, an empty list/array and the number zero (numeric or "0") will return a false value. A non-empty string, an array with 1 or more elements, or a non-zero number will return a true value.

      If you're seeing a zero, be sure it's not a string with other characters besides a zero in it. I believe spaces or other characters before or after it may cause the variable to be evaluated as a numeric zero when you're doing math, but since there's more than a number in the string, a boolean test will look at it from a string point of view, and will return 'true' since it's a non-empty string.

      If $script::debug is being set to a numeric zero (or, better, undef), there's no reason the test you're describing should return a true value. I'd like to see how you're setting $script::debug.

      (Updated 'list' vs. 'array'.)

        Your list statement is incorrect:
        print "hello\n" unless (1, 0);
        Otherwise you are right. The false scalars are:
        undef, '', '0', 0.0;
        Truth is evaluated in void (essentially scalar) context so something that looks like a list (array, hash, etc) is coerced into a scalar before the boolean test.

        EDIT
        My bad for saying void when I meant Boolean.

        But note that when I said "looks like a list" I used that phrase very, very carefully. Many list-like things exist in Perl, but there is no list per se. For an earlier post of mine on this exact topic try Arrays are not lists.

        I would give an array slice example, but tye gave a better one than the one I was going to do... :-)

          That's not a list, that's the binary comma operator. Any non-empty list is true. It may look like a list, but it's not one.

          UPDATE:

          Hmm, "Any non-empty list is true" was a dumb thing to say, as tye and tilly pointed out.

          It reminds me of an MJD quote:

          "If there is a giant purple water buffalo returned from a function, then $h = func() will always give you the length of its nose."

          In other words, any non-empty list is true because you never have a list in boolean context.

          However, I do stand by my first and third sentences in the original post. I think it's misleading to say if (1,0) or even if @array[0,2,1] is a list.

          It is, obviously, a complicated topic.

        For an in-depth look at the topic of "list context", I point you to my PerlMonth.com article, "List" Is a Four-Letter Word.

        $_="goto+F.print+chop;\n=yhpaj";F1:eval
    Re: What are all the false values in Perl ((conditional
    by kilinrax (Deacon) on Sep 22, 2000 at 19:44 UTC
      If $script::debug == "\0", then it will appear empty, but evaluate as true.
      print "\Q$script::debug" would backslash all special characters, which would at least indicate their existence, though not their nature. TMTOWTDI, though there's probably a better way ;-)

      Update:

      my @unpack = unpack('C*', $script::debug); print "Unpack info: @unpack\n";
      Should give you the ascii codes of each character in $script::debug, which will hopefully be useful.

    Re: What are all the false values in Perl ((conditional
    by japhy (Canon) on Sep 22, 2000 at 19:46 UTC
      I agree with swift -- print length($string) if you're not sure... and as for printing "unprintables":
      sub literal { require 'dumpvar.pl'; &dumpValue; }
      The dumpvar.pl library is, in fact, what the debugger uses when you tell it x $string.

      $_="goto+F.print+chop;\n=yhpaj";F1:eval
        One can always make use of Data::Dumper if you would prefer modules to libraries.
          Yes, by setting $Data::Dumper::Useqq to 1.

          $_="goto+F.print+chop;\n=yhpaj";F1:eval
    RE: What are all the false values in Perl ((conditional, boolean, whitespace, unprintable characters)
    by aardvark (Pilgrim) on Sep 22, 2000 at 20:33 UTC
      There was a great article in The Perl Journal about this: What is Truth? - The Perl Journal, Summer 1999 http://www.itknowledge.com/tpj/issues/vol4_2/tpj04020002.html Good Luck

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