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Hi, yesterday I got a strange bug that took a while to fix. It also took a while to trace down the bug enough to recreate it in the short script below (original script was too long to post).

Basically the symptom is that first I create a hash of hash of hash, then use a while(my ($key, $value) = each %hash) to loop through it, then inside the while loops, use 'last;' to break them. After that, create a second set of those while loops, and the problem is, the 2nd set of the while loops won't be entered. If I don't use 'last;', or if I use 'for my $key (keys %hash)', or if I use a dump immediately before the 2nd set of the while loops, the problem would be "fixed".

But I want to understand why the 2nd set of while loops do not work in the first place. Anyone could help? BTW I tested it on Perl on UNIX and Windows, 5.6.1 - 5.8.5.

Without further ado, here's the script:

use strict;
use Dumpvalue;

my %test = ('level1' => { 'level2' => { 'level3' => { 'level4' => 1} }
+ } );
#Dumpvalue->new->dumpValue(\%test); # when uncommented, this dump 
## doesn't help the troubled while loops

my $createtrouble = 1;
if($createtrouble)
{
  ###################################################
  # the woes of the troubled while loops further below is 
  # really caused by the 'last's in these while loops
  while(my ($key1, $sdata) = each %test)
  {
    while(my ($key2, $stats) = each %$sdata)
    {
      last;
    }
    last;
  }
}

#Dumpvalue->new->dumpValue(\%test); # when uncommented, this dump 
## mysteriously helps the troubled while loops
print "Start print out now\n\n";
my $usewhile = 1;
if($usewhile)
{
  #################################################
  # the troubled while loops
  while(my ($key1, $sdata) = each %test)
  {
    print "got in the first one!\n";
    while(my ($key2, $stats) = each %$sdata)
    {
      print "got here! key2 is $key2\n";
    }
  }
}
else
{
  #################################################
  # this one always works
  for my $key1 (keys %test)
  {
    my $sdata = $test{$key1};
    print "got in the first one!\n";
    for my $key2 (keys %$sdata)
    {
      print "got here! key2 is $key2\n";
    }
  }
}
[download]

2005-10-01 Retitled by planetscape, as per Monastery guidelines
Original title: 'A myterious bug that arises under certain innocent-looking situation'

In reply to mystery re each + last by inq123

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