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Re^5: Create unique array --the hard way!

by Anonymous Monk
on Mar 08, 2014 at 10:19 UTC ( #1077506=note: print w/replies, xml ) Need Help??

in reply to Re^4: Create unique array --the hard way!
in thread Create unique array --the hard way!

Ah ok, I thought slice would not involve pushing the unique elements into a new array, rather remove them "as-you-go" if the array is sorted, since they would be next to each other...

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Re^6: Create unique array --the hard way!
by kcott (Bishop) on Mar 08, 2014 at 20:13 UTC

    Stepping through an array and making changes "as-you-go" has unforeseen problems. We get quite a few "Why didn't this work?" questions related directly to attempting to do this type of thing.

    The issue is actually documented in "perlsyn: Foreach Loops" and specifically mentions splice:

    "If any part of LIST is an array, foreach will get very confused if you add or remove elements within the loop body, for example with splice. So don't do that."

    [In case you didn't know, for and foreach are synonymous.]

    So, you'd need to do something like the following.

    sub splice_unique2 { my @sorted = sort qw{q w e r t y q w e r t y}; my $last = ''; my @unique = @sorted; my $removed = 0; for (0 .. $#sorted) { if ($sorted[$_] eq $last) { splice @unique, $_ - $removed++, 1; } else { $last = $sorted[$_]; } } }

    I plugged that in to my earlier benchmarking script. Here's a representative result:

    Rate splice_unique2 splice_unique shift_unique +map_unique splice_unique2 83836/s -- -10% -15% + -28% splice_unique 92839/s 11% -- -6% + -20% shift_unique 99211/s 18% 7% -- + -14% map_unique 115924/s 38% 25% 17% + --

    My coding of splice_unique2() was really just intended to show a way to get around the documented issue. If you think the task has any merit, feel free to try to rewrite it to run faster.

    -- Ken

      Thank you Ken, will read up on it :)

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