Re: Find number of unique values in hash


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Re: Find number of unique values in hash

by jethro (Monsignor)

on Jan 19, 2010 at 22:11 UTC ( [id://818308]=note: print w/replies, xml )

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in reply to Find number of unique values in hash

I don't see a faster way for this. But if you need to count often, you could generate a second HoH for a one-time cost and have that and all other counts for no cost at all after that.

my %counts=();
foreach $kid (%hoh) {
  foreach my $attribute (%$kid) {
    $counts{$attribute}{$kid->{attribute})++;
  }
}

...
print $counts{'age'}{12};
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Sadly this "one-time" cost has to be paid whenever the data changes (unless you also adjust the counts in the second HoH whenever you change anything)

UPDATE: Fixed the bug in the first line thanks to warnings from johngg and chromatic. It was too much to hope that I could write even a 4-liner without a trivial bug.

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Re^2: Find number of unique values in hash
by johngg (Canon) on Jan 20, 2010 at 00:04 UTC

my %counts={};

I think you need parentheses rather than curlies there. As it is, you are assigning a single hash reference to the hash, which it will then stringify and use as a key with no corresponding value. Also, if you use warnings;, I think you will get one complaining about odd number of elements in hash assignment, or words to that effect. Trying it I get this.

$ perl -MData::Dumper -Mstrict -wle '
> my %h = {};'
> print Data::Dumper->Dump( [ \ %h ], [ qw{ *h } ] );'
Reference found where even-sized list expected at -e line 2.
%h = (
       'HASH(0x817f880)' => undef
     );
$
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I hope this is useful.

Cheers,

JohnGG

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