Re^4: Derangements iterator (callbacks)

[tye]

in reply to Re^3: Derangements iterator (callbacks)
in thread Derangements iterator

I didn't have to rewrite my iterator, just use it. I believe you are pointing out ways to make it easier to rewrite a naive recursive solution so that it can be re-implemented as an iterator instead.

By "Try to go the other way" I meant (at least in part), try to use jdporter's callback interface (as-is) in a situation where an iterator is needed.

Though, I don't even understand what you linked to yet so I still consider what I did to go from iterator to callback to be "trivial" compared to what you are proposing as "not too difficult". (:

[Roy Johnson]

I will agree that there's a distinct difference between "trivial" and "not too difficult" that applies to converting between iterators and recursive solutions. However, jd and I found it easier to write recursive solutions, so there's a trade-off in difficulty: It might be easier en toto to come up with a recursive one and then convert it to an iterator.

Not even understanding jdporter's algorithm, I converted it to be iterative (or perhaps more precisely, lazily evaluated). I made the derange function print only the first 15 results, for convenient testing of large inputs.

sub _derange_iter {
  my ($cb, $todo, @v) = @_;
  @$todo or return do { $cb->( @v ); sub {} }; # this line was wrong b
+efore
  my %seen;
  @seen{@v} = ();
  my ( $range, @todo ) = @$todo;
  my @sub_iter = map {
    my $my_ = $_;
    sub { _derange_iter ( $cb, \@todo, @v, $my_ ) }
  } grep { ! exists $seen{$_} } @$range;
  return sub {} unless (@sub_iter);
  # Grab and unwrap an iterator from the list
  my $iter = (shift @sub_iter)->();
  return sub {
      my $rval;
      $iter = (shift @sub_iter)->()
        until ($rval = $iter->() or @sub_iter == 0);
      return $rval;
  }
}

sub derange(&@)
{
    my $cb = shift;
    my $iter = _derange_iter( $cb,
        [ map { my $x = $_; [ grep { $_ ne $x } @_ ] } @_ ] );
    for (1..15) {
      $iter->();
    }
}

derange { print "@_\n" } @ARGV;

[tye]

But the iterator already existed. I find it easier to use the iterator to make a callback then to reimplement the algorithm recursively... (:

But enough good-natured intentionally talking at cross purposes... I was curious in what order your program produced the derangements and it told me:

b a d c
b a d c
b a d c
b a d c
...

So I'll still keep my iterator-from-the-get-go. ;)

Update: If this "not too difficult" technique is too difficult for its own creator to correctly apply... And the point still stands that using an iterator via a call-back interface is trivial while using a call-back interface via an interator isn't even possible in stock Perl 5.

[Roy Johnson]

Notwithstanding the fact that I bungled the code somewhere...what you call "my program" is effectively a translation of jdporter's program. Translated properly, it will return exactly the same results in exactly the same order. I've located my mistake and fixed.