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

Hello monks, I want to fill some arrays with the content of some lookup arrays, but I don't know how to get the right array. Some code: 
use DBI;
my @tables = qw(name car pet);
my @name, @car, @pet;

sub fill_array() {
   $sql = "select * from $i;";
   $dbh = DBI->connect($dsn, $dbuser, $dbupass)
             || die "Connection failed: $!\n";
   $sth = $dbh->prepare( "$lsql;" )
             || die "Couldn't prepare: $!\n";
   $sth->execute();
   while ( my @row = $sth->fetchrow_array ) {
      #Here's my problem :(
      <@(right_array)> = (<@(right_array)>, row[1]);
   }
   $rc = $dbh->disconnect || die "Disconnect Failed: $!\n";
}

[download]
Thanks!

Replies are listed 'Best First'.
Re: Fill Arbitrary Array
by tadman (Prior) on May 02, 2002 at 08:24 UTC
    Maybe you mean something like this, where you are adding a value to the @right_array listing:
    push (@right_array, $row[1]);
    Or, something like this where you are adding the row to the @right_array:
    push (@right_array, \@row);
    Probably what you mean by @right_array is that you want to specify which array to put the stuff into, correct? 
    sub fill_array(\@)
{
       my ($right_array) = @_;

       # ...

       while (my @row = $sth->fetchrow_array())
       {
             push (@$right_array, \@row);
       }

       # ...
}

# As a test
fill_array (@name);

    [download]
    This is achived by using references. Instead of knowing exactly which array you are going to be using, you have a handle to it ($right_array versus @right_array) that you can use like an array with the proper prefix (@$right_array).

    If you choose to put the entire row in there, then what you have is a multi-dimensional array. To get out data, you can either get a whole row (array) or a single entry: 
    my @row = @{$name[1]};           # Row 1
my $row_item_1 = $name[1][1];    # Row 1 column 1

    [download]
    Maybe this is what you had in mind?

    Don't forget that array numbering starts at 0, so the first element is actually $row[0]. Call it the "zeroth" row, if you will.

    Something to consider apart from this code is to use some of the specialized DBI calls, such as selectall_arrayref and selectcol_arrayref. For the latter, in particular, there is no point in fetching a whole row of data when all you want is a single column.
[reply]
[d/l]
[select]
      I don't know which arrays are filled with this code :( 
      sub fill_array(\@)
{
       my ($right_array) = @_;

       # ...

       while (my @row = $sth->fetchrow_array())
       {
             push (@$right_array, \@row);
       }

       print "Values: @$right_array\n";        #Works!
}


fill_array(@name);

foreach my $entry (@name) {
   print "$entry\n";        #Remains empty...
}

      [download]
[reply]
[d/l]
        It should fill whatever array you pass to it, which in this case is @name. The reason this can be done is because of the way the sub is declared, in particular, the (\@) definition means that the first parameter is an array reference. This way, instead of passing a copy of the @name array, a reference to it is passed, allowing you to manipulate it.

        With that in mind, I'm surprised that you get "nothing". @name should be filled with array references, these sub-arrays that contain the various rows. Using your code, since you are not de-referencing them, you should see something like this: 
        ARRAY(0x80fe5a8)
ARRAY(0x80fe5d2)
ARRAY(0x80fe6f4)

        [download]
        The reason is you are printing an array reference as a string, which isn't going to work. A solution is to either use the venerable Data::Dumper or just deference it: 
        # Data::Dumper saves the day again
use Data::Dumper;
print Dumper (\@name);

# De-reference it and treat it as an array
foreach my $row (@name)
{
     print join (',', @$row),"\n";
}

        [download]
        I'm not sure how familiar you are with references, but here's a 5 second intro: 
        my @array      = qw[ 1 2 3 ];
my $array_ref  = \@array;      # Backslash makes a reference
my @array_copy = @$array_ref;  # @ de-references array reference

$array[2]       = 4;   # Modifies @array directly
print $array_ref->[2]; # Should be '4' now
$array_ref->[1] = 5;   # Modifies @array by reference
print $array[1];       # Should be '5'
$array_copy[1]  = 6;   # Modifies @array_copy, not @array
print $array[1];       # Still '5'

        [download]
[reply]
[d/l]
[select]
      Indeed, what I meant with the right array is that I want to specify which array is going to be used. To for lookup-table "name" I want to use array @name. You're right about the selection, I just need one column.
[reply]

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