Hash to Scalar Assignment Oddity

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

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'
[download]

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

dsb
This @ISA my( $cool ) %SIG

Comment on Hash to Scalar Assignment Oddity Download Code

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Re: Hash to Scalar Assignment Oddity by philcrow (Priest) on Jan 26, 2006 at 15:34 UTC
When you put a hash into scalar context, it gives information about the hash buckets (rarely useful). Phil	[reply]
Re: Hash to Scalar Assignment Oddity by bunnyman (Hermit) on Jan 26, 2006 at 18:12 UTC
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	[reply]
Re^2: Hash to Scalar Assignment Oddity by revdiablo (Prior) on Jan 26, 2006 at 20:56 UTC
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). :-)	[reply]
Re^2: Hash to Scalar Assignment Oddity by dsb (Chaplain) on Jan 30, 2006 at 21:00 UTC
Now what doc is that in? dsb This `@ISA my( $cool ) %SIG`	[reply]
Re^3: Hash to Scalar Assignment Oddity by bunnyman (Hermit) on Feb 08, 2006 at 19:06 UTC
I already told you. It's perldata.	[reply]
Re: Hash to Scalar Assignment Oddity by borisz (Canon) on Jan 26, 2006 at 15:47 UTC
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 $db;` [download] Boris	[reply] [d/l]
Re: Hash to Scalar Assignment Oddity by ikegami (Patriarch) on Jan 26, 2006 at 17:16 UTC
In list context, the hash returns `('one', 1, 'two', 2, 'three', 3)` (but not necessarily in that order). So `print(%db, "\n");` is the same as `print('one', 1, 'two', 2, 'three', 3, "\n");` When you pass a list to `print`, it does `join($,, 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 $,	[reply] [d/l] [select]