For the first time, since I started participating the weekly challenges, I thought of doing one-liner. With handy CPAN modules, it was pretty straight forward in Perl. Even Raku with built-in features wasn’t far behind Perl. Like in the past, I learn something new in Raku every week. This week was no different. I will share what I learnt this time later.

PERL

Task #1: kth Permutation Sequence

The CPAN module Algorithm::Combinatorics is becoming very popular and favourite among Perl Hackers. For me the star module was List::Util, it gave me handy function reduce to calculate the factorial. I didn’t want to reinvent the wheel. In fact, I didn’t know that there was such thing available. It was big find for me personally. I found the syntax is out of this world to calculate the factorial. Do you think I am joking, check it out yourself.

To calculate the factorial of $n, you would have to do something like below:

reduce { $a * $b } 1 .. $n

Please check out the documentation for more informations.

I used permuations() from Algorithm::Combinatoric to do the main task.

Please check out the documentation for more informations.

#!/usr/bin/perl

use strict;
use warnings;

use List::Util qw(reduce);
use Algorithm::Combinatorics qw(permutations);

my $n = $ARGV[0];
my $k = $ARGV[1];

die "ERROR: Missing digit count.\n"        unless defined $n;
die "ERROR: Missing sequence number.\n"    unless defined $k;
die "ERROR: Invalid digit count $n.\n"     unless $n > 0;
die "ERROR: Invalid sequence number $k.\n" unless $k > 0 && ($k <= reduce { $a * $b } 1 .. $n);

print [ map { join "", @$_ } permutations([ 1..$n ]) ]->[$k-1];

Task #2: Collatz Conjecture

There is hardly anything to talk about in this solution. What you see here is just out of text book.

#!/usr/bin/perl

use strict;
use warnings;

my $n = $ARGV[0];
die "ERROR: Missing collatz starting number."    unless defined $n;
die "ERROR: Invalid collatz starting number $n." unless $n > 0;

my @collatz = ($n);
while ($n != 1) {
    $n = ($n % 2 == 0) ? ($n / 2) : ((3 * $n) + 1);
    push @collatz, $n;
}

print sprintf("%s\n", join " -> ", @collatz);

RAKU

Task #1: kth Permutation Sequence

This is the shortest Raku solutions I have ever came up with. Although it has 3 lines, each line is special to me.

I love the ability to define the parameters with checks. It is self explanatory. It lets you focus on the main task and not worry about input parameters.

sub MAIN(Int $n where * > 0, Int $k where * > 0) {

The following line does special check on $k. I found out later that this can be combined in the definition of MAIN(). However the crucial bit is to calculate the factorial like [*] 1..$n.

die “ERROR: Invalid sequence number $k.” unless $k <= [*] 1..$n;

It is time to do main task. Using the built-in permutations(), it becomes a piece of cake. What I loved the most is the use of map() to join the result of permutations().

say ( 1..$n ).permutations.map({ .join })[$k -1];

The final solution looks like below:

#!/usr/bin/env perl6

use v6.c;

sub MAIN(Int $n where * > 0, Int $k where * > 0) {
    die "ERROR: Invalid sequence number $k." unless $k <= [*] 1..$n;
    say ( 1..$n ).permutations.map({ .join })[$k -1];
}

Task #2: Collatz Conjecture

For someone new to Raku might find is copy in the MAIN() as something new. As you have guessed it already, it simply makes the parameter $n readwrite and not just readonly. By default all parameters are readonly, if I am not mistaken.

Also I have used multi keyword of Raku for the first time. It was ideal for the task in hand. It is self explanatory as well.

Please check out the documentation for more informations.

#!/usr/bin/env perl6

use v6.c;

sub MAIN(Int $n is copy where $n > 0) {
    my @collatz = ($n);
    while $n != 1 {
        $n = collatz-sequence($n);
        push @collatz, $n;
    }
    say @collatz.join(" -> ");
}

multi sub collatz-sequence(Int $n where $n  %% 2) { $n div 2   }

multi sub collatz-sequence(Int $n where $n !%% 2) { 3 * $n + 1 }

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