List slice *sometimes* ignores undefined items

[p5pRT]

Migrated from rt.perl.org#114498 (status was 'resolved')

Searchable as RT114498$

[p5pRT]

From jrw32982@yahoo.com

@​x = ( (2)[0,1,2] );
@​y = ( (2)[  1,2] );
use Data​::Dumper;
print Dumper \@​x, \@​y;

produces​:

$VAR1 = [
          2,
          undef,
          undef
        ];
$VAR2 = [];

This seems inconsistent.  The first result (for @​x) was expected. The second (for @​y) wasn't.

I have tested this on linux ActivePerl from 5.8 through 5.16 with identical results.

[p5pRT]

From @jkeenan

On Sun Aug 19 08​:27​:12 2012, jrw32982@​yahoo.com wrote​:

@​x = ( (2)[0,1,2] );
@​y = ( (2)[� 1,2] );
use Data​::Dumper;
print Dumper \@​x, \@​y;

produces​:

$VAR1 = [
��������� 2,
��������� undef,
��������� undef
������� ];
$VAR2 = [];

This seems inconsistent.� The first result (for @​x) was expected. The
second (for @​y) wasn't.

I don't think it's particularly inconsistent. In the first case the
slicing operation finds a defined element in the 0-index position of the
RH and assigns it to the array on the LH, and then (I suppose) proceeds
to auto-vivify. In the second case, the slicing operation finds no
element in the 0-index position, has nothing to assign, and stops there.

If anything, I think the second case is more intuitively plausible than
the first.

I couldn't find anything in Camel book 3rd or 4th edition that clarifies
this definitively.

Thank you very much.
Jim Keenan

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From @cpansprout

On Sun Aug 19 08​:27​:12 2012, jrw32982@​yahoo.com wrote​:

@​x = ( (2)[0,1,2] );
@​y = ( (2)[� 1,2] );
use Data​::Dumper;
print Dumper \@​x, \@​y;

produces​:

$VAR1 = [
��������� 2,
��������� undef,
��������� undef
������� ];
$VAR2 = [];

This seems inconsistent.� The first result (for @​x) was expected. The
second (for @​y) wasn't.

I have often relied on the fact that list slices return nothing for
nonexistent elements. But I have usually used a single index.

--

Father Chrysostomos

[p5pRT]

From @ap

* James E Keenan via RT <perlbug-followup@​perl.org> [2012-08-21 03​:55]​:

I don't think it's particularly inconsistent.

It is completely inconsistent with the way slices work in every other
case I can think of, and very surprising. This is a bug that should be
fixed. It has the distinct smell of an implementation accident.

[p5pRT]

From @ikegami

On Tue, Aug 21, 2012 at 3​:26 AM, Father Chrysostomos via RT <
perlbug-followup@​perl.org> wrote​:

I have often relied on the fact that list slices return nothing for
nonexistent elements.

Except they don't. Sometimes they return nothing, sometimes they return
undef.

[p5pRT]

From rick@bort.ca

On Aug 31 2012, Aristotle Pagaltzis wrote​:

* James E Keenan via RT <perlbug-followup@​perl.org> [2012-08-21 03​:55]​:

I don't think it's particularly inconsistent.

It is completely inconsistent with the way slices work in every other
case I can think of, and very surprising. This is a bug that should be
fixed. It has the distinct smell of an implementation accident.

It is no accident. From perldata​:

  A slice of an empty list is still an empty list. Thus​:

  @​a = ()[1,0]; # @​a has no elements
  @​b = (@​a)[0,1]; # @​b has no elements
  @​c = (0,1)[2,3]; # @​c has no elements

  But​:

  @​a = (1)[1,0]; # @​a has two elements
  @​b = (1,undef)[1,0,2]; # @​b has three elements

  This makes it easy to write loops that terminate when a null list is
  returned​:

  while ( ($home, $user) = (getpwent)[7,0]) {
  printf "%-8s %s\n", $user, $home;
  }

--
Rick Delaney
rick@​bort.ca

[p5pRT]

@cpansprout - Status changed from 'open' to 'rejected'

[p5pRT]

From @ap

* Rick Delaney <rick@​bort.ca> [2012-08-31 06​:10]​:

On Aug 31 2012, Aristotle Pagaltzis wrote​:

* James E Keenan via RT <perlbug-followup@​perl.org> [2012-08-21 03​:55]​:

I don't think it's particularly inconsistent.

It is completely inconsistent with the way slices work in every other
case I can think of, and very surprising. This is a bug that should be
fixed. It has the distinct smell of an implementation accident.

It is no accident. From perldata​:

I do not think your quotation says about this what you think it says.

