Location of Conditional test effects results

chakram88 has asked for the wisdom of the Perl Monks concerning the following question:

Oh revered monks, I come seeking knowledge and wisdom.

Quick description of my 'issue:

I can take the same block of code, and move one line (a conditional operator) from one point to another, and I end up with a different result.

My Data structure is an AoH, where a Hash element 'could' be an array. The conditonal is testing for the existance of that secondary array. I loop over the AoH.

With the conditional operator assignment at the top of my loop, I get the expected results based on the test in the conditional. If I move the conditional assignment to the end of my loop (no data changes within the loop), I get responses as if the conditional were always true.

Code whittled down (line marked with <------ is the offending little bugger)

#!/usr/bin/perl -l

use warnings;
use strict;

my $records = [
               {
                 'services' => [],
                 'zone_record_id' => '1',
               },
               {
                 'services' => [
                                 {
                                   'status' => 'OK',
                                   'id' => '0',
                                   'class' => 'Primary'
                                 }
                               ],
                 'zone_record_id' => '2',
               },
               {
                 'services' => [
                                 {
                                   'status' => 'OK',
                                   'id' => '1',
                                   'class' => 'Editable'
                                 }
                               ],
                 'zone_record_id' => '4',
               },
               {
                 'services' => [
                                 {
                                   'status' => 'OK',
                                   'id' => '1',
                                   'class' => 'Secondary'
                                 }
                               ],
                 'zone_record_id' => '5',
               },
 ];


my $lbr = "\n=============\n";
foreach my $record (@$records) {
    my $editor;
    print $lbr, "Record: $record->{'zone_record_id'}";

    # Conditional array defined check first
    $editor = (@{$record->{'services'}})? 'service_edit' : 'plain_edit
+'; # <--- 

    $editor = "no editor" if $record->{'services'}[0]{'class'} eq 'Pri
+mary';
    $editor = "no editor" if $record->{'services'}[0]{'class'} eq 'Sec
+ondary';

    # place here for 'Try to define all 'no editor' situations first

    print $editor;

}
[download]

Results:

Conditionaly check for defined array first:

=============
Record: 1
plain_edit

=============
Record: 2
no editor

=============
Record: 4
service_edit

=============
Record: 5
no editor


Try to define all 'no editor' situations first:


=============
Record: 1
service_edit

=============
Record: 2
service_edit

=============
Record: 4
service_edit

=============
Record: 5
service_edit
[download]

Yes, I know that the eq test is putting up 'undefined' warnings on those $records where the secondary array is not defined. I deal with it in my full code, this is just "narrowed down to the least amount of code to reproduce the problem".

P.S. I had a difficult time coming up with an appropriate title, so any suggestions welcome

Comment on Location of Conditional test effects results Select or Download Code

Replies are listed 'Best First'.
Re: Location of Conditional test effects results by ferreira (Chaplain) on Apr 10, 2007 at 19:43 UTC
That's called autovivification IIRC. See: `$ pirl Welcome to the Perl shell. Type ':help' for more information pirl @> %a = () () pirl @> print "ok" if $a{k} undef pirl @> \%a # no surprise {} pirl @> print "ok" if $a{k}[0] undef pirl @> \%a # bang! { k => [] }` [download] When accessing elements of a structure (hash or array ref) in a nested level, you may find out that Perl "helpfully" sprang some structures to life. If you're not ready for this, it may give a lot of headache. Autovivification issues happen a lot here. See for example Hash Autovivification Weirdness, Re: Array mysteriously growing (autovivification explained), Autovivification of scalars in sub calls, Autovivification for dummies (using Test::More), The Bad, the Ugly, and the Good of autovivification or look for more following the super search http://www.perlmonks.org/?node_id=3989;HIT=vivification.	[reply] [d/l]
Re^2: Location of Conditional test effects results by chakram88 (Pilgrim) on Apr 10, 2007 at 20:24 UTC
Well, sure, it's clear now. Thanks for the explanation, and lots of info to absorb.	[reply]
Re: Location of Conditional test effects results by shmem (Chancellor) on Apr 10, 2007 at 19:58 UTC
You are falling prey to auto-vivification. After the lines `$editor = "no editor" if $record->{'services'}[0]{'class'} eq 'Pri +mary'; $editor = "no editor" if $record->{'services'}[0]{'class'} eq 'Sec +ondary';` [download] the array element `@{$record->{'services'}}` contains one element - an anonymous hash holding at least the key `class` whose value might be `undef`. You are looking at the value of `$record->{'services'}[0]{'class'}`, but to do that the key `class` in the hash `%{$record->{'services'}[0]}` has to exist, so perl just allocates the hash and the key for you. After that, the anonymous array has at least one element. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply] [d/l] [select]
Re^2: Location of Conditional test effects results by chakram88 (Pilgrim) on Apr 10, 2007 at 20:23 UTC
Ding!Ding!Ding! We have a winner! And thanks to ferreira's great explanation above, I now have lots of reading to do too.	[reply]
Re^3: Location of Conditional test effects results by ikegami (Patriarch) on Apr 10, 2007 at 21:43 UTC
Yes, I know that the eq test is putting up 'undefined' warnings on those $records where the secondary array is not defined. I deal with it in my full code, Had you fixed them, you would have fixed your problem too. Do not lightly ignore warnings in Perl.	[reply]
Re^4: Location of Conditional test effects results by chakram88 (Pilgrim) on Apr 11, 2007 at 01:16 UTC
Re: Location of Conditional test effects results by graff (Chancellor) on Apr 11, 2007 at 05:06 UTC
Nobody (yet) has mentioned how your code could have been written to fix the mistake, so I'll do it ;) `for my $record (@$records) { my $editor; print $lbr, "Record: $record->{'zone_record_id'}"; if ( @{$record->{services}} == 0 ) { $editor = 'plain edit'; } elsif ( $record->{services}[0]{class} =~ /Primary\|Secondary/ ) { $editor = 'no editor'; } else { $editor = 'service edit'; } print $editor; }` [download] The point being: once you establish that you've reached an empty slot at a given point in your structure, you may want to avoid going any deeper down that particular path, unless you really do want to create new structure where none existed previously.	[reply] [d/l]


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