FAQ 4.41: How can I remove duplicate elements from a list or array?

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4.41: 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.

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

            $prev = "not equal to $in[0]";
            @out = grep($_ ne $prev && ($prev = $_, 1), @in);

        This is nice in that it doesn't use much extra memory, simulating
        uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
        guarantees that the expression is true (so that grep picks it up)
        even if the $_ is 0, "", or undef.

    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 = ();
            @out = sort keys %saw;  # remove sort if undesired

    e)  Like (d), but @in contains only small positive integers:

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

    But perhaps you should have been using a hash all along, eh?


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