Using hashes for set operations...

[LanX]

Hi

I'm trying to design a faster and more robust alternative to perlfaq: intersection,union, difference of two arrays.

One of the things I don't like is, that many such examples in the FAQ use the keys of a %count hash as results.

This is fundamentally wrong because keys are stringifications and any reference type will not be reproduced (just think about arrays of objects or a AoH or ...)

So I thought about using $set{STRINGIFICATION}=REALVALUE as as fundamental datastructure for set operations.

And to significantly speed up things I also wanted to avoid any loop constructs, restricting myself to hashslices and list flattening.

use Data::Dump qw(pp);
my @array1=(1..5);
my @array2=(3..7);

my (%union,%intersection,%difference,@tmp);

my (%set1,%set2);
@set1{@array1}=@array1;
@set2{@array2}=@array2;


@tmp= @set1{ keys %set2 };
@intersection{@tmp}=@tmp;     # warning: use of uninitialized values
delete $intersection{""} ;    # dirty hack



%union=(%set1,%set2);


%difference=%union;
delete @difference{keys %intersection};


pp \(%set1,%set2,%union,%intersection,%difference);

prints

Use of uninitialized value $tmp[0] in hash slice at /home/lanx/B/PL/PM
+/set.pl line 17.
Use of uninitialized value $tmp[3] in hash slice at /home/lanx/B/PL/PM
+/set.pl line 17.
(
  { 1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5 },
  { 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7 },
  { 1 => 1, 2 => 2, 3 => 3, 4 => 4, 5 => 5, 6 => 6, 7 => 7 },
  { 3 => 3, 4 => 4, 5 => 5 },
  { 1 => 1, 2 => 2, 6 => 6, 7 => 7 },
)

my problem is the "dirty hack" because hashslice return undef for non-existant keys and undef is later stringified to an empty string. (muting the warnings isn't the problem)

Any elegant idea how to solve this?

Or do I have to fall back to something like

%intersection = map { exists $set1{$_} ? ($_=>$set1{$_}) : () }  keys %set2;

or

 @tmp= grep {defined} @set1{ keys %set2 };

???