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

[princepawn]

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.

[swiftone]

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".

[Fastolfe]

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'.)

[tilly]

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... :-)

[dchetlin]

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.

[japhy]

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

[aardvark]

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

[kilinrax]

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.

[japhy]

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

[swiftone]

One can always make use of Data::Dumper if you would prefer modules to libraries.

[japhy]

Yes, by setting $Data::Dumper::Useqq to 1.

$_="goto+F.print+chop;\n=yhpaj";F1:eval

[mrmick]

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