Where is $# fully documented?

[LanX]

Hi

I'm struggling to find the perldocs where the usage of $# (i.e. last-index) with array-references (not symbols) is documented. Can you help me?

If not, what's the best place to augment the perldocs?

Tested behavior (5.10):

  DB<107> @a = (1..5)                             # @array
 => (1, 2, 3, 4, 5)

  DB<108> $#a                                     # classic usage
 => 4

  DB<109> $a = [1..10]                            # arrref
 => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

  DB<110> $#$a
 => 9

  DB<111> $a = [ map [0..$_],0..3 ]               # AoA
 => [[0], [0, 1], [0, 1, 2], [0, 1, 2, 3]]

  DB<112> $#{ $a->[2] }
 => 2

[ikegami]

"Using References" in perlref says:

Using References

That's it for creating references. By now you're probably dying to know how to use references to get back to your long-lost data. There are several basic methods.

1. Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a simple scalar variable containing a reference of the correct type:

[snip]

2. Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name, you can replace the identifier with a BLOCK returning a reference of the correct type. In other words, the previous examples could be written like this:

[snip]

The first documents $#$aref, the second $#{$aref}. $aref->$#* is documented by example further down in the "Postfix Dereference Syntax" section.

[tye]

I like the distillation of the rules I came up with long ago at References Quick Reference. If that distillation or something substantially similar hasn't made it into the Perl docs, I think it would be a useful addition. I certainly still often see the same questions repeated that were a big motivation for me publishing that.

[LanX]

Yes, Ikegami's table isn't bad either.

But I think as a minimum $# should be listed as one of the examples in perlref#Using-References

IMHO the only syntactic option missing.

[ysth]

The documentation for using references is at http://perldoc.perl.org/perlref.html#Using-References. It does not mention $# at all, but points 1 and 2 there address exactly what you are asking about. In general, the documentation does not strive to cover every possible combination of interacting features.

[LanX]

> It does not mention $# at all

well in these 5.20 docs this is mentioned  $aref->$#*; # same as $#{ $aref }

> but points 1 and 2 there address exactly what you are asking about.

Anywhere you'd put an identifier (or chain of identifiers) as part of a variable or subroutine name

So I suppose $# counts as a special function since its not a variable, but addressing an attribute of a variable?

Thanks anyway! :)

[ysth]

Special function? No.

The a in @a is the identifier part of the variable name; the documentation addresses anywhere you'd put that a, which includes $#a.

Update: hmm, it does say "as part of a variable...name", implying it would only work if $#a is a considered a variable. I shy away from that because, even though it is an assignable lvalue, it really is just an attribute of an array variable. So the doc could use a minor tweak.

In any case, $#$arrayref / $#{$arrayref} ought to be added to the examples there.

[ikegami]

$#a is no more a function and as much a variable as $a and @a.

[RedElk]

FWIW this might help.

[toolic]

I see several references to $# in perldata. UPDATE: I didn't notice you were focusing on references.

[LanX]

I scanned perldata and it only mentions the classic use with an @array symbol (i.e. $#array ), but no array ref (i.e. $#$aref or $#{$aref} ).

IIRC does perldsc show code using the trick, but w/o worded explanation.

Is it meant to be an undocumented feature, which can possibly change in the future?