BLOG: The Weekly Challenge #076

Sunday, Sep 6, 2020| Tags: Perl, Raku


Although I had finished the “Word Search” task a week before, still I didn’t have time to complete the “Prime Sum” task. After checking out others solution to the “Word Search” task, I was feeling low looking back my own solution. To recover from it, I delayed solving the “Prime Sum” task. At the same time, as a protest, I didn’t do Swift solutions for any of the tasks. After few days when I was feeling a little better, I decided to do at least one task better.

I really enjoyed working on the “Prime Sum” task. I got to learn about Sieve of Erastothenes. When I looked back at the end result, I felt much better. My Raku solution is even better if you look at it. At the end of the day, I was happy with my effort.

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Task #1: Prime Sum

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Let me share my solutions to the The Weekly Challenge - 076.

TASK #1 › Prime Sum

Submitted by Mohammad S Anwar

Reviewed by Ryan Thompson

You are given a number $N. Write a script to find the minimum number of prime numbers required, whose summation gives you $N.

For the sake of this task, please assume 1 is not a prime number.

To solve the task, I created the sub sieve_of_eratosthenes() to clean up the range.

sub sieve_of_eratosthenes {
    my ($i, $range) = @_;

    foreach my $j (sort { $a <=> $b } keys %$range) {
        delete $range->{$j} unless ($j % $i);

    $i  = (sort { $a <=> $b } keys %$range)[0];
    $i += 0 if defined $i;

    return ($i, $range);

Now it is time to create sub find_prime_upto() which takes the help of above subroutine.

sub find_prime_upto {
    my ($sum) = @_;

    die "ERROR: Invalid sum [$sum].\n"
        unless (($sum =~ /^\d+$/) && ($sum > 0));

    my $range = { map { $_ => 1 } 2..$sum };
    my $prime = [];

    my $i = 2;
    while (keys %$range) {
        push @$prime, $i;
        ($i, $range) = sieve_of_eratosthenes($i, $range);
        last unless defined $i;

    return $prime;

Now we have list of prime numbers upto the given $sum. We just to have make all possible unique combinations and find out which combination gives the desired result.

sub prime_sum {
    my ($primes, $sum) = @_;

    my $prime_sum = [];
    foreach my $i (1 .. $sum) {
        last if ($i > @$primes);
        foreach my $comb (combinations($primes, $i)) {
            my $_sum = 0;
            $_sum += $_ for @$comb;
            push @$prime_sum, $comb if ($_sum == $sum);

    return $prime_sum;

Finally print the combinations

sub _print {
    my ($prime_sum) = @_;

    foreach (@$prime_sum) {
        print sprintf("%s\n", join ", ", @$_);

Raku once again just showing-off.

With built-in is-prime, finding all primes up to the given number is like school boy task.

sub find-prime-upto(Int $sum) {
    return (2..$sum).grep: { .is-prime };

Once we have list of prime numbers, we again use combinations to generate possible combinations and then picked the one which gives the end result.

sub prime-sum(Int $sum) {

    my @prime = find-prime-upto($sum);
    my @prime-sum = Empty;
    for 1..$sum -> $i {
        for @prime.combinations: $i -> $j {
            my $_sum = [+] $j;
            @prime-sum.push: $j if $_sum == $sum;

    return @prime-sum;

As always, the final solution is just a wrapper around the helper subroutines.

use strict;
use warnings;
use Algorithm::Combinatorics qw(combinations);

my $SUM = $ARGV[0];

print "USAGE: perl $0 <positive_number>\n" and exit unless defined $SUM;
_print(prime_sum(find_prime_upto($SUM), $SUM));

Raku solution looks clean, I must admit.

use v6.d;

sub MAIN(Int $SUM where $SUM > 0) {

Just basic unit test to complete the task.

use strict;
use warnings;
use Test::More;
use Test::Deep;
use Algorithm::Combinatorics qw(combinations);

is_deeply(prime_sum(find_prime_upto(6), 6),
          "testing prime sum = 6");
is_deeply(prime_sum(find_prime_upto(9), 9),
          [[2, 7]],
          "testing prime sum = 9");
is_deeply(prime_sum(find_prime_upto(12), 12),
          [[5, 7], [2, 3, 7]],
          "testing prime sum = 12");


And Raku unit test as well.

use Test;

is-deeply prime-sum(6).<>,  [],                  "prime sum = 6";
is-deeply prime-sum(9).<>,  [(2, 7),],           "prime sum = 9";
is-deeply prime-sum(12).<>, [(5, 7), (2, 3, 7)], "prime sum = 12";


That’s it for this week. Speak to you soon.


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