It includes clever use of the vec operator.

Maybe there's historical reasons for using vec over substr, but nowadays it's much faster to use substr. The for my $foo ($from .. $to) { ... } construct is also slightly more efficiently than for (my $foo = $from; $foo <= $to; $foo++) { ... }. My guess is that the C-style loop was used as the "range loop" wasn't optimized back then and actually generated a list of, in this case, a million numbers.

I changed the for loop and did a direct translation to using substr instead of vec, and here's the results: 

Benchmark: running substr, vec for at least 60 CPU seconds...
    substr: 61 wallclock secs (62.41 usr +  0.02 sys = 62.42 CPU) @  1
+.09/s (n=68)
       vec: 58 wallclock secs (60.64 usr +  0.02 sys = 60.66 CPU) @  0
+.73/s (n=44)
       s/iter    vec substr
vec      1.38     --   -33%
substr  0.918    50%     --

[download]
Here's the code: 
use strict;
use warnings;
use Benchmark qw(cmpthese timethese);

my %subs;

######################################################################
+#########

my $UPPER = 1_000_000;

$subs{vec} = sub {
    my $sieve = "";
    GUESS: for my $guess (2 .. $UPPER) {
        next GUESS if vec($sieve,$guess,1);
        #print "$guess\n";
        for (my $mults = $guess * $guess; $mults <= $UPPER; $mults += 
+$guess) {
            vec($sieve,$mults,1) = 1;
        }
    }
};

$subs{substr} = sub {
    my $sieve = '0' x ($UPPER + 1);
    GUESS: for my $guess (2 .. $UPPER) {
        next GUESS if substr($sieve,$guess,1);
        #print "$guess\n";
        for (my $mults = $guess * $guess; $mults <= $UPPER; $mults += 
+$guess) {
            substr($sieve,$mults,1,1);
        }
    }
};

######################################################################
+#########

cmpthese(timethese(-60, \%subs));

[download]

lodin

In reply to Re^2: To Findout Prime number by lodin
in thread To Findout Prime number by babug_prg

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