Need help with Excel::Writer::XLSX in using 'or'

ravi45722 has asked for the wisdom of the Perl Monks concerning the following question:

my $date = $ARGV[0];
$date or $date = `date --date='1 hour ago' +%Y-%m-%d`;
chomp $date;
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In this code if the "$date" is NULL then its taking the system date.

But in this code if the hash element is NULL I need to write '0'. For that I tried this way

foreach my $sub_user_error (@db_user_errors)
{
       $worksheet->write( $row, $col, $db_error_data{$hour}{'Submissio
+n_user_error'}{$sub_user_error} or 0, $number);
       $col++;
}
$row++;
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But its not working. Later I tried like

 
foreach my $sub_user_error (@db_user_errors)
{
     if ($db_error_data{$hour}{'Submission_user_error'}{$sub_user_erro
+r} eq "")
     {
         $worksheet->write( $row, $col, 0, $number);
         $col++;
     }
     else
     {
         $worksheet->write( $row, $col, $db_error_data{$hour}{'Submiss
+ion_user_error'}{$sub_user_error}, $number);
         $col++;
     }
}
$row++;
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Its working but seems too ugly. Is there any better way for doing this.

Thanks in Advance for helping

Comment on Need help with Excel::Writer::XLSX in using 'or' Select or Download Code

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Re: Need help with Excel::Writer::XLSX in using 'or'
by marioroy (Prior) on May 03, 2016 at 04:00 UTC

Hi Ravi,

What is the value for the constant NULL?

Neither "or 0" or "|| 0" will work if NULL is not a false value.

Edit

This question no longer applies. Ravi's code had ... eq "NULL" initially when posting my reply.

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Re: Need help with Excel::Writer::XLSX in using 'or'
by Athanasius (Archbishop) on May 03, 2016 at 12:40 UTC

Hello ravi45722,

This code snippet has a precedence problem:

foreach my $sub_user_error (@db_user_errors)
{
       $worksheet->write( $row, $col, $db_error_data{$hour}{'Submissio
+n_user_error'}{$sub_user_error} or 0, $number);
       $col++;
}
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If you look at the table in perlop#Operator-Precedence-and-Associativity, you’ll see that logical or has a lower precedence than the comma operator. So if the hash element contains a false value, the expression inside the call to write evaluates to 0, $number, which in turn evaluates to $number — which is not what you want.

The fix is to use the higher-precedence version of logical or, namely ||:

#! perl
use strict;
use warnings;
use feature qw( say );

my  $hour           = 12;
my  $error          = 'Submission_user_error';
my  $sub_user_error =  3;
my  $number         = 42;
my  %db_error_data;
    $db_error_data{$hour}{$error}{$sub_user_error} =  0;

say 'With ||:';
say $db_error_data{$hour}{$error}{$sub_user_error} || 0, $number;

say 'With or:';
say $db_error_data{$hour}{$error}{$sub_user_error} or 0, $number;
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Output:

22:40 >perl 1619_SoPW.pl
Useless use of private variable in void context at 1619_SoPW.pl line 1
+7.
With ||:
042
With or:
0

22:40 >
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Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re^2: Need help with Excel::Writer::XLSX in using 'or'

by marioroy (Prior) on May 03, 2016 at 13:30 UTC

Confirmed, precedence order :)

use strict;
use warnings;
use feature qw( say );

sub _write {
   my ( $err, $num ) = @_;
   say "$err, $num";
}

my %data;

$data{error} = "";
_write( $data{error} || 0, 42 );

$data{error} = "foo bar";
_write( $data{error} || 0, 42 );

$data{error} = "foo baz";
_write( $data{error} or 0, 42 );  # <-- fails

__END__

0, 42
foo bar, 42
Use of uninitialized value $num in concatenation (.) or string at i li
+ne 7.
foo baz,
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