   A slice of an empty list is still an empty list\.  Thus​:

       @​a = \(\)\[1\,0\];           \# @​a has no elements
       @​b = \(@​a\)\[0\,1\];         \# @​b has no elements
       @​c = \(0\,1\)\[2\,3\];        \# @​c has no elements

   But​:

       @​a = \(1\)\[1\,0\];          \# @​a has two elements
       @​b = \(1\,undef\)\[1\,0\,2\];  \# @​b has three elements

The bug report was about the surprising difference between these​:

  @​x = ( (2)[0,1,2] );
  @​y = ( (2)[ 1,2] );

In both cases the slice is of a one-element list, not of an empty list,
so the “But” clause from the quoted documentation applies and neither
array should be empty.

Regards,
--
Aristotle Pagaltzis // <http​://plasmasturm.org/>

[p5pRT]

From @ikegami

On Fri, Aug 31, 2012 at 12​:51 AM, Aristotle Pagaltzis <pagaltzis@​gmx.de>wrote​:

* Rick Delaney <rick@​bort.ca> [2012-08-31 06​:10]​:

On Aug 31 2012, Aristotle Pagaltzis wrote​:

* James E Keenan via RT <perlbug-followup@​perl.org> [2012-08-21
03​:55]​:

I don't think it's particularly inconsistent.

It is completely inconsistent with the way slices work in every other
case I can think of, and very surprising. This is a bug that should be
fixed. It has the distinct smell of an implementation accident.

It is no accident. From perldata​:

I do not think your quotation says about this what you think it says.

   A slice of an empty list is still an empty list\.  Thus​:

       @​a = \(\)\[1\,0\];           \# @​a has no elements
       @​b = \(@​a\)\[0\,1\];         \# @​b has no elements
       @​c = \(0\,1\)\[2\,3\];        \# @​c has no elements

   But​:

       @​a = \(1\)\[1\,0\];          \# @​a has two elements
       @​b = \(1\,undef\)\[1\,0\,2\];  \# @​b has three elements

The bug report was about the surprising difference between these​:

@​x = \( \(2\)\[0\,1\,2\] \);
@​y = \( \(2\)\[  1\,2\] \);

In both cases the slice is of a one-element list, not of an empty list,
so the “But” clause from the quoted documentation applies and neither
array should be empty.

Actually, it is covered​:

@​c = (0,1)[2,3]; # @​c has no elements

[p5pRT]

From @perhunter

On 08/31/2012 12​:51 AM, Aristotle Pagaltzis wrote​:

* Rick Delaney <rick@​bort.ca> [2012-08-31 06​:10]​:

On Aug 31 2012, Aristotle Pagaltzis wrote​:

* James E Keenan via RT <perlbug-followup@​perl.org> [2012-08-21 03​:55]​:

I don't think it's particularly inconsistent.

It is completely inconsistent with the way slices work in every other
case I can think of, and very surprising. This is a bug that should be
fixed. It has the distinct smell of an implementation accident.

It is no accident. From perldata​:

I do not think your quotation says about this what you think it says.

    A slice of an empty list is still an empty list\.  Thus​:

        @​a = \(\)\[1\,0\];           \# @​a has no elements
        @​b = \(@​a\)\[0\,1\];         \# @​b has no elements
        @​c = \(0\,1\)\[2\,3\];        \# @​c has no elements

    But​:

        @​a = \(1\)\[1\,0\];          \# @​a has two elements
        @​b = \(1\,undef\)\[1\,0\,2\];  \# @​b has three elements

The bug report was about the surprising difference between these​:

 @​x = \( \(2\)\[0\,1\,2\] \);
 @​y = \( \(2\)\[  1\,2\] \);

In both cases the slice is of a one-element list, not of an empty list,
so the “But” clause from the quoted documentation applies and neither
array should be empty.

look at the @​c example which is the same as your @​y line. slicing only
above the last element is an empty list. if any value from the slice is
from an existing element, the whole slice has elements. been that way
forever from what i know to support that an empty list (even assigned to
a list of scalars) is false. i was thinking about this feature to add to
this thread but couldn't find the doc quote for it.

my view is the docs could use a little more clarity saying that a slice
of only indexes beyond the last one is always an empty list.

uri

[p5pRT]

From Eirik-Berg.Hanssen@allverden.no

On Fri, Aug 31, 2012 at 7​:20 AM, Uri Guttman <uri@​stemsystems.com> wrote​:

my view is the docs could use a little more clarity saying that a slice of
only indexes beyond the last one is always an empty list.

  ... or just explain why C<< (0,1)[2,3] >> is an example of "a slice of an
empty list". To me it looks like a slice of C<< (0, 1) >>, which in turn
does not look particularly empty ...

