### Re^3: Algorithm for cancelling common factors between two lists of multiplicands

by hv (Prior)
 on Aug 09, 2005 at 00:07 UTC ( #482073=note: print w/replies, xml ) Need Help??

For simple problems I usually use something like this:

```{
my @p; BEGIN { @p = (2, 3) }
sub nextprime {
my \$p = \$p[\$#p];
DIV: while (1) {
\$p += 2;
my \$pc = 0;
while (1) {
my \$d = \$p[\$pc++];
last if \$d * \$d > \$p;
next DIV unless \$p % \$d;
}
\$p[@p] = \$p;
return \$p;
}
}

sub factors {
my \$n = shift;
my(\$pc, @result) = (0);
while (\$n > 1) {
my \$d = \$p[\$pc++] || nextprime();
if (\$d * \$d > \$n) {
push @result, \$n;
\$n = 1;
} else {
while (\$n % \$d == 0) {
\$n /= \$d;
push @result, \$d;
}
}
}
\@result;
}
}

The rest of the time I use Math::Big*, sometimes with Pari or (lately) GMP.

Hugo

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Re^4: Algorithm for cancelling common factors between two lists of multiplicands
by BrowserUk (Patriarch) on Aug 09, 2005 at 00:27 UTC

Nice, thankyou. I like nextPrime(). I was kludging something together by embedding the first 1000 primes in a DATA section.

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