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qw with anonomous list ref

by John M. Dlugosz (Monsignor)
on Oct 28, 2002 at 22:36 UTC ( #208626=perlquestion: print w/replies, xml ) Need Help??
John M. Dlugosz has asked for the wisdom of the Perl Monks concerning the following question:

Which is better:
[qw(a b c)] \qw(a b c)
I was surprised to see that, with Deparse, they produced different internal parses, and that the bottom one did what I thought the top one would, and the top one seemed optimized or more direct.

What is the real distinction in the internal representations, and is it meaningful for anything?

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Re: qw with anonomous list ref
by Ovid (Cardinal) on Oct 28, 2002 at 22:54 UTC

    The bottom one creates a list of references, not a reference to a list.

    C:\>perl -e "($x,$y)=\qw(a b);print $$y" b


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      Ooff, just like I thought. I would have been really confused if it hadn't been this way.

      BTW, OP: an excellent helper tool to see what Perl does to your data, is the standard module Data::Dumper:

      use Data::Dumper; print Dumper [qw(a b c)], \qw(a b c);
      $VAR1 = [ 'a', 'b', 'c' ]; $VAR2 = \'a'; $VAR3 = \'b'; $VAR4 = \'c';
      The "$VARn" notation is the default variable name that Data::Dumper makes for each argument passed to the sub Dumper. So here you have 4 arguments: the first is for [qw(a b c)], the other for each item in \qw(a b c). It is indeed a list of references, with 3 items.
Re: qw with anonomous list ref
by fruiture (Curate) on Oct 28, 2002 at 22:56 UTC

    None is better, because they do different things! qw// is basically a compile-time construct that (as Deparse shows) returns a LIST. \ LIST is a special form of reference-list constrctor and returns a LIST where each element is a reference to the element of the original list:

    my @x = \qw(a b c); print "$_ $$_ \n" for @x;

    [] creates an anonymous ARRAY-ref, which turns out to be a scalar value in the end:

    my @x = [qw(a b c)]; #@x has one element print "$_ @$_\n" for @x;

    So when you evaluate \LIST in scalar context, you get what you alway get when forcing a list to scalar context: the last element:

    my $x = \qw(a b c); print "$x $$x\n";


      I didn't realize that \LIST went "hyper" and applied the ref element-wise. Thanks.

      Does anyone have any idea what would this be good for? I can't imagine any use for \(list).

      The only thing that comes to mind is

      $ref = \($hashref->{key}->{subkey});
      In this case the
      $ref = \{$hashref->{key}->{subkey}};
      would mean something totaly different ($ref would be a reference to an anonymous hash) and
      $ref = \$hashref->{key}->{subkey};
      is not very clear.

      So are there any other uses?


        I have used it once for setting up an AoA structure.

        I filled 3 seperate arrays from reading and processing 3 different files and then combined them

        my @AoA = \(@a, @b, @c);

        Slightly easier than initialising (or pushing) 3 anon arrays and then filling them with push @{$AoA[n]}, $stuff; or whatever.

        Nah! Your thinking of Simon Templar, originally played by Roger Moore and later by Ian Ogilvy
        Most often I use this to do something to several arrays at once:
        my %silly_example; for(\(@couple, @of, @arrays, @here)) { $silly_example{shift @$_}++; splice @$_, 7, 1; push @$_, shift @$_; }
        Occasionally it comes it handy for a similar use of map or some such.

        Makeshifts last the longest.

        If you're using DBI, then code like this:   $st->bind_columns(undef, \$a, \$b, \$c)
        can be written as   $st->bind_columns(undef, \($a, $b, $c))
        That may be slightly easier to maintain.

        However, two caveats:

        • The \(LIST) form is confusing to many who read it, so make sure you are very consistent.
        • Most code I've seen use bind_columns() is less readable than fetch_hashref(), so if you're thinking of changing your coding style, think about dropping bind_columns() altogether.

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