**Advent Calendar 2022**

### | **Day 5** | **Day 6** | **Day 7** |

The gift is presented by `Dave Jacoby`

. Today he is talking about his solution to **“The Weekly Challenge - 158”**. This is re-produced for **Advent Calendar 2022** from the original **post** by him.

## Task #1: Additive Primes

Additive primes are prime numbers for which the sum of their decimal digits are also primes.

We’re on to **Weekly Challenge #158!**. `158`

is even so not prime, but is the product of two primes, `2`

(because even) and `79`

.

Because this time, I have every expectation that I’ll have to check if a number is prime twice, I brought in `Memoize`

. Because of lack of recursion, I don’t expect it to be as much of an obvious win as, for example, fibonacci, but every little bit helps, and it’s good that I finally remember to use it, instead of just mentioning it.

So, once we know a number is prime, we then have to split it into digits `(split //, $n)`

and sum them (`sum0`

from one of my go-to’s, `List::Util`

), and then testing if that’s prime.

## Show Me The Code!

```
#!/usr/bin/env perl
use strict;
use warnings;
use feature qw{ say postderef signatures state };
no warnings qw{ experimental };
use List::Util qw{ sum0 product };
use Memoize;
memoize('is_prime');
my @aprimes;
for my $i ( 1 .. 100 ) {
if ( is_prime($i) ) {
my $sum = sum0 split //, $i;
if ( is_prime($sum) ) { push @aprimes, $i; }
}
}
say join ', ', @aprimes;
sub is_prime ($n) {
return 0 if $n == 0;
return 0 if $n == 1;
for ( 2 .. sqrt $n ) { return 0 unless $n % $_ }
return 1;
}
```

```
$ ./ch-1.pl
2, 3, 5, 7, 11, 23, 29, 41, 43, 47, 61, 67, 83, 89
```

## Task #2: First Series Cuban Primes

Write a script to compute first series Cuban Primes <= 1000. Please refer **wikipedia page** for more informations.

So, the `Cuban Prime`

is a pun on these relating to cubes.

The first form, when simplified, become:

p = 3y2 + 3y + 1, where P is the prime in question

So, what we’re doing is finding a number for `y`

.

It’s simply iteration, multiplication and addition. If we were dealing with large primes that require `Math::BigInt`

and have many more numbers between 1 and itself would require a more efficient algorithm, but for primes less than 1,000? This is fast enough.

## Show Me The Code!

```
#!/usr/bin/env perl
use strict;
use warnings;
use feature qw{ say postderef signatures state };
no warnings qw{ experimental };
use List::Util qw{ sum0 };
my @cprimes;
for my $n ( 1 .. 1000 ) {
if ( is_prime($n) ) {
my $c = is_cuban_prime($n);
push @cprimes, $n if $c;
}
}
say join ', ', @cprimes;
sub is_cuban_prime ($n) {
for my $i ( 1 .. $n ) {
my $c = sum0 1, ( 3 * $i ), ( 3 * ( $i**2 ) );
return 1 if $c == $n;
}
return 0;
}
sub is_prime ($n) {
return 0 if $n == 0;
return 0 if $n == 1;
for ( 2 .. sqrt $n ) { return 0 unless $n % $_ }
return 1;
}
```

```
$ ./ch-2.pl
7, 19, 37, 61, 127, 271, 331, 397, 547, 631, 919
```

If you have any suggestion then please do share with us perlweeklychallenge@yahoo.com.