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Perl: Finding non-duplicate elements

By MaryWeilage Editor ·
The June 3 Perl e-newsletter described how to find non-duplicate elements in a list. Will you use this technique to identify and remove duplicate elements in a list? Is there another technique you prefer? Please share what method works for you.

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There is more than one way to do it

by swstephe In reply to Perl: Finding non-duplica ...

As always, there is more than one way to do it. I would think that using grep would be more efficient than using a hash table with flags. In that case, what you want is to get a list of all elements in a2 that don't exist in a1. I wish I rememberhow to alias the $_ parameter so I could do it in two nested grep's, but that might be less readable. The complementary common function does the same, but with one small switch in the logic.

@a1 = qw(one two three four five six);
@a2 = qw(one three five seven nine);

# what is in common -> @c
foreach $a (@a2) { push @c,$a if grep {$a eq $_} @a1; }
print "\@c = (",join(',',@c),")\n";

# what is different -> @d
foreach $a (@a2) { push @d,$a unless grep {$a eq $_} @a1; }
print "\@d = (",join(',',@d),")\n";

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by zkent In reply to There is more than one wa ...

Your method is far more complex than the method in the article. Is there an advantage by using your method?

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large lists = heavy memory load

by zkent In reply to Perl: Finding non-duplica ...

I used this technique once on a list of several thousand email recipients. I loaded their user_ids into a @found array so that I would not send the email twice to the same person. but by the time the list loaded into the array (it was being loaded from a flatfile), the server killed the process. I had to find another method since arrays are memory intensive. For short lists, it is an easy, effective method.

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