RFC 120 (v3) Implicit counter in for statements, possibly $#.

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=head1 TITLE

Implicit counter in for statements, possibly $#.

=head1 VERSION

  Maintainer: John McNamara <[EMAIL PROTECTED]>
  Date: 16 Aug 2000
  Last Modified: 20 Sep 2000
  Mailing List: [EMAIL PROTECTED]
  Number: 120
  Version: 3
  Status: Frozen

=head1 ABSTRACT

The syntax of the Perl style C<for> statement could be augmented by the
introduction of an implicit counter variable. The deprecated variable C<$#>
could be used for this purpose due to its mnemonic association with C<$#array>.

Other alternatives are also proposed: an explicit counter returned by a
function; an explicit counter defined after foreach; an explicit counter
defined by a scoping statement.

=head1 DESCRIPTION

The use of C<for> and C<foreach> statements in conjunction with the range
operator, C<..>, are generally seen as good idiomatic Perl:

    @array = qw(sun moon stars rain);
    
    foreach $item (@array) {
        print $item, "\n";
    }

as opposed to the "endearing attachment to C" style:

    for ($i = 0; $i <= $#array; $i++) {
        print $array[$i], "\n";
    }


In particular, the foreach statement provides a useful level of abstraction
when iterating over an array of objects:

    foreach $object (@array) {
        $object->getline;
        $object->parseline;
        $object->printline;
    }

However, the abstraction breaks down as soon as there is a need to access the
index as well as the variable:

    for ($i = 0; $i <= $#array; $i++) { # Note
        $array[$i]->index = $i;
        $array[$i]->getline;
        $array[$i]->parseline;
        $array[$i]->printline;
    }
    
    # Note - same applies to: foreach $i (0..$#array)


Here we are dealing with array variables and indexes instead of objects.


The addition of an implicit counter variable in C<for> statements would lead to
a more elegant syntax. It is proposed the deprecated variable C<$#> should be
used for this purpose due to its mnemonic association with C<$#array>. For
example:

    foreach $item (@array) {
        print $item, " is at index ", $#, "\n";
    }


=head1 ALTERNATIVE METHODS

Following discussion of this proposal on perl6-language-flow the following
suggestions were made:


=head2 Alternative 1 : Explicit counter returned by a function

This was proposed by Mike Pastore who suggested reusing pos() and by Hildo
Biersma who suggested using position(): 

    foreach $item (@array) {
        print $item, " is at index ", pos(@array), "\n";
    }
    
    # or:
    
    foreach $item (@array) {
        $index = some_counter_function();   
        print $item, " is at index ", $index, "\n";
    }


=head2 Alternative 2 : Explicit counter defined after foreach

This was proposed by Chris Madsen and Tim Jenness, Jonathan Scott Duff made a
similar pythonesque suggestion:

    foreach $item, $index (@array) {
        print $item, " is at index ", $index, "\n";
    }
    
Glenn Linderman added this could also be used for hashes:

    foreach $item $key ( %hash ) {
        print "$item is indexed by $key\n";
    }


Ariel Scolnicov suggested a variation on this through an extension of the
C<each()>:

    foreach (($item, $index) = each(@array)) {
        print $item, " is at index ", $index, "\n";
    }

With this in mind Johan Vromans suggested the use of C<keys()> and C<values()>
on arrays.

A variation on this is an explicit counter after C<@array>. This was alluded to
by Jonathan Scott Duff:

    foreach $item (@array) $index {
        print $item, " is at index ", $index, "\n";
    }


=head2 Alternative 3 : Explicit counter defined by a scoping statement

This was proposed by Nathan Torkington.

    foreach $item (@array) {
        my $index : static = 0; # initialized each time foreach loop starts
        print "$item is at index $index\n";
        $index++;
    }
    
    # or: 

    foreach $item (@array) {
        my $index : counter = 0; # initialized to 0 first time
                                 # incremented by 1 subsequently
        print "$item is at index $index\n";
    }



=head1 IMPLEMENTATION

As Ilya says, straightforward.

=head1 REFERENCES

perlvar

Alex Rhomberg proposed an implicit counter variable on clpm:
http://x53.deja.com/getdoc.xp?AN=557218804&fmt=text and
http://x52.deja.com/threadmsg_ct.xp?AN=580369190.1&fmt=text

Craig Berry suggested C<$#>:
http://x52.deja.com/threadmsg_ct.xp?AN=580403316.1&fmt=text