http://qs1969.pair.com?node_id=602642

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

First let me explain that I've been up for more than 40 hours. I've had a few hours of sleep earlier today but my brain is not working yet.

On page 171, chapter 4, section 4.5.5 of Higher Order Perl by Mark Jason Dominus, the iterator example given shows an oddity that my brain is having trouble understanding. Specifically, it appears that the second row retrieved from the query result set is being thrown away. Is this correct?

UPDATE: This simply sets up the next iteration. That way, we will know if the result set has been entirely processed and the next iteration will be the last iteration. Lesson learned: before reading Higher Order Perl, get some SLEEP! 

      } elsif ($action eq 'nextval') {
        my $oldrow = $row;
        $row = $sth->fetchrow_arrayref;
        return $oldrow->[0];
      }

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Full subroutine code: 

  sub dbi_query_iterator {
    my ($sth, @params) = @_;

    $sth->execute(@params) or return;
    my $row = $sth->fetchrow_arrayref()

    return Iterator {
      my $action = shift() || 'nextval';

      if ($action eq 'exhausted?') {
        return ! defined $row;
      } elsif ($action eq 'nextval') {
        my $oldrow = $row;
        $row = $sth->fetchrow_arrayref;
        return $oldrow->[0];
      }
  }

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Jason L. Froebe

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Replies are listed 'Best First'.
Re: dbi_query_iterator and my misunderstanding the $row
by japhy (Canon) on Mar 01, 2007 at 04:02 UTC
    I think this is because fetchrow_arrayref uses the same location in memory every time, as the DBI docs state: Note that the same array reference is returned for each fetch, so don't store the reference and then use it after a later fetch. Also, the elements of the array are also reused for each row, so take care if you want to take a reference to an element.
    Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
    How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
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        Oh, sorry. Maybe I misunderstood. Does the code actually work, and you want to know why it works?

        $oldrow is the previous row. The function that builds the iterator stores the first row before it creates the iterator function: 

        sub dbi_query_iterator {
  my ($sth, @params) = @_;

  $sth->execute(@params) or return;

  # get the first row of results NOW
  my $row = $sth->fetchrow_arrayref();

  return Iterator {
    my $action = shift() || 'nextval';

    if ($action eq 'exhausted?') {
      return ! defined $row;
    } elsif ($action eq 'nextval') {

      # save the current row of results
      my $oldrow = $row;

      # get the next row of results
      $row = $sth->fetchrow_arrayref;

      # return the previous row (before we advanced)
      return $oldrow->[0];
    }
  }
}

        [download]
        Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
        How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart
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