Why does Perl string become undefined? - string

Question

What is the proper way to concatenate several scalar values into one Perl string?
The following code is deliberately a series of statements for debugging reasons.
my $bill_record;
$bill_record = $acct_no . " |";
$bill_record = $bill_record . defined($w_ptWtrMtrRecRef->{"mtr_addr_no"}) ? $w_ptWtrMtrRecRef->{"mtr_addr_no"} : " " . " |" ;
$bill_record = $bill_record . defined($w_ptWtrMtrRecRef->{"mtr_addr_str"}) ? $w_ptWtrMtrRecRef->{"mtr_addr_str"} : " " . " |" ;
$bill_record = $bill_record . defined($w_ptWtrMtrRecRef->{"mtr_addr_apt"}) ? $w_ptWtrMtrRecRef->{"mtr_addr_apt"} : " " . " |" ;
$bill_record = $bill_record . $issue_date . " |";
The | character is serving as a delimiter. Each line will be '\n terminated.
After the last line $bill_record = $bill_record . $issue_date . " |";
This error appears:
Use of uninitialized value $bill_record in concatenation (.) or string at /home/ics/include/WsBillFunc.pm line 1022.
at /home/ics/include/WsBillFunc.pm line 1022
$issue_date is defined when assigned.
What could be causing $bill_record to become undefined, and what is the proper way to concatenate a bunch of scalar values into one string?

Answer 1

I don't know specifically why $bill_record is undefined. But if I understand what you're trying to do, you're running into a precedence problem: the ?: operator has lower precedence than concatenation ., so that
$bill_record = $bill_record . defined($w_ptWtrMtrRecRef->{"mtr_addr_no"}) ? $w_ptWtrMtrRecRef->{"mtr_addr_no"} : " " . " |" ;
is treated as
$bill_record = ($bill_record . defined($w_ptWtrMtrRecRef->{"mtr_addr_no"})) ? $w_ptWtrMtrRecRef->{"mtr_addr_no"} : (" " . " |") ;
which I suspect is not what you want. Try adding parentheses:
$bill_record = $bill_record . (defined($w_ptWtrMtrRecRef->{"mtr_addr_no"}) ? $w_ptWtrMtrRecRef->{"mtr_addr_no"} : " ") . " |" ;
(Or use .= as another commenter suggested.)

Answer 2

I'd probably do that in one statement with a join:
$bill_record = join ' |',
map( {
defined( $_ ) ? $_ : ' '
} #{ $w_ptWtrMtrRecRef }{ qw( mtr_addr_no mtr_addr_str mtr_addr_apt ) }
),
$issue_date,
'';
In the map I limit with parens because I only want to apply it to the hash slice. After that is the $issue_date and the empty string. That empty string gets the final | you have.
But, for your problem, it looks like you have a precedence problem. One way to see this is to ask Perl to compile then deparse your program to see what it thinks you wanted. The B::Deparse module does this and I use the -p argument to add extra parentheses.
Here's a cut down version of your original program with the added call to the deparser at the top (it's the B::Deparse module but the namespace is O:
#!/usr/bin/perl
use O qw(Deparse -p);
my $b;
$b = $acct_no . " |";
$b = $b . defined($w->{"no"}) ? $w->{"no"} : " " . " |" ;
$b = $b . defined($w->{"str"}) ? $w->{"str"} : " " . " |" ;
$b = $b . defined($w->{"apt"}) ? $w->{"apt"} : " " . " |" ;
$b = $b . $issue_date . " |";
It outputs:
my($b);
($b = ($acct_no . ' |'));
($b = (($b . defined($$w{'no'})) ? $$w{'no'} : ' |'));
($b = (($b . defined($$w{'str'})) ? $$w{'str'} : ' |'));
($b = (($b . defined($$w{'apt'})) ? $$w{'apt'} : ' |'));
($b = (($b . $issue_date) . ' |'));
The key part is the (($b . defined($$w{'no'})). The current value of $b is concatenated with the return value of defined, then the conditional operator (? :) is done. If the test value is true, it returns the first value in the conditional.
When it gets to mgr_apt_no, there are probably many records that don't have that value set. However, the combined value of the previous $b and $$w{'apt'} is defined because $b is not empty. Thus, it chooses the value of $$w{'apt'} to assign to $b. When it does the last line, $b is empty for the concatenation with $issue_date.