SPUG: re-size array

[michaelrwolf at att.net]

> You can juke the "odd no. of elements in hash assigment" warning
> and maintain hash insert ordering too:

> tie my %hash, 'Tie::IxHash' or die;
> %hash = @array % 2 ? (@array,undef) : @array;
> my @odd = keys %hash; my @even= values %hash;
> $#even-- unless defined $even[-1];

For two reasons, I prefer not to use the $#array syntax.
0 - It's hard to remember (for me and my future readers)
1 - It's ugly.  Well, at least there are prettier alternatives -- pop, push, splice -- that deal with the array *directly* instead of *indirectly* through that syntactic form.
2 - It's going away in Perl6

For those reasons, I'd rewrite the final line as

pop @even unless defined $even[-1];

Cool solution.


> 
> 
> You can juke the "odd no. of elements in hash assigment" warning
> and maintain hash insert ordering too:
> 
> tie my %hash, 'Tie::IxHash' or die;
> %hash = @array % 2 ? (@array,undef) : @array;
> my @odd = keys %hash; my @even= values %hash;
> $#even-- unless defined $even[-1];
> 
> (Assuming @array won't produce key duplicates
> that'd vanish when hashified)
> 
> 
> -----Original Message-----
> From: Colin Meyer [mailto:cmeyer at helvella.org] 
> Sent: Monday, October 11, 2004 3:45 PM
> To: michaelrwolf at att.net
> Cc: SPUG; Luis Medrano
> Subject: Re: SPUG: re-size array
> 
> 
> On Mon, Oct 11, 2004 at 10:03:10PM +0000, Michael R. Wolf wrote:
> > 
> > 
> > 
> > > %hash = @array;
> > > 
> > > @even = keys %hash;
> > > @odd  = values %hash;
> > 
> > Tricky!  I like it.
> > 
> > But beware. It only works if you don't care about the ordering of the 
> > @even or @odd arrays. If you merely need to consider them as sets 
> > (collections), this will work. Otherwise, the ordering has been lost 
> > by creating a hash out of it.
> 
> Also worth noting is that you will get a bogus undef in the @odd array, if there 
> were an odd number of elements in @array.
> 
> > 
> > It'll probably take a not-so-insignificant amount of time/CPU to build 
> > that hash.
> 
> I wouldn't think so. Perl hashes are pretty speedy to use. But just to be sure, 
> here's a test of hashes vs iterating through the list:
> 
>         Rate iter hash
> iter 11962/s   -- -14%
> hash 13831/s  16%   --
> 
> Hashes 14% faster than iterating (at least by my cheap test).
> 
> -Colin.
> 
> #!/usr/bin/perl
>                                                                                 
> use Benchmark ':all';
>                                                                                 
> my @a = 1..100;
>                                                                                 
> cmpthese( 100_000, { iter => \&oddeven_iterate, hash => \&oddeven_hash } );
>                                                                                 
> sub oddeven_hash {
>   my ( $o, $e, %h );
>   %h = @a;
>   $o = [ keys %h ];
>   $e = [ values %h ];
>   return $e, $o;
> }
>                                                                                 
> sub oddeven_iterate {
>   my ( $o, $e );
>   push @{ $_ % 2 ? $o : $e }, $a[$_] for 0..$#a;
>   return $e, $o;
> }
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