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

Hello, I am writing an script that performs a series of database queries. I need to test each result set determine if any rows were returned. So, for example, my code might look like: 
my @AssignmentIDs = ();
my $GetAssignmentID_sth=$query_dbh->prepare($SQL);
$GetAssignmentID_sth->execute();
my $rows = $GetAssignmentID_sth->rows;
  if($rows > 0) {
    while (my ($AssignmentID, $UserCount) = $GetAssignmentID_sth-> fet
+chrow_array() )  {
          push(@AssignmentIDs, $AssignmentID);
    }
  }
my $AssignmentIDs = join(",",@AssignmentIDs);
$GetAssignmentID_sth->finish();

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The next query I perform, I would like to reuse the following code: 
if($rows > 0) {}

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I receive the warning, however, that: 
"my" variable $rows masks earlier declaration in same scope

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Of course, I can use some other variable than $row but it would look cleaner if I did not have a proliferation of such variables. Is there are way I can undefine the variable after I use it once? I tried adding the following code but it did not correct the issue: 
undef $row;

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Thanks!

Replies are listed 'Best First'.
Re: Reusing a variable declared earlier in the same scope
by dragonchild (Archbishop) on Apr 10, 2008 at 15:28 UTC
    There are a few different solutions. The first (and most obvious) is $rows = $some_sth->rows;, but only for the second and subsequent.

    A slightly better solution would be to create a bare scope. That could look something like: 

    y @AssignmentIDs = ();
my $GetAssignmentID_sth=$query_dbh->prepare($SQL);
$GetAssignmentID_sth->execute();
{
   my $rows = $GetAssignmentID_sth->rows;
     if($rows > 0) {
       while (my ($AssignmentID, $UserCount) = $GetAssignmentID_sth-> 
+fetchrow_array() )  {
          push(@AssignmentIDs, $AssignmentID);
       }
     }
}
my $AssignmentIDs = join(",",@AssignmentIDs);
$GetAssignmentID_sth->finish();

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    That makes it very clear what the scope of each thing should be. I've used that on many occasions. However, there's a better way - create a subroutine to do stuff. Now, you're thinking "But each while-loop is different!" Ok - encapsulate the differences. 

    sub read_sql {
    my ($dbh, $sql, $callback) = @_;

    my $sth = $dbh->prepare( $sql );
    $sth->execute();
    return unless $sth->rows;

    while ( my @row = $sth->fetchrow_array ) {
        $callback->( @row );
    }

    $sth->finish;
}

my @AssignmentIDs; # You don't need the empty assignment.
read_sql( $dbh, $sql, sub {
    my ( $AssignmentID, $UserCount) = @_;
    push @AssignmentIDs, $AssignmentID;
});

my $AssignmentIDs = join ',', @AssignmentIDs;

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    Now, you have a piece of reusable code. It should be pretty easy to extend if you want to pass optional parameters in.
    My criteria for good software:
    1. Does it work?
    2. Can someone else come in, make a change, and be reasonably certain no bugs were introduced?
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Re: Reusing a variable declared earlier in the same scope
by apl (Monsignor) on Apr 10, 2008 at 15:37 UTC
    I can't echo dragonchild strongly enough. Never clone code when you can create a subroutine!
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Re: Reusing a variable declared earlier in the same scope
by Thilosophy (Curate) on Apr 11, 2008 at 03:44 UTC 
    my $rows = $GetAssignmentID_sth->rows;
  if($rows > 0) {
    while (my ($AssignmentID, $UserCount) = $GetAssignmentID_sth-> fet
+chrow_array() )  {
          push(@AssignmentIDs, $AssignmentID);
    }
  }

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    How about completely getting rid of the whole $rows part? It seems redundant (all it does is protect a while loop, that would not be executed anyway if there is no data to loop over), and error-prone ($sth->rows is not guaranteed to be the correct number of rows until you have fetched them all). 

    while (my ($AssignmentID, $UserCount) = $GetAssignmentID_sth-> fetchro
+w_array() )  {
     push(@AssignmentIDs, $AssignmentID);
}

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      My purpose in testing whether or not any rows are returned is to prevent a situation where the final $AssignmentIDs variable point at an empty string. I am using this variable in my next SQL statement using the IN syntax. For example, 
      SELECT ... FROM ... WHERE AssignmentID IN ($AssignmentIDs)

      [download]
      This is all very helpful information though. Thanks!
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