Warning for checking with undef

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

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Re: Warning for checking with undef by splinky (Hermit) on Jul 07, 2005 at 13:10 UTC
You don't say exactly HOW you check the return value against undef, but I'm guessing you're doing something like "`if ($val eq 'undef')`" or "`if ($val == undef)`", both of which are wrong. The correct way is "`if (defined $val)`". This won't trigger any warnings.	[reply] [d/l] [select]
Re: Warning for checking with undef by mrborisguy (Hermit) on Jul 07, 2005 at 13:09 UTC
From the statement "check the return value ... against 'undef'," do you mean you are doing something like: `my ($row1,...) = $sth->selectrow_array; if( $row1 == undef ) { $row1 = 0; } # or $row1 = 0 if $row1 == undef;` [download] If you are, you may want to try something like: `if( not defined $row1 ) { $row1 = 0; } # or $row1 = 0 if not defined $row1;` [download] -Bryan	[reply] [d/l] [select]
Re: Warning for checking with undef by Hena (Friar) on Jul 07, 2005 at 13:11 UTC
Check out function defined(). Example. `if (!defined($foo)) { }` [download]	[reply] [d/l]
Re: Warning for checking with undef by anonymized user 468275 (Curate) on Jul 07, 2005 at 13:44 UTC
Whereas testing defined($var) is arguably better style; I feel I should point out that the use of undef shown in the responses above is wrong for the wrong reason with respect to the OP. undef needs to be parsed as a function call in this context, so when used in the context of a relational expression, it needs round brackets after it to prevent it from being treated as a bareword. i.e. `if ( $foo == undef() )` [download] would work just as well as `unless( defined($foo) )` [download] whereas `if ( $foo == undef )` [download] fails to satisfy for the same reason as `if ( $foo == DoesntMatterIfIamADefinedFunctionOrNot )` [download] The same programming error is often made with shift(); One world, one people	[reply] [d/l] [select]
Re^2: Warning for checking with undef by Fletch (Bishop) on Jul 07, 2005 at 14:20 UTC
Erm, `undef` without parens returns a perfectly valid value in that context. The problem is that it's being coerced into a numeric value because of the `==` which triggers the warning. What you're thinking of is using `undef` or `shift` as a hash key (`$h{undef}`) or to the left of a `=>` (`%a = ( undef => 1 )`) where there is some bareword magic going on (strict or no). In those cases it is necessary to use parens or a + to disable the magic. Update: And you can verify it by looking at the different errors produced. Bareword-ness doesn't enter into it. `freebie:~ 746> cat baz use warnings; if( 5 == undef ) { 1 } if( 5 == "undef" ) { 1 } freebie:~ 747> perl baz Argument "undef" isn't numeric in numeric eq (==) at baz line 3. Use of uninitialized value in numeric eq (==) at baz line 2.` [download] The first one gripes about comparing against `undef`, the second because you're comparing a non-numeric value numerically. -- We're looking for people in ATL	[reply] [d/l] [select]
Re^3: Warning for checking with undef by anonymized user 468275 (Curate) on Jul 07, 2005 at 15:00 UTC
I am testing on 5.6.1 and will have to look into it again, because my results look different from others. One world, one people	[reply]
Re^4: Warning for checking with undef by Fletch (Bishop) on Jul 07, 2005 at 15:05 UTC
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