http://qs321.pair.com?node_id=117928

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

I am ready to tear my hair out. I have been debugging for 5 hours now and I have found the bug in my morass of code, but I don't understand why it is occuring...

Here is a sample script that demonstrates the bug.

My element 00 in $hash doesn't print when I know it is there. I want to believe this is a perl bug, but I suspect it is really my not understanding how 00 is getting evaluated or something. 

@foo = (0,1);
$hash{00} = "data";
$hash{10} = "moredata";

print '$hash{00} is ', $hash{00} , "\n";
print '$hash{$foo[0] . "0"} is ', $hash{$foo[0] . "0"}, "\n";
print '$hash{$foo[1] . "0"} is ', $hash{$foo[1] . "0"}, "\n";
print '$hash{10} is ', $hash{10} , "\n";

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The results are:

NOTE LINE 2 after is there is nothing! 

$hash{00} is data
$hash{$foo[0] . "0"} is 
$hash{$foo[1] . "0"} is moredata
$hash{10} is moredata

[download]

What is going on?

-monkfish
The fishy monk.

Replies are listed 'Best First'.
Re: Hash element that won't print. Perl Bug???
by snowcrash (Friar) on Oct 10, 2001 at 09:12 UTC
    $hash{00} is actuallay the same as $hash{0} because the key is a number and so is not interpreted as a string. if you would use warnings or the -w switch you would have spotted this one easier.
    btw: i recently found out that $hash{1_000} is the same as $hash{1000} cos perl allows to use the underscore for legibility in larger numbers like 123_324_234.
    hth
    snowcrash
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Re: Hash element that won't print. Perl Bug???
by nardo (Friar) on Oct 10, 2001 at 09:12 UTC
    The answer is discovered with
    print join(', ', keys %hash), "\n";
    which reveals that 00 becomes 0, if you use
    $hash{'00'} = 'data';
    it will work like you want it to.
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Re: Hash element that won't print. Perl Bug???
by mirod (Canon) on Oct 10, 2001 at 09:14 UTC

    Using formatted numbers has hash keys is not a good idea. The data is not in $hash{00} but in $hash{0}, as 00 is interpreted as a number and not as a string.

    When in doubdt about your data try either the perl debugger (run perl -d) or Data::Dumper (or Data::Denter if you have it installed):

    #!/bin/perl -w
use strict;
use Data::Dumper;

my @foo = (0,1);
my %hash;

$hash{00} = "data";
$hash{"00"} = "the real one";
$hash{10} = "moredata";

print Dumper( %hash);

print '$hash{0} is ', $hash{0} , "\n";
print '$hash{00} is ', $hash{00} , "\n";
print '$hash{$foo[0] . "0"} is ', $hash{$foo[0] . "0"}, "\n";
print '$hash{$foo[1] . "0"} is ', $hash{$foo[1] . "0"}, "\n";
print '$hash{10} is ', $hash{10} , "\n";

    [download]
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Re: Hash element that won't print. Perl Bug???
by kjherron (Pilgrim) on Oct 10, 2001 at 09:14 UTC
    The 00 in the term $hash{00} is initially evaluated as a number, rather than a string, because it's not in quotes. Perl then converts the number back into a string to store into the hash. As a result, the assignment 
        $hash{00} = "data";

    [download]
    actually stores into $hash{"0"}, not $hash{"00"}.

    Later, when you try to access $hash{0 . "0"}, you do access $hash{"00"}, which of course is not the same as $hash{"0"}.

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Re: Hash element that won't print. Perl Bug???
by cLive ;-) (Prior) on Oct 10, 2001 at 09:29 UTC
    In your hash definition 00 is interpolated numerically to become 0. What you need is: 
    $hash{'00'} = "data";
$hash{'10'} = "moredata";

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    It's just "coincidence" that 10 = "10".

    When you access the hash later, the key is interpolated in string context.

    If you were doing 3 digits, things would be worse, because 010 = "8" (octal) and not "10".

    Make sense?

    cLive ;-)

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