On Aug 15, 2006, at 2:28 PM, SFantar wrote:
Thank for your quick reply.
Your solution should suit me.
Can you give me more details/ explanations about your code?
What does your code do exactly?
Sure, we are using a hash table as a way to deal with duplicates,
which is a standard idiom in Perl.
The idea is to use the elements you need to process as keys of a hash
table. Hash tables held just one entry per key, so if you reassign to
a key the entry in the hash table is overwritten, so to speak, not
repeated. That's the way hash tables work, and we leverage that
property.
For instance this is a way to count how many times each element of an
array is repeated:
my %counters = ();
$counters{$_} += 1 for @array;
After that code the keys of %counters are the distinct elements of
@array, and the hash associates the number of occurrences of each one
in @array. Do you understand that example?
In the example I sent before, we used a similar trick but using grep,
because we want to iterate over @array and filter duplicates out (you
need to know grep to understand this):
%seen = ();
@uniq = grep {
my $is_new = 0;
if not exists $seen{$_} {
$seen{$_} = 1;
$is_new = 1;
}
$is_new;
} @array;
That code is more explicit. We use a hash table called %seen to
register seen strings. The way to accomplish that is that we assign
an arbitrary value to $seen{john} the first time we encounter 'john',
and so subsequent calls to exists $seen{john} will return true. In
those cases $is_new remains 0 and grep filters that element out.
I wrote a condensed version of an analogous way to work that out.
-- fxn
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