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Re^2: Perl's feature to determine, in current point of loop, that this is the last one?

by LanX (Sage)
on Jan 24, 2022 at 02:19 UTC ( #11140764=note: print w/replies, xml ) Need Help??


in reply to Re: Perl's feature to determine, in current point of loop, that this is the last one?
in thread Perl's feature to determine, in current point of loop, that this is the last one?

> You could always watch for the last value in the list and test for that ..

this only works if the last value is unique

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

  • Comment on Re^2: Perl's feature to determine, in current point of loop, that this is the last one?

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Re^3: Perl's feature to determine, in current point of loop, that this is the last one?
by talexb (Chancellor) on Jan 24, 2022 at 02:58 UTC

    Absolutely right -- I knew that my approach had flaws, but the initial question was so vague I figured I'd throw my solution into the mix. The OP never explained why this feature was necessary .. it sounds more like a feature looking for a use case to me.

    Alex / talexb / Toronto

    Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

      WRT use case: one of the (admittedly few) places I've seen something like this (which is why I mentioned the iterator module from TT) is in templating where you want to "decorate" the first or last item in a list differently than other items. Think maybe pagination where displaying a chunk from the middle and you want those items to have a link to the next or subsequent sets of results (terrible handwavy sample follows, and yes you could do similar stuff with JS or CSS instead).

      <table> [% FOREACH item IN current_chunk %] <tr> [% IF loop.first %]<td> &lt;&lt; </td>[% ELSE %]<td>&nbsp;</td>[% END +%] <td>[% item %]</td> [% IF loop.last%]<td> &gt;&gt; </td>[% ELSE %]<td>&nbsp;</td>[% END %] </tr> [% END %] </table>

      Edit: And I realized I hadn't linked to the TT FOREACH docs which explain the loop magic (which is Template::Iterator under the hood).

      The cake is a lie.
      The cake is a lie.
      The cake is a lie.

        Yeah, that's doable. I can also think of doing it without having everything inside the loop ..

        #!/usr/bin/perl use strict; use warnings; # 2022-0125: Handle First and List iems differently that everything # in between .. { my @keys = qw/q w e r t y u i o p/; print "First is $keys[0] ..\n"; foreach my $k ( @keys[1..(scalar @keys -2)] ) { print "In the middle: $k\n"; } print "Finally, we have $keys[-1].\n"; }
        This produces the output
        First is q .. In the middle: w In the middle: e In the middle: r In the middle: t In the middle: y In the middle: u In the middle: i In the middle: o Finally, we have p.
        But I like your solution of using an Iterator -- then it has the job of handling the 'first' and 'last' issues.

        Alex / talexb / Toronto

        Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.

      It's so easy to avoid the limitation. Replace $v eq $values[-1] with \$v == \$values[-1]

        > It's so easy to avoid the limitation.

        unfortunately no, but good idea nonetheless ... :)

        DB<39> x @input 0 2 1 7 2 11 3 15 DB<40> use experimental 'refaliasing'; \$input[1] = \$input[-1] DB<41> x @input 0 2 1 15 2 11 3 15 DB<42> for my $v (@input) { say $v if \$v==\$input[-1] } 15 15 DB<43>

        I'm not even sure that experimental feature is even needed...

        IIRC one could somehow use typeglob aliasing on array elements.

        Cheers Rolf
        (addicted to the Perl Programming Language :)
        Wikisyntax for the Monastery

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