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(tye)Re: Finding all Combinations

by tye (Cardinal)
on Nov 29, 2001 at 10:02 UTC ( #128293=note: print w/ replies, xml ) Need Help??


in reply to Finding all Combinations

Sure. No need to do any recursion here. Just count in binary. Here is a version that creates a closure (an anonymous subroutine that holds the needed data inside of itself -- sort of like a tiny "object") that returns the next combination each time it is called:

sub combinations { my @list= @_; my @pick= (0) x @list; return sub { my $i= 0; while( 1 < ++$pick[$i] ) { $pick[$i]= 0; return if $#pick < ++$i; } return @list[ grep $pick[$_], 0..$#pick ]; }; } my $next= combinations( 50..59 ); my @comb; while( @comb= $next->() ) { # do work with @comb here } # Note that the empty set is a valid combination but is # the last combination returned which also indicates "no # more combinations left. So the above loop doesn't bother # processing the empty list. If you want to process the # empty set, then use: my @comb; do { # do work with @comb here } while( @comb= $next->() );

Update: My code finds combinations but the original code finds permutations even though the author asked for combinations. (See (tye)Re: Permutations if you don't know the difference between the two.)

Of course, my favorite way of finding permutations is Permuting with duplicates and no memory.

        - tye (but my friends call me "Tye")


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Re: (tye)Re: Finding all Combinations
by dragonchild (Archbishop) on Nov 29, 2001 at 19:01 UTC
    I don't understand that algorithm at all. It looks like it's just setting stuff to true so that you use it?

    ------
    We are the carpenters and bricklayers of the Information Age.

    Don't go borrowing trouble. For programmers, this means Worry only about what you need to implement.

Re: (tye)Re: Finding all Combinations
by redbeard (Beadle) on Sep 10, 2002 at 23:48 UTC
    That's fantastic - I was wondering if you could perhaps help out an initiate such as myself and comment that code - what it's doing and why, as I'm having a bit of a hard time following it and would like to learn the methodology behind it (as i believe i was trying to domsething similar but failed completely). Thanks!
      sub combinations { my @list= @_; # List of items to choose from my @pick= (0) x @list; # Whether we want each item # $pick[$i] means include $list[$i] in results. # So @pick currently describes the empty subset. # Return a closure that, each time it is called, returns # the next subset: return sub { # Treat @pick as a base-2 number and increment it. # Note that @pick started as all 0s and we stop # after it is all 1s so all cases get covered. # (See original node for handling the empty subset) # Start at least-significant bit, $pick[0]: my $i= 0; # Increment a bit. If the bit was already 1, then # set it to 0 and continue to next bit: while( 1 < ++$pick[$i] ) { $pick[$i]= 0; # If we've run out of bits, then we were at # all 1s and so are done. Return empty list: return if $#pick < ++$i; } # The grep() below returns the indices for which # $pick[$_] is not 0. The @list[...] is an array # slice that returns the list of elements of @list # at the indices returned by grep. That is, we # return all items $list[$i] where $pick[$i] is # not 0. Same as: # map { $pick[$_] ? $list[$_] : () } 0..$#list; return @list[ grep $pick[$_], 0..$#pick ]; }; } my $next= combinations( 50..59 ); my @comb; while( @comb= $next->() ) { # do work with @comb here }

      Does that help?

              - tye (but my friends call me "Tye")
        Indeed - I'm not as familiar with binary, so I'll have to ponder a bit, but thanks! I was working on this problem and was working on an algorithm that would iterate through all combos, but the checking back to see if you were REALLY done once you had reached the end of a set was killer for large sets (e.g. 30)...
Re^2: Finding all Combinations
by Limbic~Region (Chancellor) on Sep 27, 2004 at 14:06 UTC
    tye,
    I expanded the code I came up with for combinations of a fixed size:
    #!/usr/bin/perl use strict; use warnings; my $iter = combo( 30..50 ); while ( my @combo = $iter->() ) { print "@combo\n"; } sub combo { my @list = @_; return sub { () } if ! @_; my (@position, @stop, $end_pos, $done); my ($by, $next) = (0, 1); return sub { return () if $done; if ( $next ) { $by++; return () if $by > @list; @position = (0 .. $by - 2, $by - 2); @stop = @list - $by .. $#list; $end_pos = $#position; $next = undef; } my $cur = $end_pos; { if ( ++$position[ $cur ] > $stop[ $cur ] ) { $position[ --$cur ]++; redo if $position[ $cur ] > $stop[ $cur ]; my $new_pos = $position[ $cur ]; @position[ $cur .. $end_pos ] = $new_pos .. $new_pos + + $by; } } if ( $position[0] == $stop[0] ) { $position[0] == @list ? $done = 1 : $next = 1; } return @list[ @position ]; } }
    I haven't analyzed its O factor or spent a lot of time with good benchmarks, but it does appear to be considerably faster for combinations of 30..50.

