First off I would like to say thank you! This code mostly works but I get some unitialized value errors toward the end and nothing seems to be coming from file2 in the output. I'll show you that in a minute. First, discussion.

Believe it or not, the next if ( $c == 0 ) and $c++; # skip the file header worked! :) It would go to the next iteration the first time through and then it would increment $c. After that first hit though it was pretty much ignored for the duration of the script. However, I like your solution better. It is much cleaner.

You are absolutely right about the usage of a subroutine for the repetitive read()'ing. The "$_ |" was intentional. In my example I was trying to get a list in the form "var | var | var" and so I was placing the delimiter in the array with the value for the output to be correct. However, once again, the way you do it is much cleaner.

Indeed, I need to polish up my knowledge of arrays. Arrays of arrays are something that make my head hurt just mentioning. There are a ton of things I need to work on in Perl but I feel I am catching on fast considering this is my second month of coding in Perl. So, without further adieu...I start my questioning for learning process...

Reference code... 

      1 #!/usr/bin/perl -w
      2 use strict;
      3
      4 my @master_list = ();
      5
      6 readfile("f1.lst", \@master_list);
      7 readfile("f2.lst", \@master_list);
      8 readfile("f3.lst", \@master_list);
      9
     10 printf "%s | %s | %s\n", @{$_}[0..2] for(@master_list);
     11
     12 sub readfile {
     13   my $filename = shift or die "Need filename.\n";
     14   my $listref = shift; # Listed pointed to is modified in plac
+e.
     15
     16   open my $file, "< $filename" or die "Can't open $filename: $
+!\n";
     17
     18   my $header = <$file>;
     19
     20   my $c = 0;
     21   local $_;
     22   while(<$file>) {
     23     $listref->[$c] ||= []; # use strict doesn't like auto-viv.
     24     chomp;
     25     # Compare the new value with the first value stored in the
+ list.
     26     # First value to be read in for any row is assumed to be
     27     # correct.  All subsequent values must match that first on
+e.
     28     unless(@{$listref->[$c]} and $_ != $listref->[$c][0]) {
     29       push @{$listref->[$c]}, $_;
     30     } else {
     31       push @{$listref->[$c]}, ' ';
     32     }
     33
     34     ++$c;
     35   }
     36
     37   close $file or die "Can't close $filename: $!\n";
     38 }

[download]
  • Question 1:
    • In line 18, my $header = <$file>; -- I assume that this strips the header of each file? If so, how?
  • Question 2:
    • In line 21, why did you local'ize $_? What benefit does this give me?
  • Question 3:
    • I understand what you are doing with the readfile() to an extent. One thing I have not quite grasped is the whole (line 23)  $something_here->[$something_else_here] statement. What is that doing?? I know that the ||= is creating a default value ( $this = "$that" ||= "this" ) but I do not understand what the '[ ]' is doing afterward. I don't understand the open brackets by themselves. I know that brackets denote an element in an array but this eludes me. What is auto-viv? :)

  • Question 4:
    •     unless(@{$listref->[$c]} and $_ != $listref->[$c][0]) {
      push @{$listref->[$c]}, $_;
    } else {
      push @{$listref->[$c]}, ' ';
    }

    [download]
    I have a few questions here.
    Ok, I get the unless statement's purpose. I don't get how it works. This is mostly because @($listref->[$c]} totally loses me. OTOH, I have pieced together that the ... $_ != $listref->[$c][0] is probably what is actually checking to make sure we skip the 0 element (the header) in the file (which makes me question my $header = <$file>; even more curious). Now the meat! I see push() and pop() all the time. I know the basics of what they do but I have no practical understanding of their usage. I see that you are push()'ing the value from $_ to whatever @{$listref->[$c]} is :) otherwise you make @{$listref->[$c]} = to nothing (my blank if a value does not exist)?

    So, this script is great! I am still struggling to figure what everything is doing but I am going to figure it out. Now, for the part we all hate...debugging.

    When I used the code it looked like it ran beautifully, however, I started getting errors toward the end of the run and something mysteriously eludes me. Let me give you a snapshot of my output: 

    1653 |   | 1653
1654 |   | 1654
1655 |   | 1655
1656 |   | 1656
Use of uninitialized value in printf at try2.pl line 10.
1657 | 1657 |
Use of uninitialized value in printf at try2.pl line 10.
1658 | 1658 |
Use of uninitialized value in printf at try2.pl line 10.
1659 | 1659 |
    One thing that seems to be wrong here besides the obvious is that there is nothing being returned in the middle of the list (f2.lst). Therefore, for everybodys' coding pleasure I am providing the lists to work with. Aren't I nice?! =P I will try and work with the code you have provided as well to see if I can learn something.

    Again, I appreciate your help!

    ----------
    - Jim

    In reply to Re: Re: Printing out multiple array lists and more! by snafu
    in thread Printing out multiple array lists and more! by snafu

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