When art thy variables defined?

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

I've been in deep meditation over this problem for many minutes and now numbly submit the problem to you:

When this block of code is called with a REQUEST_URI of "/" it does not execute listall();

my ($username, $imagename) = $ENV{REQUEST_URI} =~ /\/(.*)\/(.*)/;
if(not defined $username || $username eq ""){
    listall();
    }
[download]

However when this block is executed with the same REQUEST_URI, listall(); is executed.

my ($username, $imagename) = $ENV{REQUEST_URI} =~ /\/(.*)\/(.*)/;
if(not defined $username){
    listall();
    }
[download]

Why is this? The only thing I can think of is that the order of precedence dictates that the "not" be after the ||.

-caedes

Comment on When art thy variables defined? Select or Download Code

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Re: When art thy variables defined? by bronto (Priest) on Jun 22, 2002 at 10:00 UTC
The problem is actually that `\|\|` is quite strong (sorry, can't find a better translation from italian), and perl is actually seeing `not defined ($username \|\| $username eq "")`. To make it work you could just change `\|\|` with a lower-precedence `or`. This snippet of code: `$username = undef ; if (not defined $username \|\| $username eq "") { print "Yep!\n" } if (not defined $username or $username eq "") { print "Yup!\n" }` [download] prints `Yup!`. Ciao! `--bronto`	[reply] [d/l] [select]
Re: When art thy variables defined? by jepri (Parson) on Jun 22, 2002 at 09:26 UTC
Update: I thought it was precedence, but now I think it's a function argument vs precednce problem. Solution should still work. Try: `if((not defined $username) \|\| ($username eq ""))` [download] Just guessing, but I think perl is seeing: `if(not defined($username \|\| ($username eq "")))` [download] It is usually a good idea to explicitly put brackets around everything you want to logically group. Old programmers are religious about this because each language has a different set of operator precedences. After you program in a few, they all mush together and you have no chance of remembering which ones work in the language you are currently programming in. ____________________ Jeremy I didn't believe in evil until I dated it.	[reply] [d/l] [select]
Re: When art thy variables defined? by abstracts (Hermit) on Jun 22, 2002 at 10:32 UTC
Are you sure this is what you want? `my ($username, $imagename) = $ENV{REQUEST_URI} =~ /\/(.)\/(.)/;` [download] The reason I'm asking is because you need to consider the following input possibilities: "" "/" "//" "///" "/a/b" "/a/b/" "/a/b/c" "/a/b/c/" "a//" "a/b/" ... You see, your regex would match any string with 2 or more slashes no matter where they occur. Also, you have a greedy `` which will match the longest string. Example: `"/a/b/c/d/e" => $user = "a/b/c/d", $img = "e"` which may not be what you want. You may need to put more constraints on the check. I think the regex you're looking for is in the neighbourhood of: `my ($user, $img) = $ENV{REQUEST_URI} =~ m#^/([^/]+)/([^/]+)$#;` [download] It may look more complex than your original regex, but should do the job. Of course, look at perlre for all the details. Hope this helps,,, Update:* If you're using `$user` as a directory name, beware of people passing it `"/../something"` as that would open a door for people to access other information.	[reply] [d/l] [select]
Re: When art thy variables defined? by Aristotle (Chancellor) on Jun 22, 2002 at 15:57 UTC
When in doubt, `Deparse`. In this case we want extra `-p`arentheses. `$ perl -MO=Deparse,-p -e'if(not defined $username \|\| $username eq ""){ +}' if ((not (defined($username) \|\| ($username eq '')))) { (); } -e syntax OK` [download] Apparently noone's gut feeling was entirely correct - mine neither. Don't try guessing. Makeshifts last the longest.	[reply] [d/l]
Re: When art thy variables defined? by particle (Vicar) on Jun 22, 2002 at 10:44 UTC
jepri and bronto have covered this subject well... but nobody's pointed to the documentation yet! see perlop for more on operators and precedence rules. ~Particle accelerates	[reply] [d/l]