    Cheers - L~R

      I don't think this is really called "combinations" because the number of elements to select isn't specified beforehand. It is really finding all possible subsets, which is the same as finding the power set.

      Another approach to it uses Algorithm::Loops and is very simple; (well, if you understand nesting loops). You loop over 0..$#set finding the first element of the subset, then loop over the next element of the subset ($_+1..$#set), etc:

      use Algorithm::Loops qw( NestedLoops ); sub powerSetGen2 { my $end= shift(@_) - 1; return NestedLoops( [ [ 0..$end ], ( sub { [ $_+1 .. $end ] } ) x $end, ], { OnlyWhen => 1, }, ); } my $size= @ARGV ? shift(@ARGV) : 40; my @set= 1..$size; $|= 1; my $start= time(); my $iter= powerSetGen2( $size ); my @subSet= (); my $count= 0; do { $count++; print "( @subSet )$/" if @ARGV; } while( @subSet= @set[ $iter->() ] ); print "$count subsets for $size in ", time()-$start, " secs.$/";

      Then you can implement this same approach directly (without using the module):

      sub powerSetGen3 { my $end= shift(@_) - 1; my @idx; return sub { if( ! @idx ) { push @idx, 0; } elsif( $idx[-1] < $end ) { push @idx, 1+$idx[-1]; } else { pop @idx; $idx[-1]++ if @idx; } return @idx; }; }

      And this code is so very simple, that I'm at a loss to explain why Limbic~Region's code is a little faster for large sets. His code goes about finding the subsets in a quite different order (and skips one subset) but the routines get called the same number of times and it appears to me that Limbic~Region's would do more work in an average call; but my benchmarks say that I'm wrong.

      - tye        

      Limbic~Region:

      I've finally got it! Thanks for the help you posted on your scratchpad. After a few hours of study, it finally paid off. I've commented it to describe how it works, and made a few changes to fix a minor bug, and remove some code that is never executed, and removed a state variable:

      #------------------------------------------------------------ # Return an iterator of all possible combinations (of all # lengths) of a set of symbols with the constraint that each # symbol in each result is less than the symbol to its right. # sub combo { # The symbols we draw our results from: my @list = @_; # The trivial case return sub { ( ) } if ! @_; # Persistent state for the closure my (@position, # Last set of symbol indices generated @stop); # Last set possible for $by symbols # Start by telling iterator that it just finished # (next=1) all results of 0 digits. my ($by, $next) = (0, 1); return sub {
      # We're done after we've returned a list of all symbols return () if @position == @list;
      if ( $next ) { # We finished all combos of size $by, now do $by+1 $by++;
      # If new size is larger than list, we're done! return () if $by > @list;
      # Start with leftmost $by symbols (except last, # which is preincremented before use) @position = (0 .. $by - 2, $by - 2); # Our stop condition is when we've returned the # rightmost $by symbols @stop = @list - $by .. $#list; $next = undef; } # Start by trying to advance the rightmost digit my $cur = $#position; { # **** redo comes back here! **** # Advance current digit to next symbol if ( ++$position[ $cur ] > $stop[ $cur ] ) { # Keep trying next-most rightmost digit # until we find one that's not 'stopped' $position[ --$cur ]++; redo if $position[ $cur ] > $stop[ $cur ]; # Reset digits to right of current digit to # the leftmost possible positions my $new_pos = $position[ $cur ]; @position[$cur .. $#position] = $new_pos .. $new_pos+$ +by; } } # Advance to next result size when we return last # possible result of this size $next = $position[0]==$stop[0]; return @list[ @position ]; } }
      Thanks again! I learned a lot from this exercise.

      UPDATE: I just tweaked the code a bit to make it check for done less frequently so it'll run a bit quicker. It munges up the code listing a bit though. Is there a better way to edit the code so it's obvious without interspersing download links?

      --roboticus

        roboticus,
        I am glad that you were able to decipher given my description of how the algorithm works. I am interested in knowing what was the minor bug and what code was never executed? If you found this interesting, might I suggest you move on to How A Function Becomes Higher Order and my iterator tutorial.

        Cheers - L~R

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