Comments on: Perl 6 and the Josephus problem

By: Konstantin

Konstantin — Thu, 07 Jan 2010 22:46:25 +0000

BTW, I copy-pasted your example, it fail with

Cannot assign to readonly variable.
in method Person::createChain (file , line )
called from Main (file ./test2.p6, line 0)

I think that this line

has ($.position, $.succ is rw);

does not define position as writable

By: Daniel

Daniel — Sat, 13 Jun 2009 15:53:20 +0000

Perl 6 is now going to have a “.rotate” method. So you will be able to solve the Josephus problem with:

my @p = 1..40;
@p.rotate(2).shift while @p > 1;
say @p

… which is both cool, and much easier to read than the push+splice version. You can readily see how the algorithm works (rotate by 2, remove 1, repeat).

By: programmingpraxis

programmingpraxis — Sat, 13 Jun 2009 15:03:30 +0000

I solved this problem in Scheme at my blog. See http://programmingpraxis.wordpress.com/2009/02/19/flavius-josephus/.

By: Daniel

Daniel — Sat, 13 Jun 2009 10:59:02 +0000

Hi wtd,

Yes, indeed. And “attr_accessor” reads better too. And with that change, Ruby becomes the shortest by character count without becoming any less legible. I’ll add a note to that effect on the post so everyone can see it.

Btw, I took the liberty of adding

tags to your post so that the code sample reads better.

By: wtd

wtd — Fri, 12 Jun 2009 16:45:10 +0000

For what it's worth, there is a simple way to make the Ruby sample marginally shorter.

class Person
	# Accessor and mutator methods
	attr_reader :position, :succ, :alive
	attr_writer :position, :succ, :alive

	# Initialize
	def initialize(pos)
		@position = pos
		@alive = true
	end

Could be rewritten using attr_accessor.

class Person
	# Accessor and mutator methods
	attr_accessor :position, :succ, :alive

	# Initialize
	def initialize(pos)
		@position = pos
		@alive = true
	end

By: Daniel

Daniel — Fri, 12 Jun 2009 16:26:58 +0000

Larry W. has proposed a more compact solution:

my @p = 1..40;
while @p > 1 {
    @p.push(@p.splice(0,2));
    @p.shift
};
say @p