This is Weekly Challenge #203

[203 - Non-Authoritative Information](

Task 1: Special Quadruplets

Submitted by: Mohammad S Anwar
You are given an array of integers.

Write a script to find out the total special quadruplets for the given array.

Special Quadruplets are such that satisfies the following 2 rules.

  • nums[a] + nums[b] + nums[c] == nums[d]
  • a < b < c < d


I tripped up on first pass, thinking that it was nums[a] < nums[b] < nums[c] < nums[d], which makes several of the examples wrong. Silly me.

I believe I could’ve made an iterative version of this, but the simpler take, to me was to think…

This looks like a job for Recursion!


There are two end cases:

  • we have four results and the sum of the first three equals the fourth
  • we have run off the end of the array and can’t throw on more numbers

We have to test them in that order, rather than do the tests before we call the next. I mean, we could, but …

So, with the calling, we pass the array we’re testing as a reference (so it could be of any size), we add one to the index, and we either add the number in the current position or not.

I also used to allow a verbose flag.

Show Me The Code!

#!/usr/bin/env perl

use strict;
use warnings;
use experimental qw{ say postderef signatures state };

use List::Util qw{ sum0 };
use Getopt::Long;

my $verbose = 0;

    'verbose'  => \$verbose,

my @examples = (

    [ 1, 2, 3, 6 ],
    [ 1, 1, 1, 3, 5 ],
    [ 3, 3, 6, 4, 5 ]

for my $e (@examples) {
    my $list = join ',', $e->@*;
    my $out  = special_quadruplets($e);
    say <<"END";
    Input:  \@array = ($list)
    Output: $out

sub special_quadruplets ( $arrayref, $pos = 0, $resultref = [] ) {
    my $output;

    # case: resultref is the right size
    if ( scalar $resultref->@* == 4 ) {
        my @results = $resultref->@*;
        my $sum     = sum0 @results[ 0, 1, 2 ];
        if ( $sum == $results[3] ) {
            say join ' ', @results if $verbose;
            return 1;
        return 0;

    # case: we've run out of data
    return 0 if !defined $arrayref->[$pos];

    my $newref = [];
    $newref->@* = $resultref->@*;
    $output +=
        special_quadruplets( $arrayref, $pos + 1, $newref ); # don't include this
    push $newref->@*, $arrayref->[$pos];
    $output +=
        special_quadruplets( $arrayref, $pos + 1, $newref );    # include this
    return $output;
$ ./ -v
1 2 3 6
    Input:  @array = (1,2,3,6)
    Output: 1

1 1 3 5
1 1 3 5
1 1 3 5
1 1 1 3
    Input:  @array = (1,1,1,3,5)
    Output: 4

    Input:  @array = (3,3,6,4,5)
    Output: 0

Task 2: Copy Directory

Submitted by: Julien Fiegehenn
You are given path to two folders, $source and $target.

Write a script that recursively copy the directory from $source to $target except > any files.


So, there are two parts: making the directories and traversing the directories. Given a directory you desire, mkdir $path_of_directory will do. So, the trick is to traverse $source, and, in each directory, find every new directory, create every directory within that directory, then go into that directory and do it again. This involves fun with opendir and directory handles and keeping track of your paths.

(I keep thinking that it would be fun and/or useful to make directory spider that takes a dispatch table, so if it finds files like this or directories like that, it would run this or that funcion on that node. The idea is still in my head, but I don’t have a strong idea for a thing I would want to do that I can’t do by abusing ls -R or find . | grep <whatever>, which yeah, is a beginner’s use of find. I never remember xargs or -exec.)

(Besides, the core of a directory spider is so quick and easy to implement that it’s easier to rewrite from scratch than make the one using subroutine refs.)

Anyway, I don’t move forward on three tests.

  • it starts with a ., which marks it as the meta “this directory”, the meta “parent directory”, or a hidden file or directory. I suppose we could want to create hidden directories, but whatever.
  • the source directory exists and is a directory (we’re not copying files or creating zero-length files of the same name, only directories)
  • the target directory exists. As I write this, I’m second-guessing myself, but if we’re making the x/y directory new, then it shouldn’t matter.

I could’ve used chdir, but I usually find it’s easier to deal with paths than try to keep track of the current directory.

Again, I used to allow arbitrary source and target paths, plus use the verbose flag. I believe it was Perl Best Practices that said you should use one standard Getopt library, and that it probably should be Getopt::Long. The book’s this far out of arm’s reach and I don’t want to stand up to look it up. You understand.

Also, This looks like a job for Recursion!

Show Me The Code!

#!/usr/bin/env perl

use strict;
use warnings;
use experimental qw{ say postderef signatures state };

use Getopt::Long;

my $source  = './a/b/c';
my $target  = './x/y';
my $verbose = 0;

    'source=s' => \$source,
    'target=s' => \$target,
    'verbose'  => \$verbose,

copy_directory( $source, $target );

sub copy_directory ( $source, $target ) {
    return unless -d $source;
    return unless -d $target;
    my @dirs;
    if ( opendir my $dh, $source ) {
        for my $f ( sort readdir $dh ) {
            my $s = join '/', $source, $f;
            my $t = join '/', $target, $f;
            next if $f =~ /^\./;
            next unless -d $s;
            next if -d $t;
            mkdir $t;
            copy_directory( $s, $t );
            say join ' => ', $s, $t if $verbose;
$ ./ -s ~/bin -v
/home/jacoby/bin/Backup => ./x/y/Backup
/home/jacoby/bin/Capital => ./x/y/Capital
/home/jacoby/bin/Fitbit => ./x/y/Fitbit
/home/jacoby/bin/LastFM => ./x/y/LastFM
/home/jacoby/bin/Old/Test/Data => ./x/y/Old/Test/Data
/home/jacoby/bin/Old/Test => ./x/y/Old/Test
/home/jacoby/bin/Old => ./x/y/Old
/home/jacoby/bin/Out_Of_Rotation/Old/Test/Data/Other => ./x/y/Out_Of_Rotation/Old/Test/Data/Other
/home/jacoby/bin/Out_Of_Rotation/Old/Test/Data => ./x/y/Out_Of_Rotation/Old/Test/Data
/home/jacoby/bin/Out_Of_Rotation/Old/Test => ./x/y/Out_Of_Rotation/Old/Test
/home/jacoby/bin/Out_Of_Rotation/Old => ./x/y/Out_Of_Rotation/Old
/home/jacoby/bin/Out_Of_Rotation/TESTING => ./x/y/Out_Of_Rotation/TESTING
/home/jacoby/bin/Out_Of_Rotation/moo-test => ./x/y/Out_Of_Rotation/moo-test
/home/jacoby/bin/Out_Of_Rotation/test => ./x/y/Out_Of_Rotation/test
/home/jacoby/bin/Out_Of_Rotation => ./x/y/Out_Of_Rotation
/home/jacoby/bin/SGT => ./x/y/SGT
/home/jacoby/bin/Toys/Dict => ./x/y/Toys/Dict
/home/jacoby/bin/Toys/Dict_old => ./x/y/Toys/Dict_old
/home/jacoby/bin/Toys => ./x/y/Toys
/home/jacoby/bin/imap => ./x/y/imap
/home/jacoby/bin/lib => ./x/y/lib
/home/jacoby/bin/shell_scripts => ./x/y/shell_scripts
/home/jacoby/bin/wordgames => ./x/y/wordgames

Maybe I should clean out a lot of that stuff.

If you have any questions or comments, I would be glad to hear it. Ask me on Mastodon or make an issue on my blog repo.