Logical expression evaluation

nghosh has asked for the wisdom of the Perl Monks concerning the following question:

Hi,
I tried the following piece of code which is not working.

$v=1;
$f=!($v);
print "**$f**\n";
[download]

The Output is ****. However, if i do

$v=0;
$f=!($v);
print "**$f**\n";
[download]

Output is **1**.

It looks like !(1) does not return zero whereas !(0) indeed returns 1. What can be the reason behind this and what should i do to get 0 from 1 by logical not.

thanks
nghosh

Edited by Ovid. 2005-07-27.

Comment on Logical expression evaluation Select or Download Code

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Re: Logical expression evaluation by ikegami (Patriarch) on Jul 27, 2005 at 15:47 UTC
You can format the result of `!` easily using the conditional operator: `$f = !$v ? 1 : 0;` [download] or just `$f = $v ? 0 : 1;` [download]	[reply] [d/l] [select]
Re^2: Logical expression evaluation by radiantmatrix (Parson) on Jul 27, 2005 at 17:00 UTC
Or, you can instruct Perl in more detail about how you'd like a boolean value to be displayed: `$v = 0; $f = !($v); printf '%1d',$f;` [download] Read the documentation on printf and sprintf for more. <-radiant.matrix-> Larry Wall is Yoda: there is no `try{}` (ok, except in Perl6; way to ruin a joke, Larry! ;P) The Code that can be seen is not the true Code "In any sufficiently large group of people, most are idiots" - Kaa's Law	[reply] [d/l]
Re: Logical expression evaluation by davorg (Chancellor) on Jul 27, 2005 at 15:46 UTC
Please use <code> tags to make your code readable. `!1` returns an empty string (which evaluates as false in a Boolean expression). -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply]
Re: Logical expression evaluation by Enlil (Parson) on Jul 27, 2005 at 15:51 UTC
one way is: `my $v=1; my $f=int( !($v) ); print "$f\n";` [download] -enlil	[reply] [d/l]
Re: Logical expression evaluation by polettix (Vicar) on Jul 27, 2005 at 17:33 UTC
Take into account that also the string `'0'` is considered as a false value in Perl, even if it's a non empty string. Regarding this, you'll probably find The best "true zero" is... poll useful for some popular workarounds. Flavio perl -ple'$_=reverse' <<<ti.xittelop@oivalf Don't fool yourself.	[reply] [d/l]
Re: Logical expression evaluation by jolande (Initiate) on Jul 28, 2005 at 02:43 UTC
You could always convert !$v into an integer before you print it. The easy way to do this is: `v=1; $f=0+!($v); print "$f\n";` [download]	[reply] [d/l]
Re: Logical expression evaluation by halley (Prior) on Jul 27, 2005 at 16:17 UTC
In Perl, the logical values are typically `1` for truth, and `undef` for falsehood. When treated as a number, `undef == 0`, when treated as a string, `undef eq ''`, all of which are treated as a false in boolean condition checks. -- `[ e d @ h a l l e y . c c ]`	[reply] [d/l] [select]
Re^2: Logical expression evaluation by ikegami (Patriarch) on Jul 27, 2005 at 16:26 UTC
As mentioned earlier in this thread, Perl uses "" for false. It does not use `undef`. `$i = undef; print(defined($i) ? "[$i]" : 'undef', "\n"); $i = !1; print(defined($i) ? "[$i]" : 'undef', "\n"); __END__ output ------ undef []` [download]	[reply] [d/l] [select]
Re: Logical expression evaluation by benizi (Hermit) on Jul 27, 2005 at 20:55 UTC
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: Read more... (833 Bytes)	[reply] [d/l]
Re^2: Logical expression evaluation (dualvar) by bart (Canon) on Jul 27, 2005 at 21:07 UTC