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=head1 TITLE
Implicit counter in for statements, possibly $#.
=head1 VERSION
Maintainer: John McNamara <[EMAIL PROTECTED]>
Date: 16 Aug 2000
Last Modified: 27 Sep 2000
Mailing List: [EMAIL PROTECTED]
Number: 120
Version: 5
Status: Frozen
Frozen since: v3
=head1 ABSTRACT
The syntax of the Perl style C<for> statement could be augmented by the
introduction of an implicit counter variable. The deprecated variable
C<$#> 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 $key\n";
}
Ariel Scolnicov suggested a variation on this through an extension of
the C<each()>:
while (($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. This behaves somewhat similarly
to Tie::Counter.
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 $index\n";
}
=head1 IMPLEMENTATION
There was no discussion about how this might be implemented. It was
pointed out by more than one person it would inevitably incur an
overhead.
=head1 REFERENCES
perlvar
Alex Rhomberg proposed an implicit counter variable on clpm:
http://x53.deja.com/getdoc.xp?AN=557218804&fmt=text and
http://x52.deja.com/threadmsg_ct.xp?AN=580369190.1&fmt=text
Craig Berry suggested C<$#>:
http://x52.deja.com/threadmsg_ct.xp?AN=580403316.1&fmt=text
