Re^2: Why is this code so much slower than the same algorithm in C?

[snowhare]

With a little analysis to omit test values that don't affect the answer and some improvement on the if..next..if..next logic, I was able to tighten that up slightly:

#!/usr/bin/perl

use integer;

my $i = 0;
while ($i += 20) {
    last unless ($i % 3
          or $i % 6
          or $i % 7
          or $i % 8
          or $i % 9
          or $i % 11
          or $i % 12
          or $i % 13
          or $i % 14
          or $i % 15
          or $i % 16
          or $i % 17
          or $i % 18
          or $i % 19
          or $i % 20);
}
print "Number: $i\n";

[Limbic~Region]

snowhare,
Can this be reduced even further?

for values of x > 20
if x % 12 == 0 then x % 6 == 0

Is there a reason to have them both?

Update: In other words, order the list in descending order removing any exact multiples of previous items. When you reach 16, one of two things will happen. Either there will be a remainder and it will exit the loop or there won't be a remainder and it will continue. There is no need to test 8 after 16.

[tye]

perl -le "do { $i += 20 } while( $i%3||$i%7||$i%11||$i%13||$i%16||$i%1
+7||$i%19 ); print $i"
77597520

or just

perl -le "print 3*5*7*11*13*16*17*19"

Update: Replace 3 with 9 in both samples above to get the correct answer, as noted and further explained in the replies below.

[snowhare]

You've got an extra factor of 3. The correct answer is 232792560. ;)

[tilly]

Why do that much analysis and then throw away so much speed by not using a lexical? Also why not finish the analysis?

perl -le 'my $i = 0;do { $i += 20 } while( $i%46558512 ); print $i'
232792560

Performance is essentially the same if you write 46558512 as (9*7*11*13*16*17*19).

[snowhare]

Good catch.

#!/usr/bin/perl

use integer;

my $i = 0;
while ($i += 20) {
    last unless (
        $i % 19
        or $i % 18
        or $i % 17
        or $i % 16
        or $i % 15
        or $i % 14
        or $i % 13
        or $i % 12
        or $i % 11
             );
}
print "Number: $i\n";

Is 17% faster than my original version.