Array iteration: foreach works, for doesn't

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

Hello.

In general my problem is that iterating trough an array using foreach works properly but using for and $awarie$iterator returns nothing.

If I didn't have to use two walues at one iteration of the last (for) loop, I would convert it to an foreach.

sub br {print "\n"};

sub parse_datetime($)
{
 $_ =~ /(\d\d\d\d)-(\d\d)-(\d\d) (\d\d):(\d\d)/;
 return($1, $2, $3, $4, $5);
}

my @awarie = ('2009-11-13 13:48', '2009-11-13 16:58',
              '2009-11-13 19:23', '2009-11-13 23:13',
              '2009-11-16 15:31', '2009-11-16 21:41',
              '2009-11-17 16:21', '2009-11-17 21:06');

if ($DEBUG)
{
 print('@awarie'); br;
 print Dumper(@awarie);
 print('parse_datetime($awarie[$i])'); br;
 #foreach(@awarie)                        # Works
 for(my $k=0; $k<scalar(@awarie); $k++)   # Dosen't
 {
  #my ($y, $m, $d, $h, $mi) = parse_datetime($_);
  # ^^^^^^^^^^^^^^^^^^^^^^^^^ Works
  #my ($y, $m, $d, $h, $mi) = parse_datetime($awarie[$k]);
  # ^^^^^^^^^^^^^^^^^^^^^^^^^ Dosen't
   my ($y, $m, $d, $h, $mi) = parse_datetime($awarie[1]);
  # ^^^^^^^^^^^^^^^^^^^^^^^^^ Dosen't!
   print("$y-$m-$d $h:$mi"); br;
 }
}

if($DEBUG)
{
 print('@array size: ' . scalar(@awarie)); br; # Everything is correct
}

for(my $j=0; $j<scalar(@awarie); $j++)
{
 if($DEBUG) { print $awarie[$j] }; br;
 my ($yy, $mm, $dd, $phh, $pmi) = parse_datetime($awarie[$j++]);
 # ^^^ Nothing returned
 if($DEBUG) { print $awarie[$j] }; br;
 my ($kyy, $kmm, $kdd, $khh, $kmi) = parse_datetime($awarie[$j]);
 # ^^^ Ditto

 if($DEBUG)
 {
  print("$yy-$mm-$dd $phh:$pmi"); br;
  print("$kyy-$kmm-$kdd $khh:$kmi"); br;
  # Both print '--:'...
 }
# (...)
}
[download]

What am I missing?

Comment on Array iteration: foreach works, for doesn't Download Code

Replies are listed 'Best First'.
Re: Array iteration: foreach works, for doesn't by moritz (Cardinal) on Nov 23, 2009 at 13:24 UTC
`sub parse_datetime($) { $_ =~ /(\d\d\d\d)-(\d\d)-(\d\d) (\d\d):(\d\d)/; return($1, $2, $3, $4, $5); }` [download] You're not using the argument you pass to `parse_datetime`, but $_, which is a global variable. Try `sub parse_datetime($) { my $s = shift; $s =~ /(\d\d\d\d)-(\d\d)-(\d\d) (\d\d):(\d\d)/; return($1, $2, $3, $4, $5); }` [download] instead. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l] [select]
Re^2: Array iteration: foreach works, for doesn't by Dźwiedziu (Initiate) on Nov 23, 2009 at 13:43 UTC
Thanks, works perfectly.	[reply]
Re: Array iteration: foreach works, for doesn't by Bloodnok (Vicar) on Nov 23, 2009 at 13:33 UTC
In the foreach loop, the `$_` variable is set implicitly, so the pattern match within `parse_datetime()` will always 'work' whereas ... in your for loop, the `$_` variable remains unset, so the match always fails. Try `(local $_ = shift) =~ ...` or `+shift =~ ....` in place of `$_ =~` at the start of `parse_datetime()` A user level that continues to overstate my experience :-))	[reply] [d/l] [select]