in reply to my $x or my ($x)

This has just caught me out in a while loop for getting information from a database

        my $query = $dbh->prepare("SELECT idFrame, name, colour FROM F
+rame");
        $query->execute();
        while (my ($frame) = $query->fetchrow_hashref) {
            push @frames, $frame;
        }

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This loop never ends!
Although $query->fetchrow_hashref returns undef once all the rows have been returned, the condition as whole doesn't so the loop continues...

This is what I really wanted (and how have)

        my $query = $dbh->prepare("SELECT idFrame, name, colour FROM F
+rame");
        $query->execute();
        while (my $frame = $query->fetchrow_hashref) {
            push @frames, $frame;
        }

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Re^2: my $x or my ($x)
by LanX (Saint) on Jan 02, 2021 at 23:41 UTC
    > This loop never ends!

    Because the iterator ->fetchrow_hashref returns one scalar undef when done. But a list assignment (...) = ... is only false if the list is empty. °

    It might have been better to return an empty list. This would have worked similarly well in list as scalar context.

    But it's only supposed to return one scalar - the mentioned hashref - anyway.

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery

    °) see also Re: list context (iterators)

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      Because the iterator ->fetchrow_hashref returns one scalar undef when done

      Usually I use fetchrow_array - I don't think I have used fetchrow_hashref before.

      The problem came because I was converting from this code 

          my @frames;
    my $query = $dbh->prepare("SELECT idFrame, name, colour FROM Frame
+");
    $query->execute();
    while (my ($id, $name, $colour) = $query->fetchrow_array) {
        my $frame = {
            'image'  => 'test_thumb.jpg',
            'id'     => $id,
            'name'     => $name,
            'colour' => $colour,
        };
        push @frames, $frame;
    }
    my $vars = {
        'frames' => \@frames,
    };
    $template->process("admin_frames.tt", $vars);

      [download]
      It seemed silly to me to be fetching the elements of an array, using those to build an anonymous hash and adding the reference to it to an array. Why not just fetch the hashref and add that to the array? 
          my @frames;
    my $query = $dbh->prepare("SELECT idFrame, name, colour FROM Frame
+");
    $query->execute();
    while (my ($frame) = $query->fetchrow_hashref) {
        $$frame{'image'} = 'test_thumb.jpg';
        push @frames, $frame;
    }
    my $vars = {
        'frames' => \@frames,
    };
    $template->process("admin_frames.tt", $vars);

      [download]
      Finding the error of my ($frame) = $query->fetchrow_hashref instead of my $frame = $query->fetchrow_hashref took some time. Not helped by developing it on a webserver so when the loop didn't end, I assumed the connection was just being ultra slow!

      I suspect that there is a more elegant solution that doesn't involve iterating through the rows from the database using fetchall_hashref but that's (possibly) for another day. There will only ever be a handful of rows returned by this query.

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