Then and now, you haven't shown us the (reduced) code and the (relevant) data that reproduce the error you're getting. Please reduce your code to a self contained (say, 20 line) script, and supply sample data (say, 5 items), so we can look at it all.

From the error message, my guess is that somewhere in your code (Perl tells you the line number, but you keep it from us as if it's some secret), you're modifying a read-only value. Normally, Perl catches such attempts right away:

perl -e "print 1++"

Can't modify constant item in postincrement (++) at -e line 1, near "1
+++"
Execution of -e aborted due to compilation errors.
[download]

... but you can be more tricky and then Perl can't detect such errors until you execute the code:

sub incr { $_[0]++ };

incr($foo);
incr(1);
[download]

Maybe you are trying to assign to other read-only values, like $1 or something. We don't know, and it's quite hard to guess without seeing the code.

I doubt that anyone here can answer this question (or this part of the earlier post) based on the information you have given in both posts combined.

Error messages like this tell you which line number in your script has caused the error. You have to show us at least that line of code, along with any other lines of code that are relevant -- that is: for whatever variables are present in the line that causes the error, show us at least the lines where those variables are declared and where values are assigned to them prior to the line that causes the error.

Nothing in the examples given to you in the previous thread would generate this error. None of the code you've posted so far would generate this error. (You haven't posted enough code to show whether it would generate this error.) So you need to show more of your code.

If the code is not ridiculously long, post the whole script. (Do it as an update or reply to this thread, please. Two places for discussing the same problem are more than enough.)

As others have suggested, you really need to post the lines of code exhibiting the error. A very good thing to do is reduce the code to a small number of lines showing the issue and post that. See I know what I mean. Why don't you? for some ideas on how to do that.

Although actually we have a pretty good track record at reading minds, sometimes we simply fail and have to be helped out with some actual information. This seems to be one of those times.

foreach $try (@text){ 
$try =~/^(.*?)\|(.*?)\|(.*?)\|(.*?)\|(.*?)\|(.*?)\|(.*?)\|(.*?)\|(.*?)
+\|(.*?)\|(.*?)$/gs or die "<br />problem with $try<br />"; 
my $Institution = $1; 
my $CourseNumber = $2;
my $Professor = $3;
my $Enrollment = $4; 
........  
if (!$CourseNumber && !$Institution){last;};
if ($CourseNumber ne $LastCourseNumber){    
$courseindex = $courseindex + 1;
$courselist[$courseindex] = [$Institution, $CourseNumber, $Professor, 
+$CourseTitle, $Enrollment];
};
$LastCourseNumber = $CourseNumber; 
}; 

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1] } @co
+urselist;
[download]

$courselist[0] = ["UM", "CS 34", "Smith", "34"]
$courselist[1] = ["AC", "PHIL 13", "Barry", "45"]
$courselist[2] = ["UM", "BIO 567", "Houlihan", "12"]
[download]

$courselist[0] = ["AC", "PHIL 13", "Barry", "45"]
$courselist[1] = ["UM", "BIO 567", "Houlihan", "12"]
$courselist[2] = ["UM", "CS 34", "Smith", "34"]
[download]

This is still not a piece of code that I can run on my machine and see your problem. Distil your code down to a stand alone sample that exhibits the problem. There are too many unknowns in the code you have provided. I say again, read I know what I mean. Why don't you?.

Note that your regex is probably much better done as a split:

my ($Institution, $CourseNumber, $Professor, $Enrollment) = split '|',
+ $try;
[download]

Oh, hold on a moment, my esp cells are kicking in. Here is the sample you should have posted:

use warnings;
use strict;

my @courselist;
my @text = 'UM|CS 34|Smith|34';
my $courseindex = 0;

foreach my $try (@text){
    my ($Institution, $CourseNumber, $Professor, $Enrollment) = split 
+'|', $try;
    
    $courseindex = $courseindex + 1;
    $courselist[$courseindex] =
        [$Institution, $CourseNumber, $Professor, $Enrollment];
};

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1]} @cou
+rselist;
[download]

And the answer is that you preincrement $courseindex so the first element is placed at index 1, but Perl generally starts arrays at 0 and trying to dereference aan undefined value in the sort (element 0) is generating your error.

Your code would be better written as:

use warnings;
use strict;

my @courselist;
my @text = 'UM|CS 34|Smith|34';

foreach my $try (@text){
    my ($Institution, $CourseNumber, $Professor, $Enrollment) = split 
+'|', $try;
    
    push @courselist, [$Institution, $CourseNumber, $Professor, $Enrol
+lment];
};

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1]} @cou
+rselist;
[download]

---

DWIM is Perl's answer to Gödel

Here is all the code I can think is relevant.

Well, you might need to think a little harder -- or read our replies more carefully... But, okay, here is a working script based on what you've posted, putting together some sample data from your earlier post, and adding some other things that you haven't included yet, like assigning data to @text. (I removed "$CourseTitle", but if you really need it, I hope you can figure out how to fit it in with this approach.) Essentially, I'm providing the thing that GrandFather asked for (except that this version does not get the run-time error that you get, so it doesn't "demonstrate the problem"):

use strict;
use Data::Dumper;

my @text = <DATA>;
my @courselist;
my $LastCourseNumber;

for (@text) {
    my ( $Institution, $CourseNumber, $Professor, $Enrollment ) = spli
+t /\|/;

    last unless ( $CourseNumber && $Institution);

    if ($CourseNumber ne $LastCourseNumber) {
        push @courselist, [ $Institution, $CourseNumber,
                             $Professor, $Enrollment];
        $LastCourseNumber = $CourseNumber; 
    }
}
print Dumper( \@courselist );

@courselist =  sort { $a->[0] cmp $b->[0] || $a->[1] cmp $b->[1] } @co
+urselist;

print Dumper( \@courselist );

__DATA__
UM|CS 34|Somebody|3
AC|PHIL 13|Another Person|8
UM|BIO 567|Someone Else|20
[download]

(update: the "last" condition in loop had been like the OP code ("if..."), but I think using "unless" makes more sense here -- this way, either variable being empty will exit the loop.)

This does what you want to do (sort the array). I expect there's still more stuff in your own script that you need to fold in, and maybe when that other stuff is included, you'll still get an error. If so, that means the problem is still somewhere in the code that you have not shown us.

Don't say

$courseindex = $courseindex + 1;
[download]

say rather

$courseindex++;
[download]

if you need $courseindex for anything other than storing the array references in the right slot of @courselist.

You should then set $courseindex initially to 0 and increment it at the proper place, i.e. after having filled an array slot.

If you don't need to keep track of the index and just want to push a value (anonymous array) onto @courselist, do so:

push @courselist, [$Institution, $CourseNumber, $Professor, $CourseTit
+le, $Enrollment];
[download]

--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}