splicing and empty rows

[muppet <scott asofyet org>]

as a followup...  you asked earlier about why SimpleList was leaving 
empty rows when you deleted a row, and we got sidetracked by the fact 
that splice wasn't fully supported.

at some point, i think in IRC, you mentioned that now that you're using 
the fixed SimpleList from CVS, you're still seeing empty rows.

after looking at your code from CVS, i think i found out why --- it's a 
pitfall for Perl list operations, so i thought i'd copy the list, i 
hope you don't mind.


you had:

  sub del_user {
      my $bywhat = shift;
      for (0   {$UserList->{'data'}}-1) {
	  splice (@{$UserList->{'data'}}, $_, 1) if 
($UserList->{'data'}[$_][1] eq $bywhat);
      }
     update_busers();
  }

the problem here is that you *always* iterate over the whole list.  say 
you have two items in the list -- you hit index zero, delete it, and 
then proceed to index 1, despite the fact that the list only has one 
element and index 1 is no longer valid.  so, perl autovivifies the 
index and the tie magic winds up leaving you with an empty row.

you actually need to do this:

  sub del_user {
      my $bywhat = shift;
      for (0   {$UserList->{'data'}}-1) {
          if ($UserList->{data}[$_][1] eq $bywhat) {
              splice @{$UserList->{'data'}}, $_, 1;
              last; # stop!
          }
      }
      update_busers();
  }

in general, if you need to modify the list while traversing it, and may 
be removing elements, either traverse it from back to front, or start 
over after every modification.

--
muppet <scott at asofyet dot org>