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How can I extract just the unique elements of an array?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:20 UTC ( #609=perlfaq nodetype: print w/ replies, xml ) Need Help??

Current Perl documentation can be found at perldoc.perl.org.

Here is our local, out-dated (pre-5.6) version:

There are several possible ways, depending on whether the array is ordered and whether you wish to preserve the ordering.

a) If @in is sorted, and you want @out to be sorted: (this assumes all true values in the array)

    $prev = 'nonesuch';
    @out = grep($_ 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
e) Like (d), but @in contains only small positive integers:

    undef @ary;
    @ary[@in] = @in;
    @out = @ary;

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