Eirik

[p5pRT]

From @ap

* Eric Brine <ikegami@​adaelis.com> [2012-08-31 07​:15]​:

On Fri, Aug 31, 2012 at 12​:51 AM, Aristotle Pagaltzis <pagaltzis@​gmx.de>wrote​:

In both cases the slice is of a one-element list, not of an empty
list, so the “But” clause from the quoted documentation applies and
neither array should be empty.

Actually, it is covered​:

@​c = (0,1)[2,3]; # @​c has no elements

D’oh. That is not a particularly empty list, the introductory verbiage
is misleading. So that means​:

  @​a = 1..2;
  @​b = @​a [4,5,6]; # @​b has 3 elements (all undef)
  @​c = (@​a)[4,5,6]; # @​c has 0 elements

It is an understatement to say I’m not wild about this design, but if
it is intended…

Patch attached.

[p5pRT]

From @ap

fix-perldata-empty-list-slice.diff

diff --git i/pod/perldata.pod w/pod/perldata.pod
index 3a4776c..9bff98f 100644
--- i/pod/perldata.pod
+++ w/pod/perldata.pod
@@ -777,13 +777,18 @@ A slice of an empty list is still an empty list.  Thus:
 
     @a = ()[1,0];           # @a has no elements
     @b = (@a)[0,1];         # @b has no elements
-    @c = (0,1)[2,3];        # @c has no elements
 
 But:
 
     @a = (1)[1,0];          # @a has two elements
     @b = (1,undef)[1,0,2];  # @b has three elements
 
+More generally, a slice yields the empty list if it indexes only
+beyond the end of a list:
+
+    @a = (1)[  1,2];        # @a has no elements
+    @b = (1)[0,1,2];        # @b has three elements
+
 This makes it easy to write loops that terminate when a null list
 is returned:
 

[p5pRT]

From jrw32982@yahoo.com

Please see the example below.

First, as the example below shows, there are inconsistencies depending on whether or not the list is held in a variable (@​a1 or @​a4) or a constant ((8) or ()) or a function return value.  I don't see how these can be anything other than one or more bugs.

Second, if there is intention (as Uri says and perldata says) to return an empty list when the list being sliced is an empty list, then there is a bug (as shown below) when the empty list is held in a variable @​a4 vs. an empty list constant or an empty list returned from a function f4.

Third, perhaps the documentation could be enhanced to make this intentional inconsistency (in the handling of list slices when the subscripts are off the end of the list and the list goes from non-empty to empty) more obvious in documentation, since it is surprising due to the inconsistency.

-- John

#!/usr/bin/perl
use warnings;
use Data​::Dumper;

sub f1 { return (9) }
@​a1 = (7);
@​x1 = (@​a1[0,1,2]);  # [ 7, undef, undef ];
@​y1 = (@​a1[  1,2]);  # [    undef, undef ];
@​x2 = ((8)[0,1,2]);  # [ 8, undef, undef ];
@​y2 = ((8)[  1,2]);  # [];
@​x3 = ((f1)[0,1,2]); # [ 9, undef, undef ];
@​y3 = ((f1)[  1,2]); # [];

sub f4 { return () }
@​a4 = ();
@​x4 = (@​a4[0,1,2]);  # [ undef, undef, undef ];
@​y4 = (@​a4[  1,2]);  # [        undef, undef ];
@​x5 = (()[0,1,2]);   # [];
@​y5 = (()[  1,2]);   # [];
@​x6 = ((f4)[0,1,2]); # [];
@​y6 = ((f4)[  1,2]); # [];

print Data​::Dumper->Dump([\@​x1, \@​y1], [qw[x1 y1]]);
print Data​::Dumper->Dump([\@​x2, \@​y2], [qw[x2 y2]]);
print Data​::Dumper->Dump([\@​x3, \@​y3], [qw[x3 y3]]);
print "\n";
print Data​::Dumper->Dump([\@​x4, \@​y4], [qw[x4 y4]]);
print Data​::Dumper->Dump([\@​x5, \@​y5], [qw[x5 y5]]);
print Data​::Dumper->Dump([\@​x6, \@​y6], [qw[x6 y6]]);

________________________________
From​: Eirik Berg Hanssen <Eirik-Berg.Hanssen@​allverden.no>
To​: Uri Guttman <uri@​stemsystems.com>
Cc​: perl5-porters@​perl.org
Sent​: Friday, August 31, 2012 1​:57 AM
Subject​: Re​: [perl #114498] List slice *sometimes* ignores undefined items

On Fri, Aug 31, 2012 at 7​:20 AM, Uri Guttman <uri@​stemsystems.com> wrote​:

my view is the docs could use a little more clarity saying that a slice of only indexes beyond the last one is always an empty list.
  ... or just explain why C<< (0,1)[2,3] >> is an example of "a slice of an empty list".  To me it looks like a slice of C<< (0, 1) >>, which in turn does not look particularly empty ...

Eirik