in reply to Clockwise or Counter-clockwise

sub direction{
     local $_;
     push @_, $_[0];
     $_ += $a->[0] * $b->[1] 
         - $a->[1] * $b->[0] 
           while ($a,$b)=(shift,$_[0]), @_;
     $_ < 0;
}
my @unitCubeAnti = ([0,0],[0,1],[1,1],[1,0]);
print direction(@unitCubeAnti) ? 'Anticlockwise' : 'Clockwise';
my @unitCubeClock = reverse @unitCubeAnti;
print direction(@unitCubeClock) ? 'Anticlockwise' : 'Clockwise';

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The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.

Replies are listed 'Best First'.
Re: Re: Clockwise or Counter-clockwise
by tall_man (Parson) on Feb 20, 2003 at 19:18 UTC
    Your example will not run as given because it destroys the @unitCubeAnti within the direction subroutine. I suggest copying the input list to a local variable first: 
    sub direction{
     local $_;
     my (@poly) = @_;
     push @poly, $poly[0];
     $_ += $a->[0] * $b->[1] 
         - $a->[1] * $b->[0] 
           while ($a,$b)=(shift @poly,$poly[0]), @poly;
     print "result is $_\n";
     $_ < 0;
}
my @unitCubeAnti = ([0,1],[1,1],[0,0],[1,0]);
print direction(@unitCubeAnti) ? 'Anticlockwise' : 'Clockwise';
print "\n";
my @unitCubeClock = reverse @unitCubeAnti;
print direction(@unitCubeClock) ? 'Anticlockwise' : 'Clockwise';
print "\n";

    [download]
    I tested a self-crossing unit "cube" in the input instead of a straight one, and got the result "0". This refutes my idea that self-crossing polygons will work with this algorithm.
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