Re: Re: Checking for value in an array (without FOR)

That faq is somewhat out of date, as recent perls have been optimized such that map and grep don't build return lists if they're not required. As the benchmarking code below shows (numbers generated with 5.6.1 on a 1.2G Athlon):

#!/usr/bin/perl
use Benchmark qw( timethese cmpthese );

my @args = ( 100_000,
             {
              void => sub {
                grep { $_ == 50 } 0..100; 1
              },
              scalar => sub {
                my $res = grep { $_ == 50 } 0..100
              },
              list => sub {
                my @res = grep { $_ == 50 } 0..100
              },
              a_foreach => sub {
                my $match;
                foreach( 0..100 ) { 
                  $match++ if $_ == 50
                }
                $match
              }
             } );

cmpthese( @args );

exit 0;

__END__
[download]

Benchmark: timing 100000 iterations of a_foreach, list, scalar, void...
 a_foreach:  4 wallclock secs ( 4.52 usr +  0.01 sys =  4.53 CPU) @ 22075.06/s (n=100000)
      list:  4 wallclock secs ( 3.35 usr +  0.00 sys =  3.35 CPU) @ 29850.75/s (n=100000)
    scalar:  3 wallclock secs ( 3.34 usr +  0.00 sys =  3.34 CPU) @ 29940.12/s (n=100000)
      void:  4 wallclock secs ( 3.29 usr +  0.00 sys =  3.29 CPU) @ 30395.14/s (n=100000)
             Rate a_foreach      list    scalar      void
a_foreach 22075/s        --      -26%      -26%      -27%
list      29851/s       35%        --       -0%       -2%
scalar    29940/s       36%        0%        --       -1%
void      30395/s       38%        2%        2%        --

There's practically no difference between grep in scalar and list context, while a foreach loop runs 26% slower.

Update: Oops, I'd said foreach was 35% slower. Changed the last paragraph above.

Update: Playing around, if you use grep EXPR, LIST rather than grep BLOCK LIST (i.e. grep $_ == 50, 0..100), that gap grows to 33% (since there's not as much overhead entering and leaving the block). If you switch comparisons from $_ == 50 to /50/ and stick with the BLOCK form, the foreach form is 45% faster.

Comment on Re: Re: Checking for value in an array (without FOR) Select or Download Code

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Re: Re: Re: Checking for value in an array (without FOR) by Anonymous Monk on Apr 19, 2002 at 20:41 UTC
a_foreach could be faster, as it is in perlfaq4.	[reply]
Re: Re: Re: Re: Checking for value in an array (without FOR) by Fletch (Bishop) on Apr 19, 2002 at 21:05 UTC
Meaning: `a_short_circuit_foreach => sub { my $present; foreach( 0..100 ) { if( /0/ ) { $present++; last } } $present },` [download] Which will best case (looking for the first item) be very much faster indeed (1178% than scalar grep), worst case just as bad as the non-short-circuit foreach, and "average" case (in the middle of the list) 12% faster. (All numbers are for the data in question for this benchmark, not in general). Of course if you're really serious about performance, just use a hash. #!/usr/bin/perl use Benchmark qw( timethese cmpthese ); my @data = ( 0 .. 100 ); my %cheat; @cheat{ @data } = (1) x @data; my @args = ( -1, { void => sub { grep /50/, @data; 1 }, scalar => sub { my $res = grep /50/, @data }, list => sub { my @res = grep /50/, @data }, a_foreach => sub { my $match; foreach( @data ) { $match++ if /50/ } $match }, a_short_circuit_foreach => sub { my $present; foreach( @data ) { if( /50/ ) { $present++; last } } $present; }, a_hash => sub { exists $cheat{ 50 }; } } ); cmpthese( @args ); exit 0; __END__ [download] Benchmark: running a_foreach, a_hash, a_short_circuit_foreach, list, scalar, void, each for at least 1 CPU seconds... a_foreach: 1 wallclock secs ( 1.06 usr + 0.00 sys = 1.06 CPU) @ 20286.79/s (n=21504) a_hash: 1 wallclock secs ( 1.11 usr + 0.01 sys = 1.12 CPU) @ 7080227.68/s (n=7929855) a_short_circuit_foreach: 2 wallclock secs ( 1.07 usr + 0.00 sys = 1.07 CPU) @ 40193.46/s (n=43007) list: 1 wallclock secs ( 1.04 usr + 0.00 sys = 1.04 CPU) @ 25845.19/s (n=26879) scalar: 1 wallclock secs ( 1.02 usr + 0.00 sys = 1.02 CPU) @ 26351.96/s (n=26879) void: 2 wallclock secs ( 1.04 usr + 0.00 sys = 1.04 CPU) @ 25845.19/s (n=26879) Rate a_foreach void list scalar a_short_circuit_foreach a_hash a_foreach 20287/s -- -22% -22% -23% -50% -100% void 25845/s 27% -- 0% -2% -36% -100% list 25845/s 27% 0% -- -2% -36% -100% scalar 26352/s 30% 2% 2% -- -34% -100% a_short_circuit_foreach 40193/s 98% 56% 56% 53% -- -99% a_hash 7080228/s 34801% 27295% 27295% 26768% 17515% --	[reply] [d/l] [select]