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Re: Re: Perl Idioms Explained - keys %{{map{$_=>1}@list}}

by Jasper (Chaplain)
on Aug 04, 2003 at 22:21 UTC ( #280823=note: print w/replies, xml ) Need Help??

in reply to Re: Perl Idioms Explained - keys %{{map{$_=>1}@list}}
in thread Perl Idioms Explained - keys %{{map{$_=>1}@list}}

You can also grep lists for certain 'numbers' of entries (so if you wanted only the items that were in a list twice)
@doubles = grep ++$seen{$_} == 2, @list;

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Re: Re: Re: Perl Idioms Explained - keys %{{map{$_=>1}@list}}
by cees (Curate) on Aug 05, 2003 at 03:32 UTC

    That would be 2 or more times right? Once ++$seen{$_} == 2 is true, you are immediatelly copying $_ to @doubles. So if it appeared a third time, you couldn't magically remove it again. Your code would be clearer if you used >= to emphasize that.

    The following would probably do if you only wanted entries with exactly 2 occurences:

    @doubles = grep $seen{$_} == 2, grep !$seen{$_}++, @list;

    But it is not pretty, or obvious. The first grep counts all occurences and only passes the first found entry to the next grep which will check to see how many were actually found.

    There must be a better way!

    - Cees

Re^3: Perl Idioms Explained - keys %{{map{$_=>1}@list}}
by Aristotle (Chancellor) on Aug 05, 2003 at 17:51 UTC
    No, that won't fly. You need
    $seen{$_}++ for @list; my @doubles = grep $seen{$_} == 2, @list;

    Makeshifts last the longest.

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