I thought I knew this, but...

Often I will test some base-level part of a Perl routine I am writing,
even if it is something I already know, just to make sure it really
works as expected. I find that this makes later debugging easier.

Anyway, I need to count how many 'records' I have in a hash after my
program gets done stuffing values into it. So I wrote a piece of test
code that goes like this:
------------------------
use warnings;

%parts = {};

$parts{'ms51957-101'} = 1207;
$parts{'ms51957-102'} = 17;
$parts{'ms51957-103'} = 1;

print "Number of Records in hash: ", scalar keys %parts, "\n";

Number of Records in hash: 4
------------------------

To my surprise, this is wrong; there are only 3 records in the hash.

NOW for the weird part. On a hunch, I changed the line:

%parts = {};

....to...

%parts = ();

Now I get:

Number of Records in hash: 3

....which is correct. Anyone out there know what gives?? And which is the
"correct" way to define an empty hash, {} or ()?

Barry Brevik
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On Thu, 26 Feb 2009, Barry Brevik wrote:
>
> Often I will test some base-level part of a Perl routine I am writing,
> even if it is something I already know, just to make sure it really
> works as expected. I find that this makes later debugging easier.
>
> Anyway, I need to count how many 'records' I have in a hash after my
> program gets done stuffing values into it. So I wrote a piece of test
> code that goes like this:
> ------------------------
> use warnings;
>
> %parts = {};
>
> $parts{'ms51957-101'} = 1207;
> $parts{'ms51957-102'} = 17;
> $parts{'ms51957-103'} = 1;
>
> print "Number of Records in hash: ", scalar keys %parts, "\n";
>
> Number of Records in hash: 4
> ------------------------
>
> To my surprise, this is wrong; there are only 3 records in the hash.

Didn't you get a warning from the program, something like " Reference
found where even-sized list expected"?

> NOW for the weird part. On a hunch, I changed the line:
>
> %parts = {};

This is the same as

%parts = ({}, undef);

Which creates a single hash element that has the stringified version of a hash
reference, e.g. "HASH(0x239bac)" and a value of undef.

> ...to...
>
> %parts = ();
>
> Now I get:
>
> Number of Records in hash: 3
>
> ...which is correct. Anyone out there know what gives?? And which is the
> "correct" way to define an empty hash, {} or ()?

() is an empty list, {} is a reference to an anonymous empty hash.

Cheers,
-Jan

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Barry Brevik wrote:

> ...which is correct. Anyone out there know what gives?? And which is the
> "correct" way to define an empty hash, {} or ()?

{} is a hashref, so assigning {} to a hash will lead this empty hashref to be a key and value is undefined.
So it's like my %parts = ( {} => undef ); which is obviously wrong.

You may want to just declare this hash using "my %parts;", it will be empty.

By the way, if you "use warnings", you should get a warning about it, "Reference found where even-sized list expected".

--
Serguei Trouchelle
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use warnings;

%parts = {};

$parts{'ms51957-101'} = 1207;
$parts{'ms51957-102'} = 17;
$parts{'ms51957-103'} = 1;

print "Number of Records in hash: ", scalar keys %parts, "\n";

Number of Records in hash: 4
------------------------

> To my surprise, this is wrong; there are only 3 records in the hash.

not quite - {} is an empty, anonymous hash, assigned as a key to the first
entry in %parts. undef is the value. Should've seen:
Reference found where even-sized list expected at -e line 3.

%parts = ();

clears it but isn't needed if parts is new:
my %parts;

will ensure a new, empty hash.

a
-------------------
Andy Bach
Systems Mangler
Internet: andy_bach [at] wiwb.uscourts.gov
Voice: (608) 261-5738; Cell: (608) 658-1890

Entropy just ain't what it used to be
--=_alternative 007BEA9E86257569_=
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<tt>use warnings; 
 
%parts = {}; 
 
$parts{'ms51957-101'} = 1207; 
$parts{'ms51957-102'} = 17; 
$parts{'ms51957-103'} = 1; 
 
print "Number of Records in hash: ", scalar keys %parts, "\n"; 
 
Number of Records in hash: 4 
------------------------ 
 
> To my surprise, this is wrong; there are only 3 records in the hash.</tt>
 
 not quite - {} is an empty, anonymous
hash, assigned as a key to the first entry in %parts. undef is the value.
Should've seen:
 Reference found where even-sized list
expected at -e line 3.
 
 %parts = ();
 
 clears it but isn't needed if parts
is new:
 my %parts;
 
 will ensure a new, empty hash.
 
 a
 ------------------- 
Andy Bach 
Systems Mangler 
Internet: andy_bach [at] wiwb.uscourts.gov 
Voice: (608) 261-5738; Cell: (608) 658-1890 
 
Entropy just ain't what it used to be
--=_alternative 007BEA9E86257569_=--

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