Comma Operator, BUG or feature ???

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

... After only 3 hours searching, finally found a mistake in my code ...

I am somewhat disappointed actually getting no warnings from the compiler - please consider the following code:

#!/usr/bin/perl

use strict;
use warnings;

use Data::Dumper;

my %hash;

$hash{'A'.'B'} = 1;
# note a mistakenly written comma between 'C' and 'D'  
$hash{'C','D'} = 2;
$hash{'E','F'} = 3;

print Dumper(\%hash);
print exists $hash{'CD'} ? "YES\n" : "NO\n";
[download]

And here is the output

$VAR1 = {
          'CD' => 2,
          'EF' => 3,
          'AB' => 1
        };
NO
[download]

My xterm shows nothing between C and D in the output from Dumper, but with hexdump i could see that there is actually HEX: '1c' there

24 56 41 52 31 20 3d 20  7b 0a 20 20 20 20 20 20  |$VAR1 = {.      |
20 20 20 20 27 43 1c 44  27 20 3d 3e 20 32 2c 0a  |    'C.D' => 2,.|
20 20 20 20 20 20 20 20  20 20 27 45 1c 46 27 20  |          'E.F' |
3d 3e 20 33 2c 0a 20 20  20 20 20 20 20 20 20 20  |=> 3,.          |
27 41 42 27 20 3d 3e 20  31 0a 20 20 20 20 20 20  |'AB' => 1.      |
20 20 7d 3b 0a 4e 4f 0a                           |  };.NO.|
[download]

Any ideas, what's really happened? Why is it '1c' and not 'f5'? Is it a bug that perl produces no warnings in this case?

Comment on Comma Operator, BUG or feature ??? Select or Download Code

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Re: Comma Operator, BUG or feature ??? by moritz (Cardinal) on Sep 22, 2009 at 15:35 UTC
use `use Data::Dumper; $Data::Dumper::Useqq = 1; ...` [download] To properly escape non-printing characters in the output. See the documentation for $; to see what's happening. Perl 6 - links to (nearly) everything that is Perl 6.	[reply] [d/l]
Re^2: Comma Operator, BUG or feature ??? by nif (Sexton) on Sep 22, 2009 at 16:21 UTC
Thank you for the explanation - I didn't know that $foo{$a,$b,$c} means $foo{join($;, $a, $b, $c)} Thank you again	[reply]
Re: Comma Operator, BUG or feature ??? by ikegami (Patriarch) on Sep 22, 2009 at 16:01 UTC
Is it a bug that perl produces no warnings in this case? "`.`" is the concatenation operator, and you used it properly. There's no reason for it to give a warning. As for a hash element index, it accepts multiple comma-separated values. This was a means of doing multilevel hashes before Perl had true references. These days, one would create a hash or hashes. `$hash{A}{B} = 1; $hash{C}{D} = 2; $hash{E}{F} = 3;` [download] Again, it was used properly, so there's no reason for it give a warning. Why is it '1c' and not 'f5'? F5? As in LATIN SMALL LETTER O WITH TILDE? Why do you expect that? It can be done, though. Just change `$;` to `"\xF5"`.	[reply] [d/l] [select]
Re: Comma Operator, BUG or feature ??? by JavaFan (Canon) on Sep 22, 2009 at 17:13 UTC
You might want to look up the `$;` entry in perlvar. `$hash{'C','D'}` is short for `$hash{join($;,'C','D')}`, with `$;` defaulting to `"\034"`, which is the same as `"\x1c"`.	[reply] [d/l] [select]