How embarrassing. My first version of problem 9 (packing lists into sublists) had a bug. Here's a cleaner version which actually works. I know the bug in problem 9. Can you spot it?

sub pack (@array) returns Array {
    my @unpacked = @array;
    my (@list, @packed);
    while @unpacked {
        @list.push(@unpacked.shift) while !@list || @list[0] eq @unpac
+ked[0];
        @packed.push([@list]);
        @list = ();
    }
    return @packed;
}
pack(<a a a a b c c a a d e e e e>).perl.say;
[download]

And for the Lisp weenies who claim Lisp is so much better, here's one way to do it in Lisp (can you make it shorter?):

(defun pack (lista)
    (if (eql lista nil) 
        nil
        (cons (pega lista) (pack (tira lista)))
    )
)

(defun pega (lista)
    (cond ((eql lista nil) nil)
      ((eql (cdr lista) nil) lista)
          ((equal (car lista) (cadr lista))
              (cons (car lista) (pega (cdr lista))))
          (t (list (car lista)))
    )
)

(defun tira (lista)
    (cond ((eql lista nil) nil)
      ((eql (cdr lista) nil) nil)
          ((equal (car lista) (cadr lista))
              (tira (cdr lista)))
          (t (cdr lista))
    )
)
[download]

But the Prolog folks still have a neat solution:

pack([],[]).
pack([X|Xs],[Z|Zs]) :- transfer(X,Xs,Ys,Z), pack(Ys,Zs).

transfer(X,[],[],[X]).
transfer(X,[Y|Ys],[Y|Ys],[X]) :- X \= Y.
transfer(X,[X|Xs],Ys,[X|Zs]) :- transfer(X,Xs,Ys,Zs).
[download]

And Haskell still makes us all look like chumps:

group (x:xs) = let (first,rest) = span (==x) xs
               in (x:first) : group rest
group [] = []
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.