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

I'm trying to get an array sorted into 3 parts, along the lines of (1,4,7) (2,5,8) (3,6,9). I thought that changing $linkscount = 4 to 0 would be the way to go but that command seems to be being ignored and I get the error message after the first pass. What is going wrong and what is the best way to do this? 
foreach (@online) {
  if ($linkscount == 4) {
    $linkscount = 0;
  }
  $linkscount++;
  print "$_ :: $linkscount<br>\n";
  if ($linkscount == 1) {
    push @links1, $_;
  } elsif ($linkscount == 2) {
    push @links2, $_;
  }  elsif ($linkscount == 3) {
    push @links3, $_;
  } else {
    print "error!";
    exit;
  }
}

[download]

Replies are listed 'Best First'.
Re: sorting an array
by bart (Canon) on Apr 07, 2007 at 11:35 UTC
    Change 
    if ($linkscount == 4) {
    $linkscount = 0;
}

    [download]
    to 
    if ($linkscount == 3) {
    $linkscount = 0;
}

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    because you increment the counter after you do this test.

    And your code works in a way that feels very "unperlish" to me, but I'm too lazy to completely rewrite it right now. Perhaps later. For example, there's no need to use 3 independent arrays, use an array of arrays instead, and directly use the counter as an index for what subarray to use. 

    push @{$links[$linkscount]}, $_;

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    And I'd prefer to let $linkscount go from 0 to 2, instead of from 1 to 3. Because now you'll get an empty slot for index 0. All you have to do is swap the $linkscount++ statement and the above test.
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[d/l]
[select]
Re: sorting an array
by shmem (Chancellor) on Apr 07, 2007 at 12:13 UTC
    A better way might be 
    while (@online) {
    push @links1, shift @online;
    push @links2, shift @online;
    push @links3, shift @online;
}

    [download]

    or as per bart's idea of using an array of anonymous arrays (see perlref) 

    while (@online) {
    push (@{$links[$_]}, shift @online) for 0..2;
}

    [download]
    To access the 2nd element (index 1) in the 3rd sub-array (index 2) you would use 
    $var = $links[2]->[1];

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    --shmem 

    _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
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[d/l]
[select]
      That's really interesting, thanks very much. I've proved to myself it works by using the following: 
      while (@online) {  
  push (@{$links[$_]}, shift @online) for 0..2;
}

@links1 = @{ $links[0] };
@links2 = @{ $links[1] };
@links3 = @{ $links[2] };

print "@links1<br>@links2<br>@links3";

      [download]
      I shall try to work with the anonymous arrays going forward rather than the named ones but it does all start to get rather obscure and difficult for a simpleton such as me...
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[d/l]
        I remember I had a hard time trying to grok references, creating references and dereferencing. But I got through it, and so will you. And you have to, since references are a crucial concept in perl5.

        Cheers from another simpleton,

        --shmem 

        _($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}
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