Beefy Boxes and Bandwidth Generously Provided by pair Networks
go ahead... be a heretic
 
PerlMonks  

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 ( #11140768=note: print w/replies, xml ) Need Help??


in reply to Re^2: 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?

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.

  • Comment on Re^3: Perl's feature to determine, in current point of loop, that this is the last one?

Replies are listed 'Best First'.
Re^4: Perl's feature to determine, in current point of loop, that this is the last one?
by Fletch (Bishop) on Jan 25, 2022 at 17:05 UTC

    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.

        Edge case behaviour might need some changes:

        #!/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/; my @keys = qw/q/; 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"; }
        B:\>perl 11140865-single.pl First is q .. Finally, we have q. B:\>

        Alexander

        --
        Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
Re^4: Perl's feature to determine, in current point of loop, that this is the last one?
by ikegami (Patriarch) on Jan 24, 2022 at 04:01 UTC

    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

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

        Good catch. I'll add a warning to my answer that first suggested the idea.

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

        Correct.

        my $inputs = sub { \@_ }->( 0, $x, $x ); for my $v (@$inputs) { say \$v; }
        SCALAR(0x559bb5b21960) SCALAR(0x559bb5b33228) SCALAR(0x559bb5b33228)

        I imagine that would Data::Alias can also achieve this.

Log In?
Username:
Password:

What's my password?
Create A New User
Domain Nodelet?
Node Status?
node history
Node Type: note [id://11140768]
help
Chatterbox?
and the web crawler heard nothing...

How do I use this? | Other CB clients
Other Users?
Others musing on the Monastery: (4)
As of 2022-05-23 12:23 GMT
Sections?
Information?
Find Nodes?
Leftovers?
    Voting Booth?
    Do you prefer to work remotely?



    Results (82 votes). Check out past polls.

    Notices?