if not defined

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

Replies are listed 'Best First'.
Re: if not defined by agianni (Hermit) on Mar 23, 2007 at 18:11 UTC
not defined is absolutely valid Perl. Just be careful about precedence . Note that `!` and `not` both mean not, but have varying precedence. So if you're going to use the `not` form, you should probably also use the `and` form of the AND operator, as `&&` has a higher precedence than `not`, but `and` has a lower precedence, which is likely causing you trouble in your example. When in doubt, use parenthesis. If you find this confusing, consider reading this Perl Journal article on the topic by Domimus, which covers the topic nicely. `perl -e 'split//,q{john hurl, pest caretaker}and(map{print @_[$_]}(joi +n(q{},map{sprintf(qq{%010u},$_)}(2*23074993,51016415261,75979 +36997,13177145131,3*26789167*181))=~/\d{2}/g));'` [download]	[reply] [d/l] [select]
Re: if not defined by Eimi Metamorphoumai (Deacon) on Mar 23, 2007 at 18:15 UTC
`not defined` is fine, but `not` is very low precedence. That is, `if (not defined($pname) && $pname ne ''){` [download] is parsed the same as `if (not (defined($pname) && $pname ne '')){` [download] instead of `if ((not defined($pname)) && $pname ne ''){` [download] Also, if `$pname` isn't defined, then it can't possibly equal `''`, so you probably need to change your conditions a bit (probably you mean something like `if (!defined($pname) \|\| $pname eq ''){` [download] but it's possible you want something else.)	[reply] [d/l] [select]
Re: if not defined by Old_Gray_Bear (Bishop) on Mar 23, 2007 at 18:01 UTC
`if (!defined $pname) { do something with $pname } else { do something in the way of a polite error message }` [download] ---- I Go Back to Sleep, Now. OGB	[reply] [d/l]
Re^2: if not defined by bart (Canon) on Mar 23, 2007 at 18:44 UTC
No need to switch to `!`, `not` is legal Perl.	[reply] [d/l] [select]
Re^3: if not defined by ikegami (Patriarch) on Mar 23, 2007 at 19:41 UTC
But there is a precedence problem `if (not defined($pname) && $pname ne '')` means `if (not (defined($pname) && ($pname ne '')))` while `if (!defined($pname) && $pname ne '')` means `if ((!defined($pname)) && ($pname ne ''))` The OP wants `if (!defined($pname) && $pname ne '')` or `if ((not defined($pname)) && $pname ne '')`	[reply] [d/l] [select]
Re^4: if not defined by bart (Canon) on Mar 23, 2007 at 19:47 UTC
Re^4: if not defined by pKai (Priest) on Mar 23, 2007 at 20:51 UTC
Re: if not defined by TGI (Parson) on Mar 23, 2007 at 18:17 UTC
Your problem is in operator precedence. After adding parenthesis to match precedence, you have: `not ( ( ( ( ( defined($pname) && $pname ne '' ) && defined($policy_ur) ) && $policy_ur ne '' ) && not defined($odate) ) && $odate ne '' )` [download] I think you want to use the higher precedence logical not operator (`!). That way your condition will work out to this:` `( ( ( ( (!defined($pname) && $pname ne '') && defined($policy_ur) ) && $policy_ur ne '' ) && not defined($odate) ) && $odate ne '' ) [download] TGI says moo`	[reply] [d/l] [select]
Re^2: if not defined by pKai (Priest) on Mar 23, 2007 at 19:11 UTC
I'd say the former (resembeling the OP) makes sense: It's testing for all vars at first if it is defined and then if it also isn't the empty string. This partial results is inverted for $odate and then chained on the same level. The result of that is inverted by the leading "not". If you ignore the (possible) warnings, when a string function is applied to `undef` `!(length($pname) && length($policy_ur) && !length($odate))` [download] will yield the same. That may be what he wants or not. Your `(!defined($pname) && $pname ne '')` on the other hand is only true f�r $pname being "undef", so contains a redundant test.	[reply] [d/l] [select]
Re: if not defined by pKai (Priest) on Mar 23, 2007 at 18:10 UTC
