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

Simple question.
What will dumper do if I ask it for @_ within a sub? e.g.
print Dumper(@_); or perhaps
print Dumper(\@_);

A different subroutine is calling dbAction()
That subroutine is passing an array into dbAction:
@params = ($sql,$_[0],$_[1]);
dbAction(@params);

sub dbAction(\@){
print Dumper(@_);
print Dumper(\@_);
}

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The $sql may have two arguments on an update, but 6 on an insert, therefore I want to make sure that the references get placed accordingly to do the $dbh->prepare() and execute(). Using print statements 
print "$_[0]\n";
print "$_[1]\n";

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I can see that the data is there, but I want to use a for loop to make sure how many items are being passed in to the sub, and I get -1 when I ask for $#{@_}.
Dumper is breaking and I can't tell why.

Replies are listed 'Best First'.
Re: Dumper Won't print @_ ?
by Anonymous Monk on Jul 30, 2012 at 06:30 UTC

    What will dumper do if I ask it for @_ within a sub? e.g.

    T.I.T.S - Try It To See 

    $ perl -l
use Data::Dumper;
my $sql = " junk ";
Junk( $sql, 2,4 );
sub Junk {
    my @params = ($sql,@_);
    dbAction(@params);
}

sub dbAction(\@){
    print Dumper(@_);
    print Dumper(\@_);
}
^Z
$VAR1 = ' junk ';
$VAR2 = ' junk ';
$VAR3 = 2;
$VAR4 = 4;

$VAR1 = [
          ' junk ',
          ' junk ',
          2,
          4
        ];

    [download]

    Dumper is breaking and I can't tell why.

    That's because Dumper isn't breaking -- you've shown no proof

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Re: Dumper Won't print @_ ?
by kingram (Acolyte) on Jul 30, 2012 at 07:04 UTC
    Jeezus. In theory I'm a decent programmer, but this is ridiculous.
    print Dumper(@_);

    [download]
    What I had:
    print Dumper(@_):

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    I think I need glasses....
    Once again, thanks for the extra eyes, on yet another stupid mistake.
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Re: Dumper Won't print @_ ?
by Marshall (Canon) on Jul 30, 2012 at 07:16 UTC
    Dumper takes a reference to "something", could be a scalar, an array or even an arbitrary complex data structure. 
    #!usr/bin/perl -w
use strict;
use Data::Dumper;

my $sql = "some string";
dbAction($sql, 55, 66);

sub dbAction  # DO NOT use a prototype!
{
   my (@inputs) = my ($sql, $parm1, $parm2) = @_;

   print Dumper (\@inputs);
   print Dumper (\@_);      #this works also
                            #but I would prefer to give
                            #the @_ inputs a name
                            #choice is yours
   print Dumper (\$sql, \$parm1, \$parm2);
   print Dumper (\$sql, \@inputs[1,2]);
}

__END__
prints:
$VAR1 = [
          'some string',
          55,
          66
        ];
$VAR1 = [
          'some string',
          55,
          66
        ];
$VAR1 = \'some string';
$VAR2 = \55;
$VAR3 = \66;
$VAR1 = \'some string';
$VAR2 = \55;
$VAR3 = \66;

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Re: Dumper Won't print @_ ?
by Athanasius (Archbishop) on Jul 30, 2012 at 06:35 UTC

    sub dbAction is prototyped to take a reference to an array; but you call it with an array (not a reference). Update: See the reply by AnomalousMonk below; also Prototypes.

    Without the prototype:

    #! perl
use strict;
use warnings;
use Data::Dumper;

my @params = qw(homer marge bart lisa maggie);
dbAction(@params);

sub dbAction
{
    print Dumper( @_), "\n";
    print Dumper(\@_);
}

    [download]

    Outputs:

    $VAR1 = 'homer';
$VAR2 = 'marge';
$VAR3 = 'bart';
$VAR4 = 'lisa';
$VAR5 = 'maggie';

$VAR1 = [
          'homer',
          'marge',
          'bart',
          'lisa',
          'maggie'
        ];

    [download]

    At least one of these is what you wanted?

    HTH,

    Athanasius <°(((>< contra mundum

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      sub dbAction is prototyped to take a reference to an array; but you call it with an array (not a reference).

      Actually,  dbAction() is prototyped to take a single explicit array (i.e., something with a  @ sigil) as an argument and pass a reference to the array to the function. It will refuse to take an array reference, which is a scalar – won't even compile. 

      >perl -wMstrict -le
"use Data::Dump;
 ;;
 sub S (\@) {
   my ($array_ref) = @_;
   dd $array_ref;
   }
 ;;
 my @ra = (qw(foo bar baz), [ 1, 2, 3 ]);
 S(@ra);
"
["foo", "bar", "baz", [1, 2, 3]]

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