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Re^5: Derangements iterator (callbacks)

by Roy Johnson (Monsignor)
on Dec 30, 2005 at 18:22 UTC ( #520038=note: print w/replies, xml ) Need Help??

in reply to Re^4: Derangements iterator (callbacks)
in thread Derangements iterator

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;

Caution: Contents may have been coded under pressure.

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Re^6: Derangements iterator (callbacks)
by tye (Sage) on Dec 30, 2005 at 19:11 UTC

    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.

    - tye        

      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.

      Caution: Contents may have been coded under pressure.

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