Beefy Boxes and Bandwidth Generously Provided by pair Networks
Perl Monk, Perl Meditation

Comment on

( #3333=superdoc: print w/replies, xml ) Need Help??
Update If one needed to allow for deranging a list which contains duplicates, one could simply derange the list of its indices.

Um, not really. A derangement algorithm that "doesn't handle duplicates" is one that deranges "1 4 4" into "4 1 4" because it doesn't notice that the duplicates are the same and so doesn't realize that replacing the last 4 with the second 4 didn't actually cause the last item to be different. It also, when deranging "1 1 2 2" returns "2 2 1 1" four times instead of just once because it doesn't realize that reversing "1 1" (nor "2 2") gives the same thing.

So your first algorithm simply breaks when presented with duplicates (it always finds zero derangements, for those who didn't notice) but your work-around just prevents this breakage while not actually correctly handling the duplicates. (So I'd suggest you document how it doesn't handle duplicates rather than suggest that work-around.)

Just for fun, here is your algorithm modified to correctly handle duplicates:

sub _derange { my( $cb, $av, $todo, @i ) = @_; return $cb->( @$av[@i] ) if ! @$todo; my( %iseen, %vseen ); @iseen{@i}= (); @vseen{@$av[@i]}= @i; my( $range, @todo )= @$todo; for( @$range ) { _derange( $cb, $av, \@todo, @i, $_ ) if ! exists $seen{$_} and ! exists $vseen{$av->[$_]} || $vseen{$av->[$_]} < $_; } } sub derange(&@) { my $cb= shift @_; _derange( $cb, \@_, [ map { my $x = $_[$_]; [ grep { $_[$_] ne $x } 0..$#_ ] } 0..$#_ ], ); } derange( sub { print "@_\n" }, 1,1,2,2,3 );

Of course, if you try to look up a definition for "derangement", you won't find anything that makes much sense when considering duplicates because mathematicians define derangements in term of permutations which they define without considering duplicates either (though they usually don't use language that actually makes that clear, either).

But the extension of these two concepts to cover lists with duplicate elements is natural, even obvious, as well as useful.

- tye        

In reply to Re^2: Derangements iterator (duplicates) by tye
in thread Derangements iterator by tye

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • 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! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, 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, 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:
    & &amp;
    < &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.
  • Log In?

    What's my password?
    Create A New User
    and all is quiet...

    How do I use this? | Other CB clients
    Other Users?
    Others having an uproarious good time at the Monastery: (5)
    As of 2018-05-27 16:13 GMT
    Find Nodes?
      Voting Booth?