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Re^2: counting backward

by vagabonding electron (Hermit)
on Mar 03, 2013 at 16:33 UTC ( #1021516=note: print w/ replies, xml ) Need Help??


in reply to Re: counting backward
in thread counting backward

It seems that your output and the output of BrowserUk brings the different results ( Re^5: counting backward (optimize foreach reverse low .. high ).

I noticed that you print in the loop and BrowserUk calls '1'. I tried to run the both versions and found that it brings the different results indeed:

Loops with print (I used a NULL device to keep it clear, s.below): Rate foreach_loop c_for_loop foreach_loop 256/s -- -8% c_for_loop 278/s 8% -- Loops with '1': Rate c_for_loop foreach_loop c_for_loop 1314/s -- -22% foreach_loop 1674/s 27% --
The code (your code :-):
#! perl use strict; use warnings; use IO::Null; =pod PM Thread perlquestion [id://1021418] =cut use Benchmark qw( cmpthese ); my $n = 1e4; my $null = IO::Null->new; cmpthese ( 10000, { foreach_loop => \&foreach_loop, c_for_loop => \&c_for_loop, } ); sub foreach_loop { for my $i (-$n .. 0) { # $null->print( -$i ); 1; } } sub c_for_loop { for (my $j = $n; $j >= 0; --$j) { # $null->print( $j ); 1; } }


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Re^3: counting backward
by BrowserUk (Pope) on Mar 03, 2013 at 18:40 UTC

    You caught that I use 1; as the body of the loop to avoid confusing things with the efficiency of the system null device driver (not particularly efficient on Win32). But you missed the point I was trying to make; namely that you cannot use the C-style for as a modifier; which means you must always pay the penalty of creating a new scope for each iteration of the loop.

    Add that to the benchmark to see that cost:

    #! perl use strict; use warnings; #use IO::Null; use Benchmark qw( cmpthese ); my $n = 1e4; #my $null = IO::Null->new; cmpthese 10000, { foreach_loop => \&foreach_loop, foreach_modifier => \&foreach_modifier, c_for_loop => \&c_for_loop, }; sub foreach_loop { for my $i (-$n .. 0) { # $null->print( -$i ); 1; } } sub foreach_modifier { 1 for -$n .. 0; } sub c_for_loop { for (my $j = $n; $j >= 0; --$j) { # $null->print( $j ); 1; } } __END__ C:\test>junk Rate c_for_loop foreach_loop foreach_modi +fier c_for_loop 922/s -- -41% +-52% foreach_loop 1573/s 71% -- +-18% foreach_modifier 1922/s 108% 22% + --

    Of course, some people eschew the use of modifier forms; but then they are probably the same people that favor their own one-off developer time over the every-user, every-time runtime costs.

    As for much of my work I am both user and developer, I don't have that luxury.


    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority".
    In the absence of evidence, opinion is indistinguishable from prejudice.
Re^3: counting backward
by Athanasius (Monsignor) on Mar 04, 2013 at 02:20 UTC

    vagabonding electron,

    You are correct that a foreach loop is faster than a C-style for loop, other things being equal. As BrowserUk has explained, this is because with the latter, but not the former,

    you must always pay the penalty of creating a new scope for each iteration of the loop.

    But my point was that if you count upwards from -$n, you add the overhead of a unary negation operator when you come to actually use the original $n:

    #! perl use strict; use warnings; use Benchmark qw( cmpthese ); my $n = 1e6; my $foreach_loop_total = 0; my $c_for_loop_total = 0; cmpthese ( 1000, { foreach_loop => \&foreach_loop, c_for_loop => \&c_for_loop, } ); print "\$foreach_loop_total = $foreach_loop_total\n"; print "\$c_for_loop_total = $c_for_loop_total\n"; sub foreach_loop { $foreach_loop_total += -$_ for -$n .. 0; } sub c_for_loop { for (my $j = $n; $j >= 0; --$j) { $c_for_loop_total += $j; } }

    Output:

    12:00 >perl 555_SoPW.pl Rate foreach_loop c_for_loop foreach_loop 5.62/s -- -8% c_for_loop 6.13/s 9% -- $foreach_loop_total = 500000500000000 $c_for_loop_total = 500000500000000 12:05 >

    So, it appears that the overhead of the additional negation outweighs the benefit of not having to create a new scope on each iteration.

    Athanasius <°(((><contra mundum Iustus alius egestas vitae, eros Piratica,

      There are still plenty more ways to skin that cat :)

      Why subtract and add; when you can just subtract. And why compare to zero when it is unnecessary:

      #! perl use strict; use warnings; use Benchmark qw( cmpthese ); my $n = 1e6; my $foreach_loop_total = 0; my $c_for_loop_total = 0; my $while_loop_total = 0; my $while_loop_total2 = 0; cmpthese 100, { foreach_loop => \&foreach_loop, c_for_loop => \&c_for_loop, while_loop => \&while_loop, while_loop2 => \&while_loop2, }; print "\$foreach_loop_total = $foreach_loop_total\n"; print "\$c_for_loop_total = $c_for_loop_total\n"; print "\$while_loop_total = $while_loop_total\n"; print "\$while_loop_total2 = $while_loop_total2\n"; sub foreach_loop { $foreach_loop_total -= $_ for -$n .. 0; } sub c_for_loop { for (my $j = $n; $j >= 0; --$j) { $c_for_loop_total += $j; } } sub while_loop { my $n = $n; $while_loop_total += $n-- while $n; } sub while_loop2 { use integer; my $n = $n; $while_loop_total2 += $n-- while $n; } __END__ C:\test>junk Rate c_for_loop while_loop while_loop2 foreach_loo +p c_for_loop 4.94/s -- -22% -39% -44 +% while_loop 6.31/s 28% -- -22% -28 +% while_loop2 8.07/s 63% 28% -- -8 +% foreach_loop 8.76/s 77% 39% 8% - +- $foreach_loop_total = 50000050000000 $c_for_loop_total = 50000050000000 $while_loop_total = 50000050000000 $while_loop_total2 = 50000050000000

      With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.

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