DAJ 198

1 files changed, 27 insertions, 0 deletions

diff --git a/challenge-198/dave-jacoby/perl/ch-2.pl b/challenge-198/dave-jacoby/perl/ch-2.pl
new file mode 100644
index 0000000000..044a2f68a4
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+++ b/challenge-198/dave-jacoby/perl/ch-2.pl
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+#!/usr/bin/env perl
+
+use strict;
+use warnings;
+use experimental qw{ say postderef signatures state };
+use Algorithm::Permute;
+
+my @examples = ( 10, 15, 1, 25 );
+
+for my $e (@examples) {
+    my $out = prime_count($e);
+    say <<"END";
+    Input:  \$n = $e
+    Output: $out
+END
+}
+
+# 1 is not a prime, but 2 is, but the regex in is_prime accepts 1, so
+# we start on 2 and go to the given max
+# grep ( is_prime($_) ) gives only the list of primes 
+# scalar gives us the count, which is what we want
+sub prime_count ( $e ) {
+    return scalar grep { is_prime($_) } 2 .. $e;
+}
+
+# https://catonmat.net/perl-regex-that-matches-composite-numbers
+sub is_prime ( $n ) { ( '1' x $n ) !~ /\A(11+?)\1+\z/ }