The Weekly Challenge ‐ Perl and Raku

CY's Take on The Weekly Challenge #112

If you want to challenge yourself on programming, especially on Perl and/or Raku, go to, code the latest challenges, submit codes on-time (by GitHub or email).

Do tell me, if I am wrong or you strongly oppose my statements!

Task 1: Canonical Path

For the . cases, we can choose to ignore them (grep { $_ ne "."  }).

For the .. cases, naturally they are handled by a stack ‐ pop and push operations in Perl arrays.

For cases with multiple consecutive slashes, we can choose to ignore the empty string (grep { $_ ne "" }).

my $origin = $ARGV[0] || "/a//b/c/../../";

my @directories = grep { $_ ne "" && $_ ne "."  } split "/"$origin;

my @new_dirs = ();

for (@directories) {
    if ($_ ne "..") {
        push @new_dirs$_;
    else {
        pop @new_dirs;

print ( "/" . (join "/"@new_dirs));
print "\n";

The followings are some output of the program, it is good that the Perl interpreter does not complain on pop @empty_array:

$ perl "/../../"
$ perl "/a/"
$ perl "/a/b//c/"
$ perl "/a/b/c/../.."

Task 2: Climb Stairs

If you had studied a bit about the Fibonacci sequence, did the domino-monomino covering problem cross your mind?

Consider a 1-by-n chessbaord. You have plenty of monomino(1×1) pieces and domino(1×2) pieces. How many ways are there to cover the chessboard?

It is a standard exercise of programming to display the n-th Fibonacci number. Let us go further. Can we display the details of the options?

After some thoughts, I realized it can be accomplished by combinations:

use Algorithm::Combinatorics qw/combinations/;
for my $i ($n%2+$n/2 .. $n-1) {
    my $iter = combinations([0..$i-1] , ($n-$i) );
    my $str = "1" x $i;
    while (my $c = $iter->next) {
        my $str_clone = $str;
        substr($str_clone$_1) = "2" for (@{$c});
        push @all$str_clone;

The followings are some output of the program:

$ perl 3
For 3 steps to climb, the number of options is 3:

$ perl 5
For 5 steps to climb, the number of options is 8:

$ perl 10
For 10 steps to climb, the number of options is 89:

As a dessert for this blogpost: a math question popped in my mind: what is the total number of 2's occurs on the options? Then I came to this integer sequence:

    A001629 Self-convolution of Fibonacci numbers.

The recent weekly challenge tasks contain some sophisticated math sequences ‐ refer to FUSC Sequence in Week 104 (Task 1) and Bell Numbers in Week 108 (Task 2).

Besides, I wish I have knowledge and time to write Common Lisp solution(s) for the tasks, but I am stuck on exam preparation. Therefore, leave the "guest language conquer" after May.

Stay alert and healthy! □

link for codes:,

Contact on twitter: @e7_87.

Created Date: 16th May, 2021; last updated: 21:50 HKT