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Questions and Answers
Amy Rich
October 2001
Q: I have an array in Perl with a large number of duplicate
entries. How do I remove the duplicates?
A: This is actually answered in the perldoc information. Run the following
on a system that has Perl installed:
perldoc -q duplicate
And you get the following output:
How can I remove duplicate elements from a list or array?
There are several possible ways, depending on whether the array is ordered
and whether you wish to preserve the ordering. (this assumes all true values
in the array)
a) If @in is sorted, and you want @out to be sorted:
$prev = 'nonesuch';
@out = grep($_ ne $prev && ($prev = $_), @in);
This is nice in that it doesn't use much extra memory, simulating uniq(1)'s
behavior of removing only adjacent duplicates. It's less nice in that it won't
work with false values like undef, 0, or ""; "0 but true" is OK, though.
b) If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$_}++, @in);
c) Like (b), but @in contains only small integers:
@out = grep(!$saw[$_]++, @in);
d) A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
integers:
e) Like (d), but @in contains only small positive
undef @ary;
@ary[@in] = @in;
@out = grep {defined} @ary;
But perhaps you should have been using a hash all along, eh?
Q: I'm from a Sun/Solaris background, but my new position is at an
HP shop. Is there HP hardware comparable to the Starfire?
A: HP's top of the line machine is the Superdome. In a single node
it can have up to 64-way 550 MHz, 4-way PA-8600 CPUs with 1.5
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