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### Re^2: To Findout Prime number

by lodin (Hermit)
 on Feb 08, 2008 at 13:45 UTC ( #666965=note: print w/replies, xml ) Need Help??

in reply to Re: To Findout Prime number
in thread To Findout Prime number

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%     --
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));

lodin

Replies are listed 'Best First'.
Re^3: To Findout Prime number
by BrowserUk (Pope) on Feb 08, 2008 at 15:20 UTC
Re^3: To Findout Prime number
by andreas1234567 (Vicar) on Feb 11, 2008 at 06:57 UTC
nowadays it's much faster to use substr
I re-ran your benchmark on Linux 2.6.9 i686, perl v5.8.5 built for i386-linux-thread-multi. The difference was much less than what you encountered.
```\$ perl -w 666965.pl
Benchmark: running substr, vec for at least 60 CPU seconds...
substr: 67 wallclock secs (60.69 usr +  0.03 sys = 60.72 CPU) @  0
+.58/s (n=35)
vec: 66 wallclock secs (61.33 usr +  0.04 sys = 61.37 CPU) @  0
+.54/s (n=33)
s/iter    vec substr
vec      1.86     --    -7%
substr   1.73     7%     --
On Linux 2.6.22-10-386 i686, perl v5.10.0 (different hardware than above):
```Benchmark: running substr, vec for at least 60 CPU seconds...
substr: 62 wallclock secs (61.69 usr +  0.05 sys = 61.74 CPU) @  0
+.44/s (n=27)
vec: 61 wallclock secs (60.81 usr +  0.05 sys = 60.86 CPU) @  0
+.39/s (n=24)
s/iter    vec substr
vec      2.54     --   -10%
substr   2.29    11%     --
Update Mon Feb 11 12:13:38 CET 2008: Added perl 5.10 benchmark on lodin's request.
--
Andreas

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