For one, it's valid Perl syntax: `D:\Temp>perl -c -e "if (not defined($pname) && $pname ne '' && defined +($policy_ur) && $policy_ur ne '' && not defined($odate) && $odate ne +''){}" -e syntax OK` [download] But it's probably not doing what you expect: `D:\Temp>perl -MO=Deparse,p -e "if (not defined($pname) && pname ne '' +&& defined($policy_ur) && $policy_ur ne '' && not defined($odate) && +$odate ne ''){}" if (not defined $pname && $pname ne '' && defined $policy_ur && $polic +y_ur ne '' && !(defined $odate && $odate ne '')) { (); } -e syntax OK` [download] `not` has a much lower precedence than `&&` ~~use either `not` and `and` or `!` and `&&` in conjunction to get the intended effect.~~ Update: Seems like newly accuired syntax highlighting was distracting me ;-). Though a little bit complicated the assembly of the leading `not` (inverting the result of the whole test) with the `&&` (chaining all the inner sub-tests together) makes sense.	[reply] [d/l] [select]
Re: if not defined by andye (Curate) on Mar 23, 2007 at 18:17 UTC
Sure. You have a choice of: `! defined` which is high precedence, or `not defined` which is low precedence. If you don't want to have to think about precedence, just use loads of brackets. You can read more about this in: perlop. By the way, you mention CGI: it doesn't make any difference whether the Perl script is being used for a website (CGI), or not. The language remains the same. Hope that helps, andye	[reply] [d/l] [select]
Re: if not defined by ikegami (Patriarch) on Mar 23, 2007 at 19:46 UTC
If there's no difference between undef and the nul string (`""`), you could canonize the values. `for ($pname, $policy_ur, $odate) { if (!defined($_)) { $_ = ''; } } if ($pname ne '' && $policy_ur ne '' && $odate ne '') { ... }` [download] Or the other way around: `for ($pname, $policy_ur, $odate) { if (defined($_) && $_ eq '') { undef $_; } } if (!defined($pname) && !defined($policy_ur) && !defined($odate)) { ... }` [download]	[reply] [d/l] [select]
Re: if not defined by Trihedralguy (Pilgrim) on Mar 23, 2007 at 18:18 UTC
Thats very odd that you have to do either "not..and" or "!..&&". `if (not defined($pname) and $pname eq '' && defined($policy_ur) and $p +olicy_ur eq '' and not defined($odate) && $odate eq'') {` [download] That works famously! - Thanks everyone and pKai :)	[reply] [d/l]
Re^2: if not defined by TGI (Parson) on Mar 23, 2007 at 18:36 UTC
Is this really what you mean? `( ( ( not defined($pname) ) and ( $pname eq '' && defined($policy_ur) ) ) and $policy_ur eq '' ) and ( not ( defined($odate) && $odate eq'' ) )` [download] Please take the time to read about operator precedence. That perl has logical operators with two levels of precendence is very useful, but it is a trap for the unwary or unaware. If you don't pay attention to precedence you'll think the answer to 2+35 is 25. Even worse, if you have code to implement X+YZ that uses bad precedence, and you test your code with X, Y and Z all being 1, you'll believe the code works. Then six months later, Bob in accounting is wanting to know where $5 million went... TGI says moo	[reply] [d/l]
Re^2: if not defined by pKai (Priest) on Mar 23, 2007 at 18:38 UTC
hm, `... and not defined($odate) && $odate eq '')` [download] ~~is testing if it's "not defined" (i. e. `undef`) and then if it's also equals the empty string.~~ ~~Thinking about it, both can never be true at the same time.~~ Update: Oh well, twice in a row on the wrong track. :-( TGI has the precedence right and the problem lies in the first part, not in the second I cited.	[reply] [d/l] [select]
Re: if not defined by Trihedralguy (Pilgrim) on Mar 23, 2007 at 18:08 UTC
That doesn't appear to work: `if (!defined $pname && !defined $policy_ur && !defined $odate)` [download] The values ARE defined.	[reply] [d/l]
Re^2: if not defined by TGI (Parson) on Mar 23, 2007 at 18:24 UTC
The above condition is not equivalent to what you posted earlier. `NOT( DEFINED(A) AND DEFINED(B) AND DEFINED(C) ... )` [download] is equivalent to: `( NOT DEFINED(A) ) OR ( NOT DEFINED(B) ) OR ( NOT DEFINED(C) ) ...` [download] That's De Morgan's theorem. TGI says moo	[reply] [d/l] [select]


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