gepebril69 has asked for the wisdom of the Perl Monks concerning the following question:

Hi there

What does the code: my @x= (undef)x7; in de example below do?

Are they trying to empty the array?

#!/usr/bin/perl -w

use strict;
use Win32API::File qw(
    getLogicalDrives GetVolumeInformation );

my @drives= getLogicalDrives();
foreach my $d (  @drives  ) {
    my @x= (undef)x7;
    GetVolumeInformation( $d, @x );
    print "$d $x[0] ($x[5])\n";

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Re: What does my @x= (undef)x7; do?
by Corion (Patriarch) on Nov 09, 2015 at 14:34 UTC 
    use Data::Dumper;
my @x= (undef)x7;
warn Dumper \@x;

    [download]

    It initializes an array with 7 instances of undef.

    The rest should be clear from reading the documentation on Win32API::File::GetVolumeInformation.

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      @Corion

      Thanks very much for this info!!

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Re: What does my @x= (undef)x7; do?
by toolic (Bishop) on Nov 09, 2015 at 14:35 UTC
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Re: What does my @x= (undef)x7; do?
by BrowserUk (Patriarch) on Nov 09, 2015 at 14:53 UTC

    Update: In most cases...(but not this one. Thanks, Athanasius.)

    FWIW: The same thing can be achieved by my @x; $#x = 6;

    With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
    Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
    "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
    In the absence of evidence, opinion is indistinguishable from prejudice.
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      Yes, it should work; but, strangely, it doesn’t:

      #! perl
use strict;
use warnings;
use Data::Dump;
use Win32API::File qw(getLogicalDrives GetVolumeInformation);

my @drives = getLogicalDrives();

for my $d (@drives)
{
#   my @x = (undef) x 7;
    my @x;
    $#x = 6;
#   dd \@x;
    GetVolumeInformation($d, @x);
    print "$d $x[0] ($x[5])\n";
}

      [download]

      Output:

       1:16 >perl 1442_SoPW.pl
Modification of a read-only value attempted at C:/Perl/Strawberry/stra
+wberry-perl-5.22.0.1-64bit-PDL/perl/lib/Win32API/File.pm line 269.

 1:16 >

      [download]

      But with the line dd \@x; uncommented, it does work:

       1:16 >perl 1442_SoPW.pl
[undef, undef, undef, undef, undef, undef, undef]
C:\  (NTFS)
[undef, undef, undef, undef, undef, undef, undef]
D:\  ()
[undef, undef, undef, undef, undef, undef, undef]
E:\  ()

 1:16 >

      [download]

      What’s going on?

      Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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        What’s going on?

        The explicit method populates the array with 7 physical pointers to undef:

        [0] Perl> @x = (undef)x7;;
[0] Perl> use Devel::Peek;;
[No such file or directory] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 7
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    SV = NULL(0x0) at 0x3cb4b98
      REFCNT = 1
      FLAGS = ()
    Elt No. 1
    SV = NULL(0x0) at 0x3c73578
      REFCNT = 1
      FLAGS = ()
    Elt No. 2
    SV = NULL(0x0) at 0x3cb49b8
      REFCNT = 1
      FLAGS = ()
    Elt No. 3
    SV = NULL(0x0) at 0x3cb4af0
      REFCNT = 1
      FLAGS = ()

        [download]

        Whereas, when you pre-extend the array, no physical pointer are populated; the undef's are implied by the absence of pointers:

        [0] Perl> undef @x;;
[0] Perl> $#x = 6;;
[0] Perl> Dump \@x;;
SV = RV(0x3c735d0) at 0x3c735c0
  REFCNT = 1
  FLAGS = (TEMP,ROK)
  RV = 0x3c6ac88
  SV = PVAV(0x3cfafd8) at 0x3c6ac88
    REFCNT = 2
    FLAGS = (RMG)
    MAGIC = 0x3cee198
      MG_VIRTUAL = &PL_vtbl_arylen_p
      MG_TYPE = PERL_MAGIC_arylen_p(@)
      MG_FLAGS = 0x02
        REFCOUNTED
      MG_OBJ = 0x3c6a410
      SV = PVMG(0x3b1a078) at 0x3c6a410
        REFCNT = 1
        FLAGS = (GMG,SMG,pIOK)
        IV = 6
        NV = 0
        PV = 0
        MAGIC = 0x3ceea08
          MG_VIRTUAL = &PL_vtbl_arylen
          MG_TYPE = PERL_MAGIC_arylen(#)
          MG_OBJ = 0x3c6ac88
    ARRAY = 0x3cf1398
    FILL = 6
    MAX = 6
    ARYLEN = 0x3c6a410
    FLAGS = (REAL)
    Elt No. 0
    Elt No. 1
    Elt No. 2
    Elt No. 3

        [download]

        Perl code would treat the two the as the same, but user-written XS code makes assumptions.

        Thanks for checking it.

        With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
        Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
        "Science is about questioning the status quo. Questioning authority". I knew I was on the right track :)
        In the absence of evidence, opinion is indistinguishable from prejudice.
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