Re: Logical expression evaluation

Wrong. Perl false is !1. (PL_sv_no in the C source) It's a dualvar with a string value of "" and a numerical value of 0.

See: Re^4: m//g behaves strange... (!1). And this snippet:

$ perl -lwe 'for (qw/undef 0 "" !1 1==2/) { print "\n\$a = eval $_;"; 
+$a = eval $_; print "$_:\t|$a|", 1+$a; }'

$a = eval undef;
Use of uninitialized value in concatenation (.) or string at -e line 1
+.
Use of uninitialized value in addition (+) at -e line 1.
undef:  ||1

$a = eval 0;
0:      |0|1

$a = eval "";
Argument "" isn't numeric in addition (+) at -e line 1.
"":     ||1

$a = eval !1;
!1:     ||1

$a = eval 1==2;
1==2:   ||1
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Re^2: Logical expression evaluation (dualvar) by bart (Canon) on Jul 27, 2005 at 21:07 UTC
And nowadays, you can have the magic of dualvars for your own ~~evil plots~~ applications, using dualvar() from Scalar::Util. BTW another typical magical dualvar value is $!, which returns an error message as a string, and an error number as a number. For example: `local $\ = "\n"; $! = 3; # error number print $!; # error message print 0+$!; # error number` [download]	[reply] [d/l]