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

Perl revision 5 version 16 subversion 3

Greetings, oh Great Ones!

Here is a sort command that is barfing on me: 

my @ptn_sortd = sort {    ${$partition_h{a}}{full_tabname}
                      cmp ${$partition_h{b}}{full_tabname}} @parnums;

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This is greeted with a barrage of this error message: 
Use of uninitialized value in string comparison (cmp) at ./<script nam
+e> line 156.
 at ./<script name> line 156.
Use of uninitialized value in string comparison (cmp) at ./<script nam
+e> line 156.
 at ./<script name>l line 156.
Use of uninitialized value in string comparison (cmp) at ./<script nam
+e> line 156.
 at ./<script name> line 156.
....

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I have already proven that no item in the hash has an undefined {full_tabname} member. I used 

  grep {defined(${$partition_h{$_}}{full_tabname}) }

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to weed out the one such holdout. (That undefined name is separate mystery, out of scope for this question.) I have also confirmed that when I supply an actual value, the syntax does indeed return a string.

By the way, the hash keys are large-ish integers.

What could I possibly be doing wrong to deserve this frustrating punishment?

Thanks much for help here! -- RP

Replies are listed 'Best First'.
Re: Debug a sort block
by AnomalousMonk (Archbishop) on Jan 31, 2020 at 20:05 UTC
    ${$partition_h{a}}{full_tabname}

    Do you really mean  ${$partition_h{a}}{full_tabname} and not  ${$partition_h{$a}}{full_tabname} (a vice $a)?

    Update: It is possible (though usually very tedious) to put debug statements into a sort block as long as the comparison is the last expression evaluated: 

    c:\@Work\Perl\monks>perl -wMstrict -e
"use Data::Dump qw(dd);
 ;;
 my @sorted = sort { dd $a, $b;  $a cmp $b; } (qw(d a c e b), undef);
 dd \@sorted;
"
("d", "a")
("c", "e")
("b", undef)
Use of uninitialized value in string comparison (cmp) at -e line 1.
("a", "c")
("c", "d")
("d", "e")
("a", undef)
Use of uninitialized value in string comparison (cmp) at -e line 1.
("a", "b")
("c", "b")
[undef, "a", "b", "c", "d", "e"]

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    (Update: Changed example code to use Data::Dump::dd() instead of print and to include an undef element in the list to be sorted.)

    Give a man a fish:  <%-{-{-{-<

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      > Update: It is possible (though usually very tedious) to put debug statements into a sort block

      In this case I'd rather replace the sort block with a function, it's match easier to set breakpoints and watch expressions then.

        DB<1> sub by_num { return $a <=> $b }

  DB<2> x [sort by_num 1,11,2,12]
0  ARRAY(0x334b888)
   0  1
   1  2
   2  11
   3  12
  DB<3>

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      update

      added return to make mechanism obvious.

      Cheers Rolf
      (addicted to the Perl Programming Language :)
      Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

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Re: Debug a sort block
by LanX (Saint) on Jan 31, 2020 at 21:22 UTC
    It should be $a and $b not a and b °

    On a side note:

    > ${$partition_h{$a}}{full_tabname}

    your dereferencing syntax is kind of weird, the following is much easier to read and does the same:

     $partition_h{$a}{full_tabname}

    Cheers Rolf
    (addicted to the Perl Programming Language :)
    Wikisyntax for the Monastery FootballPerl is like chess, only without the dice

    °) oh AnomalousMonk already told you! :)

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Re: Debug a sort block
by karlgoethebier (Abbot) on Feb 01, 2020 at 14:37 UTC
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