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ppm-guest pushed a commit to annotated tag v0.38
in repository libmath-prime-util-perl.
commit cc4ccdf648ac83a0efd8fb09622e0dcf398d9fb2
Author: Dana Jacobsen <d...@acm.org>
Date: Fri Jan 31 22:27:38 2014 -0800
Update PE 357 example
---
examples/project_euler_357.pl | 26 +++++++++++++++++++-------
1 file changed, 19 insertions(+), 7 deletions(-)
diff --git a/examples/project_euler_357.pl b/examples/project_euler_357.pl
index b805d15..501c90f 100644
--- a/examples/project_euler_357.pl
+++ b/examples/project_euler_357.pl
@@ -6,21 +6,33 @@ use List::MoreUtils qw/all/;
my $maxn = shift || 100_000_000;
-# Takes ~ 5 seconds
-my $sum = 0;
+prime_precalc($maxn); # Speeds up is_prime, but not necessary
+my($sum, $n) = (1, 0);
forprimes {
- my $n = $_-1; # 1+$n/1 is prime (hence n=1 or even)
- if (is_prime(2+($n>>1))) { # 2+$n/2 is prime (noting n is even or 1)
+ $n = 2*$_ - 4; # 2+$n/2 is prime
+ if (is_prime($n+1)) { # 1+$n/1 is prime
if (moebius($n) != 0) { # n should be square free
$sum += $n if all { is_prime($_+$n/$_) } divisors($n);
}
}
-} $maxn;
+} int($maxn/2);
print "$sum\n";
+
+# This version is a little more direct.
+# my($sum, $n) = (0, 0);
+# forprimes {
+# $n = $_-1; # 1+$n/1 is prime (hence n=1 or even)
+# if (is_prime(2+($n>>1))) { # 2+$n/2 is prime (noting n is even or 1)
+# if (moebius($n) != 0) { # n should be square free
+# $sum += $n if all { is_prime($_+$n/$_) } divisors($n);
+# }
+# }
+# } $maxn;
+# print "$sum\n";
+
+
# We could additionally check these:
# if ( (($n+2) % 4) == 0 || $n == 1) {
# Using all is more efficient, but this works:
# $sum += $n unless scalar grep { !is_prime($_+$n/$_) } divisors($n);
-# We could alternately generate primes to $maxn/2,
-# my $n = 2*$_-4; if (is_prime($n+1)) { ... }
--
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