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Re^3: counting backward

by Athanasius (Abbot)
on Mar 04, 2013 at 02:20 UTC ( #1021565=note: print w/ replies, xml ) Need Help??


in reply to Re^2: counting backward
in thread counting backward

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,


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Re^4: counting backward
by BrowserUk (Pope) on Mar 04, 2013 at 03:28 UTC

    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

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