Reusing a variable declared earlier in the same scope

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();
[download]

The next query I perform, I would like to reuse the following code:

if($rows > 0) {}
[download]

I receive the warning, however, that:

"my" variable $rows masks earlier declaration in same scope
[download]

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;
[download]

Thanks!

Comment on Reusing a variable declared earlier in the same scope Select or Download Code

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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); $GetAssignmentID_sth->finish();` [download] That makes it very clear what the scope of each thing should be. I've used that on many occasions. However, there's a better way - create a subroutine to do stuff. Now, you're thinking "But each while-loop is different!" Ok - encapsulate the differences. `sub read_sql { my ($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;` [download] 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: Does it work? Can someone else come in, make a change, and be reasonably certain no bugs were introduced?	[reply] [d/l] [select]
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!	[reply]
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, $AssignmentID); } }` [download] How about completely getting rid of the whole `$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); }` [download]	[reply] [d/l] [select]
Re^2: Reusing a variable declared earlier in the same scope by reluctant_techie (Novice) on Apr 14, 2008 at 15:30 UTC
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!	[reply] [d/l]


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