Every other

[Bob MacCallum]

I clearly have more urgent things to do...

@new = @old[map $_*2, 0..$#old/2];


On Fri, Oct 30, 2009 at 9:21 AM, Bob MacCallum <uncoolbob at gmail.com> wrote:
> If your array is numeric:
>
> use PDL;
> my @old = (1..10);
> my @new = pdl(@old)->slice("0:-1:2")->list;
>
> although this is unlikely to be the most efficient way either (due to
> the conversion between arrays and piddles).
>
>
>
> On Fri, Oct 30, 2009 at 9:05 AM, Mark Fowler <mark at twoshortplanks.com> wrote:
>> Hello,
>>
>> I have in the past coded things to only discover later that someone
>> else has already written what I have toiled away on, only better.  So
>> this time, I'm asking the experts[1] first.
>>
>> I have an array in Perl 5 [2].  I want *every* *other* element from
>> it.  There is, of course, more than one way to do it:
>>
>> my @new;
>> foreach (my $i = 0; $i < @old; $i++) {
>>  push @new, $old[ $i ];
>> }
>>
>> Or
>>
>> my $i;
>> my @new = grep { $i = !$i } @old;
>>
>> Or so on.  None of these are particularly readable, or for that
>> matter, blindly efficient (they still use quite a few ops for such a
>> simple operation)
>>
>> What I would prefer is this:
>>
>> my @new = every_other @old;
>>
>> Which I guess could be generalised like so:
>>
>> i.e.
>>
>> everyother @array;
>> everyother @array, $offset;
>> everyother @array, $offset, $take_how_many;
>> everyother @array, $offset, $take_how_many, $skip_how_many;
>>
>> (with the default being everyother @array, 0, 1, 1)
>>
>> e.g.
>>
>> Ideally this would be a utility in List::MoreUtils or suchlike, but
>> it's not.  Ideally it'd be implemented in C as well as in Perl so that
>> it doesn't burn ops for such a simple idea.
>>
>> Before I get going with the coding, does anyone know of anything else
>> that can do this?
>>
>> Mark.
>>
>> [1] experts on Buffy that is.   Who might also happen to know some Perl.
>> [2] There's very nice syntax for this in Perl 6, isn't there?  I'm not
>> using that language yet.
>>
>
>
>
> --
> http://evolectronica.com - survival of the funkiest
>



-- 
http://evolectronica.com - survival of the funkiest