Hash to Scalar Assignment Oddity

[dsb]

A coworker and I were trying to work out some of the code when tie'ing a hash to a Berkeley DB file. I noticed that one of the lines called for the return value from a tie() to be assigned to a scalar variable. So I tried assigning a regular old hash - not a reference to a hash - to a scalar. I didn't get what i expected.

I half expected the code to bomb. Instead the assignment evaluated and when I printed the contents of the scalar, I got a weird result. See the code.

%db = ( one => 1, two => 2, three => 3 );
print %db, "\n";     # prints 'three3one1two2'

$db = %db;
print $db, "\n";     # prints '3/8'

Anyone have an idea of what's going on here and why I get a fraction when I print the contents of $db?

[philcrow]

When you put a hash into scalar context, it gives information about the hash buckets (rarely useful).

Phil

[bunnyman]

This isn't odd. It's supposed to do that.

If you evaluate a hash in scalar context, it returns false if the hash is empty. If there are any key/value pairs, it returns true; more precisely, the value returned is a string consisting of the number of used buckets and the number of allocated buckets, separated by a slash. This is pretty much useful only to find out whether Perl's internal hashing algorithm is performing poorly on your data set. For example, you stick 10,000 things in a hash, but evaluating %HASH in scalar context reveals "1/16" , which means only one out of sixteen buckets has been touched, and presumably contains all 10,000 of your items. This isn't supposed to happen.

-- man perldata

[revdiablo]

This isn't odd. It's supposed to do that.

Are you saying defined behavior is never odd?!? That's quite a bold statement, and one I'm certainly not prepared to make (especially in this case, where I do indeed find the defined behavior a bit odd, even if it's well documented and I already knew what it was). :-)

[dsb]

Now what doc is that in?

[bunnyman]

I already told you. It's perldata.

[borisz]

The first value is the number of used entries and the second the number of slots. Read it as we use currently 3 from 8 entries. You can set the slots if you like with:

keys %db = 1000;
$db = %db;
print $db;

[ikegami]

In list context, the hash returns
('one', 1, 'two', 2, 'three', 3)
(but not necessarily in that order). So
print(%db, "\n");
is the same as
print('one', 1, 'two', 2, 'three', 3, "\n");
When you pass a list to print, it does
join($,, LIST)
So the above print is equivalent to
print(join($,, 'one', 1, 'two', 2, 'three', 3, "\n"));

However, in scalar context, the hash some information about its buckets. To do emulate the
behaviour of print, do
$db = join($,, %db);

References:
hash
print
$,