On Thursday 12 October 2006 23:52, John W. Krahn wrote: > Write it in C. :-) Tempting, but I'm prototyping here. Perl is much faster to write than C, especially when dealing with complex data structures (for the more complex algorithms I'm playing with, some of them would be downright painful in C)
> Yes, Perl uses references which are conceptually similar to pointers.
yeah, I understand Perl references, but you can't (afaik) do C-style pointer
arithmetic with them. Without pointer arithmetic, this access:
$details->[ $j ]
is something akin to (in C/Perl/psuedocode mishmash):
address_of($details) + sizeof(void *) * $j
That multiply is happening 100 million times. I wonder if using foreach is any
faster, I'd expect that it's optimised for skimming though arrays, and it may
wind up faster even if I skip every second one perhaps. I guess DProf will
tell me...
My original method (main::learn is the function I posted, I've flipped the
lines around so they're always in the same order):
%Time ExclSec CumulS #Calls sec/call Csec/c Name
44.6 66.01 142.31 1 66.009 142.31 main::learn
50.5 74.78 74.780 480189 0.0002 0.0002 main::get_user_details
Using a foreach with an 'if' to skip each second one (I didn't understand the
$i ^= 1 trick until after I'd run this, don't know how much of a difference
it'd make):
55.6 91.53 161.97 1 91.539 161.97 main::learn
41.9 68.90 68.909 480189 0.0001 0.0001 main::get_user_details
Paul's method where the operation is a one-liner using grep:
41.9 52.99 122.67 1 52.999 122.67 main::learn
53.8 68.11 68.119 480189 0.0001 0.0001 main::get_user_details
I think it's somewhat clear what the winner is here :)
> 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.
I don't follow. Where was my version copying @$users?
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