Perl Style: Use Hashes for Sets

Jan 1, 1998 by Tom Christiansen

• Consider finding the union and intersection of two unique arrays @a and @b:

      foreach $e (@a) { $union{$e} = 1 }
      foreach $e (@b) {
          if ( $union{$e} ) { $isect{$e} = 1 }
          $union{$e} = 1;
      }
      @union = keys %union;
      @isect = keys %isect;
  

• This would be more idiomatically written as:

      foreach $e (@a, @b) { $union{$e}++ && $isect{$e}++ }
      @union = keys %union;
      @isect = keys %isect;
  

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Copyright © 1998, Tom Christiansen All rights reserved.