Robin Sheat wrote:
> I have a section of a program that is this:
>
> my $users = get_all_users();
> foreach my $user (@$users) {
> my $details = get_user_details($user);
> my $sum=0;
> my $count=0;
> for (my $i=1; $i<@$details; $i+=2) {
> $sum += @$details[$i];
> $count++;
> }
> $means{$user} = $sum/$count;
> }
>
> The $users var is a reference to an array that is about 450,000 entries. The
> $details var is an array reference typically of size around 500 or so (note
> however, the inner loop goes up in twos). Overall the innermost part is
> executed 100 million times. How can I optimise it?
Write it in C. :-)
> If I was using C, I'd use
> pointer arithmetic on the array to save the multiplication on the array
> indexing. A naive reading of this implies that Perl would be doing a multiply
> for every array access, is this the case, or is it optimised away? If not, is
> there a way to use pointer arithmetic (or something that behaves similarly,
> but probably safer)?
Yes, Perl uses references which are conceptually similar to pointers.
my $users = get_all_users();
for ( my $i = 0; $i < @$users; ++$i ) {
my $details = get_user_details( $users->[ $i ] );
my $sum = 0;
my $count = 0;
for ( my $j = 1; $j < @$details; $j += 2 ) {
$sum += $details->[ $j ];
$count++;
}
$means{ $user } = $sum / $count;
}
Although in my tests that takes about the same time to execute but uses a
*lot* less memory as it doesn't have to make a copy of the @$users array like
your version does.
> It is probably possible to restructure the array so that
> it can go up in ones, and do a foreach, but in most cases it'll be common to
> want both values, which is why I didn't do that.
>
> Anyone know any good resources for this kind of thing?
perldoc -q "How do I profile my Perl programs"
http://www.ccl4.org/~nick/P/Fast_Enough/
John
--
Perl isn't a toolbox, but a small machine shop where you can special-order
certain sorts of tools at low cost and in short order. -- Larry Wall
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