Re: Re: Product of a list of numbers?

Why don't you Benchmark it?

Updated: Added my own Benchmark result.

use strict;
use Benchmark qw/ cmpthese timethese /;
use List::Util qw/ reduce /;

my @list = 1..10;

cmpthese(
  timethese(1000000, {
    'map_void'    => '&map_void',
    'for_loop'    => '&for_loop',
    'list_reduce' => '&list_reduce',
  }));
  
# printf "%d, %d, %d\n", map_void(), for_loop(), list_reduce();

sub map_void {
    my $result = 1;
    map {$result *= $_} @list;
    return $result;
}

sub for_loop {
    my $result = 1;
    $result *= $_ foreach @list;
    return $result;
}

sub list_reduce {
    return reduce { $a * $b } @list;
}

__END__

Benchmark: timing 1000000 iterations of for_loop, list_reduce, map_voi
+d...
  for_loop:  4 wallclock secs ( 5.43 usr +  0.00 sys =  5.43 CPU) @ 18
+4229.92/s
(n=1000000)
list_reduce:  9 wallclock secs ( 8.46 usr +  0.00 sys =  8.46 CPU) @ 1
+18175.37/s
 (n=1000000)
  map_void:  5 wallclock secs ( 4.52 usr +  0.00 sys =  4.52 CPU) @ 22
+1385.88/s
(n=1000000)
                Rate list_reduce    for_loop    map_void
list_reduce 118175/s          --        -36%        -47%
for_loop    184230/s         56%          --        -17%
map_void    221386/s         87%         20%          --
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Comment on Re: Re: Product of a list of numbers? Download Code

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Re: Re: Re: Product of a list of numbers? by davido (Cardinal) on Jan 23, 2004 at 06:19 UTC
Ok.... `use strict; use warnings; use Benchmark; my @list = ( 1 .. 500 ); my $count = 25000; timethese ( $count, { 'Map' => sub { my $result = 1; map { $result = $_ } @list; }, 'For' => sub { my $result = 1; $result = $_ for @list; } } ); __OUTPUT__ Benchmark: timing 25000 iterations of For, Map... For: 11 wallclock secs (10.62 usr + 0.00 sys = 10.62 CPU) @ 2355.16/s + (n=25000) Map: 11 wallclock secs (11.18 usr + 0.00 sys = 11.18 CPU) @ 2236.94/s + (n=25000)` [download] The moral... There's no way to perform a mathematical operation based on the value of each element of a list of arbitrary values, without looking at each element of the list. And that implies (and requires) some from of iteration. map and `for` seem to be pretty close to each other in terms of efficiency in iterating over the list. On the other hand, if the list consists of a known sequence, it is possible to break out the math-fu to derive a non-iterative solution. Brute force is necessary in the absence of a known mathematical sequence though. Dave	[reply] [d/l] [select]
Re: Product of a list of numbers? by Abigail-II (Bishop) on Jan 23, 2004 at 10:45 UTC
I'm quite surprised by your results. It shows the reduce version to be the slowests, and it suggests that using a map with a block is faster than a for statement modifier. The former needs to enter/leave a block for each iteration, and the latter doesn't. A benchmark on my box shows different results: #!/usr/bin/perl use strict; use warnings; use Benchmark qw /cmpthese timethese/; use List::Util qw /reduce/; our @list = 1 .. 50; our ($map_b, $map_e, $for_m, $for_b, $red, $bc, $eval); print "Perl version: $].\n"; system "cat /proc/version"; cmpthese -10 => { map_block => '$::map_b = 1; map {$::map_b = $_} @::list', map_expr => '$::map_e = 1; map $::map_e = $_, @::list', for_mod => '$::for_m = 1; $::for_m = $_ for @::list', for_block => '$::for_b = 1; foreach (@::list) {$::for_b = $_}', reduce => '$::red = reduce {$a * $b} @::list', bc => q !local $" = ""; $::bc = `echo '@::list' \| bc`!, eval => '$::eval = eval join "" => @::list', }; print "Map block: $map_b\n"; print "Map expres: $map_e\n"; print "For modifier: $for_m\n"; print "For block: $for_b\n"; print "Reduce: $red\n"; print "bc: $bc\n"; print "Eval: $eval\n"; __END__ Perl version: 5.008003. Linux version 2.4.18-3 (bhcompile@daffy.perf.redhat.com) (gcc version +2.96 20000731 (Red Hat Linux 7.3 2.96-110)) #1 Thu Apr 18 07:37:53 ED +T 2002 Rate bc eval map_block for_block map_expr for_m +od reduce bc 2779/s -- -55% -88% -94% -95% -9 +5% -95% eval 6199/s 123% -- -73% -87% -88% -8 +8% -89% map_block 22904/s 724% 269% -- -53% -55% -5 +7% -60% for_block 49041/s 1664% 691% 114% -- -3% - +9% -13% map_expr 50669/s 1723% 717% 121% 3% -- - +6% -11% for_mod 53638/s 1830% 765% 134% 9% 6% +-- -5% reduce 56651/s 1938% 814% 147% 16% 12% +6% -- Map block: 3.0414093201713378e+64 Map expres: 3.0414093201713378e+64 For modifier: 3.0414093201713378e+64 For block: 3.0414093201713378e+64 Reduce: 3.0414093201713378e+64 bc: 30414093201713378043612608166064768844377641568960512000 +000000000 Eval: 3.0414093201713378e+64 [download] Abigail	[reply] [d/l]