"defined" function fails to evaluate expression; feature or fault?

w8lle has asked for the wisdom of the Perl Monks concerning the following question:

Hi Monks!

I am running Perl v5.10.0 (built for x86_64-linux-thread-multi) and I have an issue with the "defined" function. I am not sure this is actually a fault, but it have worked in previously releases (unfortunately I don't know which).

The issue seems to happen when I've got a defined variable without a value, e.g. $var1 = "".

If I need to check if (one of) two variables are defined, I need to do that like this:

if (defined $var1 || defined $var2)
if (defined $var1 && defined $var2)
[download]

While I've previously been able to do it like this:

if (defined ($var1 || $var2))
if (defined ($var1 && $var2))
[download]

Running the following fails for both OR and AND (test 1 and 3):

./test.pl ""
Test 1: NOK
Test 2: OK
Test 3: OK
Test 4: NOK
[download]

When test.pl contains:

#!/usr/bin/perl

use strict;
use warnings;

for my $i (1 .. 4) {
    my $result = (&successfulValidation ($i))? "OK": "NOK";
    print "Test ", $i, ": ", $result, "\n";
}

sub successfulValidation {
    my $test = shift;

    if ($test == 1) {
        return defined ($ARGV[0] || $ARGV[1]);

    } elsif ($test == 2) {
        return defined $ARGV[0] || defined $ARGV[1];

    } elsif ($test == 3) {
        return defined ($ARGV[0] && $ARGV[1]);

    } elsif ($test == 4) {
        return defined $ARGV[0] && defined $ARGV[1];
    }
}
[download]

The following works just fine, though, so maybe the first (and only) variable to check can't be empty!?

./test.pl "" ""

Is it a bug that test 1 and test 3 fail? I mean, test 1 should have returned TRUE ($ARGV[0] was defined, although with an empty value) and test 3 should have returned "FALSE" ($ARGV[1] wasn't defined)? Maybe I am using "defined" in the wrong way!?

Many thanks in advance,

/Johan

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Re: "defined" function fails to evaluate expression; feature or fault? by stevieb (Canon) on Jan 26, 2017 at 20:50 UTC
It's not a feature or bug, it just is :) defined() returns a bool, (either false/true) when it evaluates something, so when you do this: `defined ($x \|\| $y);`, it's not doing what you think it does. Here's an example that illustrates the issue: `perl -wMstrict -E 'my $x = defined $ARGV[5]; say defined $x' 1` [download] Now, `$ARGV[5]` is not set in any way, but since we're `return`ing the value of the call to `$x`, `$x` will most definitely be defined, because it now evaluates to `false` as the variable was not defined (`define()` returns false, if something wasn't defined). You need to call a define on each side of the comparison operator: `my $x = defined $x \|\| defined $y;` [download] ...as per your solutions that work. Then, check `$x` for truth: `die "blah...\n" if ! $x;` [download] ...or combined: `die "blech...\n" if ! defined $x \|\| defined $y;` [download]	[reply] [d/l] [select]
Re^2: "defined" function fails to evaluate expression; feature or fault? by AnomalousMonk (Archbishop) on Jan 26, 2017 at 21:08 UTC
Not that it makes any difference in this case, but defined returns `''` (the empty string; false) or `1` (true): `c:\@Work\Perl>perl -wMstrict -le "print 'perl version ', $]; ;; print 'undefined A: >', defined(), '<'; print 'undefined B: >', defined(undef), '<'; print 'defined: >', defined(42), '<'; " perl version 5.008009 undefined A: >< undefined B: >< defined: >1< perl version 5.014004 undefined A: >< undefined B: >< defined: >1<` [download] Give a man a fish: `<%-{-{-{-<`	[reply] [d/l] [select]
Re^3: "defined" function fails to evaluate expression; feature or fault? by haukex (Archbishop) on Jan 27, 2017 at 07:44 UTC
Hi AnomalousMonk, defined returns `''` (the empty string; false) or `1` (true) To be even more specific, Perl has a "special" value for "false" - see Truth and Falsehood. The difference between an empty string and the special "false" value: `$ perl -wMstrict -le 'my $x=""; print 3+$x;' Argument "" isn't numeric in addition (+) at -e line 1. 3 $ perl -wMstrict -le 'my $x=!!""; print 3+$x;' 3 $ perl -wMstrict -le 'my $x=defined(); print 3+$x;' 3` [download] And looking even more closely with Devel::Peek: Read more... (862 Bytes) Regards, -- Hauke D Update 2019-08-17: Updated the link to "Truth and Falsehood".	[reply] [d/l] [select]
Re^3: "defined" function fails to evaluate expression; feature or fault? by stevieb (Canon) on Jan 26, 2017 at 21:13 UTC
Yep, I caught that after doing some testing, and had updated my reply (at least I thought I fixed it all). Thanks for the heads-up, as if someone is expecting a zero and is getting the empty-string type false instead, it could cause head-scratching.	[reply]
Re: "defined" function fails to evaluate expression; feature or fault? (truth) by Anonymous Monk on Jan 26, 2017 at 20:35 UTC
While I've previously been able to do it like this: if (defined ($var1 && $var2)) Hi, That expression does not check of $var1 and $var2 are defined, it checks if the result of `$var1 && $var2` is defined The result of && is always defined even if both $var1 and $var2 are undefined && tests for truth, and the result is always defined definedness is not truth $var1 \|\| $var2 also tests for truth, but the result is the value of $var1 or the value of $var2 , the result can be undef the empty string is false, its not true, its not undef On truthiness What is the correct definition of False and Null in Perl? Validation: is defined whether the variable? True or False? A Quick Reference Guide What is truth? (Curiosity corner) When does while() test for defined vs truth	[reply] [d/l]
Re: "defined" function fails to evaluate expression; feature or fault? by w8lle (Novice) on Jan 27, 2017 at 06:07 UTC
OK, so I actually WAS using defined in the wrong way! :-( Many thanks for the prompt response...	[reply]