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From: ziegast@eng.umd.edu (Eric W. Ziegast)
Newsgroups: comp.lang.perl
Subject: Re: A question on associative arrays
Message-ID: <1991May31.011755.4305@eng.umd.edu>
Date: 31 May 91 01:17:55 GMT
References: <PRZEMEK.91May30164101@rrdstrad.nist.gov> <1991May30.205555.14490@convex.com>
Sender: news@eng.umd.edu (C-News)
Reply-To: ziegast@eng.umd.edu (Eric W. Ziegast)
Organization: University of Merryland, Engineering Computing Services
Lines: 51

(Tom Christiansen) writes:
> From the keyboard of (Przemek Klosowski):
>       How does one check if the associative array was assigned at all?
> :My program runs a loop initializing an array, but each individual
> :initialization is conditioned on something. On a rainy day all conditions
> :could be false: is there a way to find out? The obvious $#ary doesn't work,
> :since it refers to @ary. defined(%ary) is no good, since %ary might be
> :pre-defined in a local(). Ideas, anyone?
> 
>     if (%a) {
> 
> will actually work, but I don't think that Larry has ever documented
> this.  More portably, and more slowly would  be
> 
>     if (@my_keys = keys %a) {

Perhaps this should be documented.  I gave it a few tests of wort-case
scenarios...

1. Null assignment (an assignment nonetheless)

	$assoc{''}='';
	print %assoc ? "true" : "false";

   Results in true.

2. Defining the assoc as a local

	sub foo {
		local(%assoc);
		print %assoc ? "true" : "false";
	}
	&foo

   Results in false.

3. A new dbm file

	dbmopen(%assoc,"newdbmfile",0600);
	print %assoc ? "true" : "false";

   Results in false.


I performed these tests with Perl 3.0 (41) on a SunOS 4.1 system.
I believe ndbm was compiled into our perl.  Just FYI.

Someone more knowledgable might want to explain why/how this works.

-- 
Eric Ziegast