Re: [FWP] Interleaving arrays

[tomr@aceldama.com]

On Wed, Nov 10, 1999 at 05:56:13PM -0600, Brand Hilton wrote:
> On Wed, Nov 10, 1999 at 03:03:06PM -0600, I wrote:
> > Oddly, benchmarking indicates that Tom's way is about twice as fast as
> > mine, which was:
> > 
> >   my $half = $#cats/2;
> >   @cats = @cats[map (($_, $_+$half+1), 0..$half)];
> > 
> > Knowing next to nothing about Perl internals, I'm guessing that
> > it's something to do with growing the list at each iteration of
> > the map.

I suppose that it would be a good optimization for Perl to
automagically pre-allocate $#source elements for the target array.

> Looks like I guessed right.  I changed my subroutine from:
> 
>   sub mine {
>     my $half = $#array/2;
>     map (($_, $_+$half+1), 0..$half);
>   }

Mmm, good choice.

> to the substantially more verbose:
> 
>   sub mine2 {
>     my $half = $#array/2;
>     my @retval;
>     $#retval = $#array;
>     my $i = 0;
>     foreach (0..$half) {
>       $retval[$i++] = $_;
>       $retval[$i++] = $_+$half+1;
>     }
>     @retval;
>   }

Ermmm ... doesn't that fill @retval with numbers rather than the
values from the input array? That is, shouldn't the lines in the loop be:
      $retval[$i++] = @array[$_];
      $retval[$i++] = @array[$_+$half+1];

Or am I on the wrong drugs?

> The speedup was quite noteworthy:
> 
> Benchmark: timing 100 iterations of Mine, Mine improved, Tom's, Tom's improved...
>       Mine: 42 wallclock secs (41.01 usr +  0.05 sys = 41.06 CPU)
> Mine improved: 10 wallclock secs ( 9.91 usr +  0.01 sys =  9.92 CPU)
>      Tom's: 23 wallclock secs (22.25 usr +  0.01 sys = 22.26 CPU)
> Tom's improved: 18 wallclock secs (16.98 usr +  0.00 sys = 16.98 CPU)
> 
> I hope someone besides me is having fun with this :-)

Let's just say I'm sorry I don't need a columnizer anymore :)

Cheers,

Tom

-- 
--    Tom Rathborne    tomr@aceldama.com
--                     http://www.aceldama.com/~tomr/
--    "I seem to be having tremendous difficulty with my life-style."

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